QA-QC of IMRT: American Perspective
Jean M. Moran, Ping Xia
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Contents
11.1 The Paradigm of Quality Assurance . . . 129
11.1.1 Verification of Patient Position . . . 129
11.1.2 Dosimetric Verification as Validation of IMRT Plans . . . 130
11.1.3 Computational vs Experimental Dosimetry . . 130
11.1.4 Reliability and Validity of Tests for QA . . . 131
11.1.5 QA of Total Treatment Chain . . . 131
11.1.6 QA as a Familiarization Tool when Starting IMRT . . . 131
11.1.7 How much QA Is Enough? . . . 132
11.2 Treatment-Method Specific QA . . . 132
11.2.1 Penumbra Modeling . . . 132
11.2.2 Small Field Scatter Factors . . . 133
11.2.3 Radiation Field Offset . . . 133
11.2.4 Leakage and Transmission: Interleaf, Intraleaf, Leafend . . . 133
11.2.5 Leaf Sequencer . . . 134
11.3 Machine Specific QA . . . 134
11.3.1 MLC-based Systems . . . 134
11.3.2 Sequential Tomotherapy . . . 135
11.3.3 Record and Verify System . . . 136
11.4 Patient Specific Quality Assurance . . . 136
11.5 Methods of Dosimetry of IM Beams . . . 137
11.5.1 Film . . . 137
11.5.2 EPID . . . 138
11.5.3 2D Arrays . . . 139
11.6 Methods of Dosimetry of Complete IMRT Treatments . . . 139
11.6.1 2D Film . . . 139
11.6.2 3D Gel . . . 139
References . . . 140
11.1 The Paradigm of Quality Assurance
In the United States, the professional societies of the American Association of Physicists in Medicine and the American Society of Therapeutic Radiolo- gists have collaborated within their organizations and between organizations to provide guidance in deter- mining the appropriate use of IMRT, beginning an
IMRT program, and maintaining the appropriate qual- ity assurance to safely use IMRT [1–3]. The complex beam intensity modulation in each IMRT field has required new systems for treatment planning and de- livery and therefore, a paradigm shift has occurred in quality assurance. Each institution should have a com- prehensive quality assurance program tailored to their software, delivery system, and patient planning and treatment process for IMRT. Such a program typically involves a team approach to quality assurance includ- ing physicians, physicists, dosimetrists, therapists, and nurses. This chapter focuses on system, machine, and patient-specific quality assurance requirements in IMRT delivery.
11.1.1 Verification of Patient Position
For IMRT to fulfill its promise to reduce normal tissue complications while improving local control, verification of target position is critical. Verification methods may vary by institution and depend on the specific disease site and whether or not the tu- mor volume involves organ motion. Other chapters in this book address adaptive approaches to patient positioning and the development and potential of image-guided therapy. Therefore, this section focuses on how patient immobilization and verification should be addressed within a comprehensive quality assurance program.
As in conventional conformal radiotherapy, the pa- tient model for IMRT is developed based on image data such as CT scans, additional diagnostic scans, and information about patient setup and organ motion.
A well-defined process should be used for patient immo- bilization prior to acquisition of the treatment planning CT scan. For each treatment site, it is helpful to de- velop and follow a checklist prior to the CT scan with details such as slice thickness, region of interest, and immobilization aids clearly listed.
The degree of positioning accuracy depends on the immobilization, localization method, and motion of the organ. When using thermoplastic materials, the manu-
facturer instructions for heating and cooling should be followed carefully. An improperly cooled patient mask may shrink after the CT scan and be uncomfortable for the patient during the entire course of treatment.
For head and neck patients, it may be necessary to cut regions out of the mask material to reduce skin reac- tions because of increased surface dose due to the mask material [4].
At the treatment unit, patient anatomy can be verified with orthogonal images using electronic portal imagers or film [5]. For head and neck patients, bony landmarks are sufficient as reference points for the location of the tumor. When a mask is used over the head and shoul- ders along with other immobilization aids (such as bite blocks), a setup accuracy of 1–3 mm is possible [6]. For prostate, organ motion has been shown to be significant with respect to the pelvis bony structure and therefore, another method is required for accurate target position- ing. Daily localization with gold implanted markers [7]
or ultrasound [8] are methods that are used in the US and permit localization to within 2–3 mm. Use of such a daily positioning method allows for a reduction in dose delivered to the bladder and rectum when treating prostate cancer.
For other organs that move significantly with respi- ration (lung, liver, breast), gating and active breathing control methods that limit respiratory motion while the beam is on allow for significant improvements in target positioning [9–11]. Prior to using such methods, ad- ditional QA is required on the immobilization method (see chapter II. 11).
Other improvements have been made to the treat- ment couch. Patient immobilization devices that can be fixed directly to the treatment couch improve the precision and efficiency of the patient setup process.
Treatment table tops made of carbon fiber permit beam delivery at multiple angles without metal parts in the beam. Measurement of attenuation through such treatment couches should be conducted to determine whether the attenuation should be taken into consid- eration or can be safely ignored. While published data from other institutions provide a guideline of what is achievable, it is important that each institution assess its immobilization and localization method to determine the accuracy of its own immobilization and position- ing process for each treatment site [11]. It may be necessary to change the process prior to the start of an IMRT program if the positioning accuracy does not meet the treatment goals. The positioning and organ motion information is used in the treatment planning system to expand target and organ-at-risk volumes [12, 13]. This information sets the limits of what is achievable for normal tissue sparing with IMRT. Additional checks, such as comparing measured patient treatment SSDs with the treatment planning system, can also be an effective part of a QA pro- gram.
11.1.2 Dosimetric Verification as Validation of IMRT Plans
AAPM Task Group 40 report recommends an in- dependent check of monitor units prior to patient treatment [14]. In conventional therapy, this can be done through hand calculations based on measurements in water. For IMRT plan verification, the situation is more complicated due to the hardware and software involved in IMRT planning and delivery. Individual fields are composed of many small segments (ranging from a few to several hundred) of varying intensity located on and off the central axis of the beam. Therefore, hand calcu- lations are unreasonable for verification of IMRT plans.
The main approaches used for verification of IMRT plans are dosimetric measurements and monitor unit calculations.
Dosimetric measurements typically encompass two types of checks for pre-treatment quality assurance.
One measurement is a composite delivery of all IMRT fields at the correct delivery angles on a phantom (of- ten cylindrical or cubic) with an ion chamber or other detectors to verify the dose at a single point or multi- ple points [15,16]. The chamber position (and hence the phantom position in plastic phantoms) may need to be adjusted to determine an appropriate high dose, shallow gradient region for the measurement. At many centers, additional verification is performed of individual fields with film or other 2D imaging systems. Verification can be a qualitative visual check of the intensity pat- terns or it can be done in a phantom with dosimetric measurements to provide additional information on the delivery [16].
Measurements of individual patient treatment fields test the transfer of patient information from the planning system to the record-and-verify system, the deliverability of individual fields, and dosimetric eval- uation of the delivery compared to the dose calculation in the phantom geometry. However, a limitation of dosi- metric measurements in a homogeneous phantom is that errors from the planning process are not checked.
For example, if the table top in the CT scanner is improp- erly contoured as part of the patient surface that error would not be caught with dosimetric measurements.
A careful review of the patient CT scan and contoured structures is still necessary.
11.1.3 Computational vs Experimental Dosimetry
No single quality assurance check provides enough in- formation to verify that IMRT delivery for a patient will be accurate. Dosimetric measurements are time- consuming and only verify the dose in a phantom geometry. Also, discrepancies between planar measure- ments and calculations may be difficult to interpret in
terms of the clinical significance on a patient by patient basis. Independent monitor unit calculations provide a complementary verification of the dose to a single point at isocenter in the patient when compared to the treatment planning system calculation.
Methods based on a Clarkson integration for in- dividual segments or on scatter calculations have been developed for dynamic, step-and-shoot delivery, MIMiC, and mMLC delivery methods [17–19]. The user should understand the method of calculation and ap- proximations in the algorithm. Approximations include ignoring the contour of the patient surface or limited modeling of scatter interactions resulting in less accu- rate calculations in regions where the contribution of dose from scatter is high [18]. Prior to clinical use, the calculation method should be verified against phantom measurements and the treatment planning system algo- rithm. Once a calculational method is properly verified, significant time can be saved when compared to mea- surements. However, additional QA checks are needed to verify accurate transfer of patient data and accurate delivery.
11.1.4 Reliability and Validity of Tests for QA
Quality assurance tests should be designed to ensure that the sensitivity is appropriate to the impact of potential errors [2, 15, 16]. Standard plans should be developed and validated for testing during commissioning and af- ter software upgrades of the treatment planning and inverse planning systems, data transfer software, and the record and verify system. For MLC-based IMRT systems, accuracy of delivery depends on the num- ber of segments, leaf sequencing (with or without leaf synchronization to minimize tongue-and-groove) and machine-related parameters such as leaf position tol- erance, delivery method (SMLC, DMLC), leaf speed (for DMLC) and dose rate [2]. For serial tomotherapy with the MIMiC collimator, positional problems with the alignment of the MIMiC collimator with the linear accelerator or with the couch indexing can lead to sig- nificant treatment delivery errors and therefore must be checked [20].
11.1.5 QA of Total Treatment Chain
The entire treatment chain for IMRT must be verified.
When compared with 3D conformal therapy, stricter requirements may be placed on frequency of testing and accuracy because of new planning and delivery methods. Thorough commissioning of individual com- ponents should be followed with checks of the entire process from the CT scan and derivation of the pa- tient model (including setup and motion) to treatment planning and delivery.
The first part of the QA chain is information from the CT-simulation. The beam angles and sharp gradi- ents within fields require accurate models of the patient.
As fields become more conformal, organ motion can degrade the delivered dose distribution. As discussed above, proper immobilization is required for the CT scan. Setup uncertainty and organ motion must be considered for input into the treatment planning pro- cess [21, 22]. As with 3D conformal therapy, CT and other image datasets must be transferred accurately to the treatment planning system and the spatial and density information must be verified. The expansion of surfaces from generated contours can be verified using phantom tests.
The second part of the treatment chain is the treat- ment planning system. Tools, such as dose volume histograms, beam placement, dose calculation, and dig- ital reconstructed radiograph generation, should all be verified following published guidelines [2,3,14,23]. The user should assess how dose changes with increased in- tensity modulation. In addition to standard checks, the generation of leaf sequences from the inverse planning system for delivery must be checked. Some systems al- low for recalculation of the dose using the sequenced MLC files. Tools should be readily available to transfer the patient plan to a phantom geometry, recalculate the doses, and import measured 2D distributions.
The final part of the chain is patient treatment. This step relies on accurate transfer of patient treatment data. Data should be transferred digitally from the plan- ning system to the record-and-verify system. All IMRT fields should be delivered with the level of accuracy considered to be acceptable and achievable during com- missioning of the IMRT process. Beam angles through the table top should be evaluated to determine if atten- uation through the table top is modeled correctly in the planning process. The patient position should be eval- uated on the first few days of treatment using EPIDs or film [24].
11.1.6 QA as a Familiarization Tool when Starting IMRT
When first beginning an IMRT program, quality assur- ance is an important part of determining the limits of a planning and delivery system. QA tests can be run more frequently to determine the reproducibil- ity of leaf positioning. Process QA should begin with simple geometric fields measured in a flat phantom geometry and progress to more complicated intensity modulated fields. In addition to simple checks, treat- ment planning studies should be done on patient models prior to treating with IMRT. Target and normal struc- tures in the treatment fields should be contoured, the dosimetrist should gain experience in adjusting the ob- jective functions in the inverse planning system to meet the physician needs. Once an acceptable plan is reached,
the delivery should be tested along with dosimetric mea- surements and verification calculations if appropriate.
At each step, the process should be verified. For ex- ample, the structures derived from the drawn contours should be evaluated in 3D to make sure that the con- tours are drawn consistently. Because the output of the inverse planning system is dependent on the treatment objectives and prescribed dose, each new treatment site should be commissioned from treatment planning, op- timization, to delivery. Mechanical checks should also be in place to evaluate reproducibility of mechanical systems over time. Finally, it is important that all mem- bers of the implementation team understand how long each part of the process takes so that the expectations of the IMRT program are reasonable, achievable, and safe.
11.1.7 How much QA Is Enough?
Each institution’s QA program should be designed to encompass the entire IMRT process. The checks should be divided into hardware, software, and data transfer checks. Procedures should be in place to modify the institution’s existing daily, weekly, monthly, and annual QA to include appropriate tests for IMRT. The physicist should be prepared with the appropriate tests to run after software and hardware upgrades. As an institution gains more experience with the IMRT process, hardware, and software, it may be appropriate to adapt the process accordingly.
The frequency of tests depends on the significance of errors as well as institution’s experience, equipment, and frequency of software and hardware upgrades. For example, since errors in leaf positioning and leaf gap affect all IMRT patients, routine tests should evaluate the accuracy of both parameters. Later sections address the QA checks in detail.
The QA program must also be designed considering the software and hardware at the institution. For ex- ample, individual pre-treatment QA measurements are a critical part of a QA program if the institution does not verify patient monitor unit calculations with an in- dependent dose calculation algorithm. If an institution does use an independent dose calculation algorithm, then it also needs specific tests to verify data trans- fer from the planning system to the record-and-verify system. If limitations are set on the smallest monitor unit fraction, number of segments, or segment size, it is important that checks are in place to adhere to those limits.
After gaining years of experience, several centers have decreased their individual patient-specific QA measure- ments. The decrease in patient-specific measurements was possible because the system was thoroughly tested and appropriate individual QA tests are in place to identify specific problems [16].
11.2 Treatment-Method Specific QA
Quality assurance tests of an inverse treatment plan- ning system, leaf sequencing algorithm, and delivery technique involves a subset of checks from the commissioning process [2, 3]. The tests outlined in this section focus primarily on SMLC-IMRT, DMLC- IMRT, and sequential tomotherapy since these are the main delivery methods currently in clinical use.
For SMLC-IMRT and DMLC-IMRT, fields are com- posed of many small segments superimposed during delivery at a fixed gantry angle. For sequential tomother- apy, a special multileaf intensity modulating collimator (MIMiC, NOMOS Corp., Sewickley, Pennsylvania) con- sisting of two banks of 20 tungsten vanes with a leaf length of 1 or 2 cm and leaf width of 1 cm when pro- jected 100 cm from the X-ray target to the isocenter for a maximum field size of 2× 20 cm at isocenter for a sin- gle arc. The beam intensity is usually modulated every 5 or 10 degrees of gantry rotation by moving leaves into and out of the fan beam. The beam intensity produced at each leaf position is proportional to the fraction of time the leaf is held in the open position. Prior to ver- ifying individual IMRT fields, it is important to verify the separate parts of the planning system that will affect the overall accuracy. It is important that QA includes the following checks:
1. Penumbra modeling
2. On- and off-axis small field collimator scatter factors 3. Leaf offset factor to correct discrepancies between
the light field and radiation field
4. Transmission through the MLC leaves or vane col- limator – inter-leaf leakage, intra-leaf leakage, and leaf-end leakage
5. Leaf sequencer accuracy
6. Additional checks for sequential tomotherapy
11.2.1 Penumbra Modeling
For conventional therapy, the impact of the modeling inaccuracies in the penumbra is typically limited to the region encompassing the outside of the target volume.
For IMRT delivery, fields are composed of many sub- fields. Therefore, modeling inaccuracies affect multiple regions across the target volume and normal tissues.
Measurements with ionization chambers that have an inner diameter greater than 3 mm will over-estimate the penumbra width [2, 15]. Care should be taken that measurements are made with an appropriate detec- tor for determining the penumbra width such as film or small detector with high spatial resolution such as a diode, diamond detector, or pinpoint ionization chamber [2].
11.2.2 Small Field Scatter Factors
The small sub-fields that make up IMRT fields affect the output factor of the beam on and off the central axis.
As sub-fields decrease in size to less than 3× 3 cm2, the output sharply decreases due to lack of electronic equi- librium [25]. These fields are often irregularly shaped as well. It is important to verify the dose calculation algo- rithm against reliable beam measurements made with an appropriate small field detector. Special care must be taken to verify that leaf positioning is accurate because of sensitivity of the output factor on leaf positioning for small fields. For example, a 2 mm deviation in field size changes the dose per monitor unit by 2% for a 2×2 cm2 field and 15% for a 1× 1 cm2field for a 6-MV photon beam [25]. Discrepancies between measurements and calculations off-axis can be due to lack of inclusion of off-axis softening of the photon energy spectrum in the model [25]. The commissioning process should include determining the minimum acceptable segment size for IMRT and QA tests based on these results and then setting this limit in the treatment planning system if pos- sible. This should be coupled closely with determination of the necessary leaf position accuracy for delivery along with the appropriate QA that the accuracy requirement is met.
11.2.3 Radiation Field Offset
Leaf position accuracy not only affects the machine out- put, but also affects the dose delivery in the overlap and underlap regions of sub-fields within an IMRT field.
A systematic leaf position error may be introduced if dif- ferences between the radiation field and the light field are not taken into account in the planning system. The dosimetric effect of the differences depends on how the radiation and light field are calibrated. The radiation field offset is a significant factor for MLC designs that have curved leaf-ends and move linearly with respect to the radiation beam instead of moving divergently with the beam. The radiation field offset is also found in double-focused MLCs (where the motion of MLC leaves follows the divergence of the radiation beam) [26]. Sev- eral institutions have studied the radiation field offset and found values ranging from of 0.7 to 1 mm depending on the characteristics of multi-leaf collimators, beam en- ergy, and gantry angles [27, 28]. Ignoring the radiation field offset in IMRT delivery has been shown to result in significant discrepancies between calculations and
Manufacturer Inter-leaf (%) Intra-leaf (%) Leaf-end (%)
Siemens 1.1 0.8 1.6
Elekta 2.5 1.6 > 20%
Varian 1.8 1.2 > 20%
Table 1.Values of inter-leaf, intra-leaf, and leaf-end trans- mission for a 6-MV beam measurements of IMRT fields, especially in the vicin- ity of critical structures [28, 29]. Ideally, this issue will be addressed more directly in future versions of com- mercial treatment planning systems. The radiation field offset should be determined by measurement of the ra- diation field over multiple gantry angles (e.g., 0, 90, and 270◦) with film. An average correction value should be chosen.
11.2.4 Leakage and Transmission: Interleaf, Intraleaf, Leafend
For MLC-based systems, leakage and transmission are important factors that should be modeled directly when possible. The main types of transmission for MLC sys- tems are interleaf leakage, intraleaf transmission, and transmission between opposed leafends. The values for the different systems are dependent on the design, posi- tion in the collimator head, and whether or not the leaf ends are double-focused, moving in an arc with respect to the beam or curved, moving linearly. The amount of leakage per field can also vary with the leaf sequenc- ing algorithm and the maximum beamlet intensity of a field.
Figure 1 shows the leakage patterns for the three major MLC collimators [30]. The inter-leaf and the intra- leaf leakage are represented by the crests and troughs of the graphs, respectively. Table 1 shows the values for Siemens, Elekta, and Varian accelerators [30]. Notice that the most significant difference is in the values of the leaf-end leakage. Siemens has a double focused de-
Fig. 1. Leakage patterns for the major MLC collimators. From: Huq MS, Das IJ, Steinberg T, Galvin JM (2002) A dosimetric comparison of various multileaf collimators. Phys Med Biol 47(12):N159–N170.
Reprinted with permission
sign where the leaves more in an arc with respect to the beam resulting in leakage similar to that between ad- jacent leaves. Because the Elekta and Varian leaves are curved, the leakage between opposed leaves is approxi- mately 20% [31]. The Elekta system also has a backup jaw immediately above the MLC. Users should measure their machine’s transmission values with film and compare the results to published values.
For all MLCs, the percent transmission value for conventional treatment is better than the leakage of a conventional cut block. However, the low delivery efficiency ratio (defined as the MU for a physical com- pensator to the MU for MLC-IMRT field) for IMRT fields can result in a significant portion of the total dose being contributed from the MLC transmission, particularly as the IMRT field complexity increases [32]. At this time, many treatment planning systems only model the transmission with an average value. More sophisticated models which take into account the different types of transmission are actively under investigation.
11.2.5 Leaf Sequencer
The optimization system derives intensity maps for each treatment angle based on the input objectives in the planning system. These intensity maps can be continu- ous profiles or discretized into beamlets of specific size (e.g., 0. 5×0.5 cm2or 1×1 cm2). Hardware and software delivery constraints may or may not be accounted for in the derivation of the intensity maps. The leaf sequencer (or leaf motion calculator) converts the intensity pro- file into a series of control points for delivery by the MLC that include all leaf positions and fractional moni- tor units based on delivery restrictions such as MLC leaf width, MLC step size, and intensity level.
As part of treatment planning commissioning for IMRT, the physicist should understand the basic prin- ciples of converting intensity profiles into a deliverable set of leaf sequences. The constraints for leaf sequenc- ing are dependent on the MLC design such as leaf width, over-travel distance, maximum field size, and interdigitation of leaves. User options in leaf sequenc- ing algorithms typically include the MLC step size and the number of intensity levels. As the step size is de- creased and the number of intensity levels is increased, the leaf sequenced file will more closely resemble the continuous profile. However, for some delivery systems, the number of control points or segments of the field is directly proportional to the delivery time. For these de- livery systems, one has to consider the practical delivery time when choosing these parameters and determine the accuracy limits of the chosen leaf sequencing pa- rameters. Another potential drawback of using a finer MLC step size and increased number of intensity levels, especially for SMLC-IMRT delivery, is the inclusion of many segments with small monitor units and|or small
field sizes. As noted earlier, the delivery accuracy for such small segments with low (or fractional) monitor units is much more strongly dependent on positioning accuracy due to machine and modeling limitations of these small segments.
For DMLC-IMRT delivery, some algorithms synchro- nize leaves to minimize tongue-and-groove effects while others minimize total travel time. The significance of effects such as tongue-and-groove depends on the mod- ulation within the field. Developing an efficient leaf sequencer that can minimize total delivery MUs, total number of segments, and total MLC leaf travel distance, remains an active research area [33, 34].
11.3 Machine Specific QA
Tests for accelerator quality assurance of IMRT delivery should verify proper functioning of delivery equipment at an appropriate level of accuracy and reproducibil- ity. Individual patient pre-treatment QA measurements do not serve as a valid substitute for routine eval- uation of the delivery equipment since those checks are designed to validate the overall process and treat- ment planning system/leaf sequencer output. To aid in problem-solving, delivery system checks should be kept distinct from process checks involving delivery of files from the treatment planning system and leaf se- quencer algorithm. The tests in this section focus on QA for MLC-based IMRT systems (SMLC-IMRT and DMLC-IMRT) and sequential tomotherapy IMRT de- livery systems.
11.3.1 MLC-based Systems
The frequency of tests depends on the impact of an error on patient planning and on the likelihood of an error occurring. Several tests are an important part of acceptance testing and annual quality assurance. Also, verification may be required after service if parts of the delivery system have been affected. Physical constraints of the MLC should be verified such as the maximum IMRT field width, software-controlled opposed leaves gap, and leaf inter-digitation (if allowed). The specific tests will depend on the design of the MLC [35]. For MLC-based systems, mechanical and dosimetric checks should verify the alignment and positioning of the MLC carriage and leaves to the central axis of the beam and to opposing leaves. The carriage skew affects the orien- tation of all leaves with respect to the central axis of the beam and should be measured with film [16]. Another check of the carriage involves checking the physical gap between the carriages with feeler gauges [16]. Be- cause carriage adjustments are needed infrequently, the physicist may want to make any changes with a service
engineer. Any adjustments should then be verified and documented.
MLC-related quantities that are incorporated into the treatment planning system are inter- and intra- leaf transmission|leakage, leaf end transmission, the radiation field offset, and small field output factors as described in Section 11.2. Tests of those parameters were described in Section 11.2. Figure 2a shows an example film for a 6 MV photon beam from a Siemens accelera- tor. To test the leakage and the gap between leaves, all leaves are closed at different locations across the field with the Y jaws held open by creating a small open- ing with the top and bottom mega-leaves (specific to the Siemens MLC design). If there is additional leakage dose between opposed leaves, unnecessary dose may be delivered in SMLC-IMRT where closed leaf pairs may appear in the middle of the field (if the closed leaves cannot be placed under a backup jaw). The position of closed leaves may also depend on the leaf sequencing al- gorithm. Another example test film for a Siemens 6 MV photon beam is shown in Fig. 2b to determine the radia- tion field offset. A series of abutting strips were delivered at a 0 degrees gantry angle resulting in a radiation field offset of 0.5 mm for this example. In addition to film tests, the special case of small fields on and off the cen- tral axis should be investigated. The machine output, beam flatness, and symmetry should be measured for small fields with few (or fractional) monitor units to set the limits in the planning and leaf sequencing system.
More frequent tests should be conducted on leaf posi- tioning accuracy and leaf gap in both static and DMLC- or SMLC-IMRT modes. Figure 2c depicts a film exposed to a test pattern consisting of eight strips with 2 cm width separated by 1 cm gaps. This test pattern can be used for numerical analysis of leaf position accuracy across the field by measuring the full width at half maximum for each strip. A sensitive visual test of leaf position- ing and gap accuracy and gap is the band test where a 1-mm field is delivered every few cm over the MLC range [16, 36]. Similar test patterns can be used for all manufacturers. The effect of gravity on leaf positioning should be evaluated by measuring the test fields at mul-
Fig. 2a–c. Three MLC patterns designed to test for static MLC leaf position accuracy:(a)MLC leaves closed at various locations across the IMRT field;(b)an MLC pattern with a series of abutting strips;
(c)an MLC pattern with a series of 2-cm strips and 1-cm gaps in between
tiple gantry angles. Such tests can be part of a routine QA program by alternating the gantry angle on a regular basis.
Additional tests are required for DMLC-IMRT. Leaf pair speed can be evaluated using a simple ramp test where strips of different doses are delivered [36]. The effect of dose rate on leaf position accuracy should be evaluated at multiple dose rates if more than one dose rate is used clinically. The effects of leaf accelera- tion and deceleration can be evaluated using machine log files when available [37]. The effect of the leaf position tolerance value on the dosimetric accuracy should be tested or an appropriate value investigated and used [28, 37].
11.3.2 Sequential Tomotherapy
Because the MIMiC collimator is an add-on device to the linear accelerator, additional quality assurance checks are required to verify that it is correctly positioned. The collimator fits into a block tray slot of a linear accelerator with its long axis oriented along the patient’s transaxial direction. Positional adjustment screws and bolts en- able the collimator slit to be aligned accurately with the cross-wires. Figure 3a shows an example alignment veri- fication of the radiation field along the inplane direction (parallel to the axis of rotation of the gantry) obtained with film positioned 15 cm off the isocenter and parallel to the inplane. Two exposures were delivered at gantry angles of 90 and 270 degrees with the MIMiC vanes fully open (2× 20 cm field size at isocenter). The amount of misalignment of the MIMiC in the inplane direction is the distance between the lines in the Fig. 3a. This is a sen- sitive test because it amplifies the effect caused by both gantry and collimator sag. A test of the collimator align- ment in the crossplane direction is shown in Fig. 3b.
A film was positioned at isocenter (parallel to the axis of the gantry) with every other leaf of the MIMiC open and then two exposures were delivered at gantry angles of 90 and 270 degrees. When the two exposures are superim- posed, the result is a checkerboard as shown in Fig. 3b.
Misalignments are seen by evaluating the matchline. In this example, the checkerboard is not perfectly matched at the field edge due to the amplified effect of gantry and collimator sag.
Another important aspect of QA for sequential to- motherapy involves verification of the couch indexing.
Fig. 3a,b. Quality assurance films to verify the alignment of the MIMiC collimator when it is attached to the accelerator:(a)align- ment verification of the radiation field in the inplane direction (parallel to the axis of rotation of the gantry); (b)alignment verification of the radiation field in the crossplane direction
Fig. 4a,b.Film dosimetry used to determine the cranial-caudal in- crements:(a)multiple abutting fields superimposed on a single film with all leaves open at nominal index spacing from 1.64 cm to 1.70 cm;(b)uniform field from a film taken by superimposing five abutting fields with 1.66 cm index spacing
Because each gantry rotation about a patient treats two 1 cm slices, the treatment couch must be indexed cran- iocaudally to treat targets with dimensions greater than 2 cm. It is critical that the fields treated by each suc- cessive arc are matched precisely. The actual field size projected at 100 cm SAD may vary slightly for different accelerators since the field width depends on the beam penumbra and the distance from the radiation source to the block tray slot. As shown in Fig. 4a, the actual in- dex spacing between successive treatment fields can be measured by abutting multiple fields on a single film (at isocenter and a depth of 5 cm) with all leaves open at nominal index spacing ranging from 1.64 to 1.70 cm.
For this example, the index spacing for successive treat- ment fields was determined to be 1.66 cm. Then, the index spacing can be verified by superimposing multi- ple fields on a single film as shown in Fig. 4b. Five fields with 1.66 cm index spacing were delivered and resulted in a fairly uniform field.
Additional quality assurance checks are required each time the couch indexing device, the Crane, is at- tached. The Crane consists of a large vertical column that supports two arms each equipped with an electronic digital micrometer that has a precision of 0.01 mm. The crane is locked to the side rail of the treatment couch by two clamps which are mounted on one of the arms. If the clamps are not mounted correctly on the rail, the con- trol of the couch movement will be inaccurate. Since the Crane is installed and removed daily, a simple daily QA procedure assures correct mounting of the Crane device to the couch. In addition, a quarterly check of the treat- ment indexing may be performed using the same film method mentioned above.
With respect to accuracy of the MIMiC collimator, leaf position accuracy and the leaf switch rate should be evaluated.
11.3.3 Record and Verify System
At each institution, the interface between information used by the accelerator and the record-and-verify sys- tem for all IMRT delivery systems should be tested.
The value of the leaf position tolerance and limits on gantry, collimator, and table position accuracy should be tested.
11.4 Patient Specific Quality Assurance
Individual patient quality assurance should take place at each step of the treatment planning and delivery process as discussed in Section 11.1. When possible, procedures or checklists should be followed to ensure consistency in the overall process.
Patient quality assurance begins with immobiliza- tion of the patient for the CT scan. Patients should be set up in a comfortable position since IMRT treatment may take longer than conventional treatment. Stringent patient immobilization is required for IMRT treatments because the plans are highly conformal and often include high dose gradient regions at the boundaries between the tumor and sensitive structures. The dosimetric effect of patient movement and setup uncertainties in IMRT treatment is greater than those in conventional treat- ment. Any treatment aids that are used should be noted in the patient chart. The isocenter is usually placed at the center of the tumor volume if IMRT is delivered with conventional multi-leaf collimators. If IMRT is delivered with an add-on collimator, placement of the isocenter must also consider clearance between the patient and the add-on collimator.
In treatment planning CT acquisition, a thinner im- age slice (i.e., 3 mm) is preferred for IMRT treatment so that radiation oncologists and treatment planners can outline the tumor volume and sensitive structures more accurately. Furthermore, thinner image slices re- sult in better quality digital reconstructed radiographs (DRRs).
After the patient CT scan is transferred, the target and structures of interest should be drawn by the ra- diation oncologist and dosimetrist. Any structure that will be assigned an objective function in the inverse planning system must be contoured. Target and organ- at-risk structures should then be expanded based on the institution’s organ motion and setup data. All struc- tures and expansions should be reviewed by a physicist, dosimetrist, or oncologist in 3D prior to beginning the optimization process. The number of beams should be selected and the beam angles reviewed. The cost or ob- jective functions that are used for treatment planning should also be reviewed. Ideally, clinical protocols will be used to derive the values of the cost or objective function.
After an optimized plan has been accepted by the ra- diation oncologist, the dosimetrist or physicist should use the planning system to sequence the fields. The transfer of patient treatment data, such as gantry angle, collimator angle, and jaw positions, from the planning system to the record-and-verify system should be ver- ified. The plan should also be validated to make sure that no collisions will take place. After the patient infor- mation is downloaded, the institution’s pre-treatment QA program should be followed. This will involve
either a monitor unit verification check or dosimet- ric measurements as discussed in Section 11.1. For dosimetric measurements, the beam technique data are then used for dose calculations in the phantom geometry. Proper export of the dose calculation in- formation or import of the measured data should be verified. The physicist should then determine if the QA fits within the institution’s criteria for acceptabil- ity. The AAPM guidance document suggests an action level of 3 to 4% in high dose, low gradient regions for verification in a phantom compared with measure- ments [2]. If the measurement exceeds the action level, then it is the responsibility of the physicist to iden- tify the problem. Additional measurements may be required along with a thorough review of the patient plan.
Once a plan successfully passes the pre-treatment QA, the patient setup should be verified on the first day of treatment. If the treatment isocenter is different from the isocenter marked at the time of the CT-simulation, special care must be taken to make sure that the cor- rect shift is made when setting up the patient. To verify patient position, a set of orthogonal images should be acquired and compared with digitally reconstructed radiographs. If a patient move is required, then an addi- tional set of images should be acquired and verified by the radiation oncologist. The institution should follow its individual site protocol to determine if an adaptive approach will be used for patient setup (multiple mea- surements over the first few days to determine an average displacement) or if the precision of daily imaging is required.
The AAPM guidance document on IMRT recom- mends verification of individual ports on the first day of treatment when supported by the delivery and verifica- tion system. Individual ports may be constructed from the outline of the IMRT field and acquired with film or an EPID [2]. In addition, some EPIDs allow for ac- quisition of images during IMRT delivery. These fields can be superimposed on the patient anatomy seen in the portal image. It may be difficult to interpret such results apart from clearly defining the borders of the field and large differences in field intensity. Additional QA checks on the first day include verification of the IMRT field delivery without anatomic information. One method is to measure the fluence intensity patterns by taping a piece of film to the reticule during the patient’s delivery for comparison to the intensity patterns from the treatment plan. This method shows gross errors in the treatment delivery. Another method is to record the machine log files which track the MLC position and re- view the reconstructed delivery of the intended vs actual position [37]. Weekly physics checks should verify that the correct dose is delivered for every treatment. Tools in record-and-verify systems can be used to look for anomalies in the patient treatment such as variations in table position.
11.5 Methods of Dosimetry of IM Beams
As described in many publications, individual IMRT fields have typically been verified with film. An accu- rate film program requires stringent quality assurance and the process is very time consuming for film delivery (plus acquisition of the dose response curve), process- ing, digitizing, and data analysis. Digital approaches are therefore desirable. Electronic portal imager devices (EPIDs) and 2D diode arrays offer the potential to sig- nificantly reduce the time it takes to do individual field dosimetry by digital acquisition. Appropriate software tools are essential for quantitative evaluation of the ac- curacy between dose calculations and measurements. It is important to make sure that any dosimetry system has proper quality assurance associated with it since decisions about patient care are made based on the re- sults. This section addresses the use of film, EPIDs, and 2D diode arrays for 2D evaluation of individual IMRT fields.
11.5.1 Film
Film is an essential part of commissioning an IMRT system because of its excellent spatial resolution and is used by many centers for a number of daily, weekly, and monthly quality assurance tests. Components of a film dosimetry program include film, water equivalent phantom, processor, digitizer (or scanner), and analy- sis software. For a reliable film dosimetry program, all aspects must be quality assured.
In the U.S., Kodak XV and EDR (extended dose range) film are primarily used. The response of each film type has been compared for energy and depth de- pendence [38–40]. When compared to XV film, EDR film shows less dependence on the processor and field size, and less response to low energy photons. EDR film also has been found to have better reproducibility and agreement with ion chamber measurements than XV film. Because of the decreased sensitivity of EDR film, it can be used to measure a complete fraction of an IMRT delivery.
Radiochromic film has also been investigated in a limited way for IMRT field dosimetry. The advantage of radiochromic film is that it changes color as a result of radiation and therefore no processing is required. How- ever, great care must be taken when using radiochromic film as well to achieve accurate results [41, 42].
Due to the number of measurements required, ready pack film is often used. Ready pack film can conve- niently be placed at any depth between slabs in a plastic phantom. Air trapped in the package can be removed by placing a pinhole in the package and forcing the air out. The phantom should be easy to use and to set up reproducibly for dosimetric measurements. When the
phantom is set up, fiducial marks should be placed on the film. These are built in to some phantom designs. If they are not, fiducial marks can be added using a jig and placing pinholes with a specific orientation outside of the beam. When using slabs of water equivalent mater- ial perpendicular to the beam, a jig can be used which allows placement of fiducials with pinholes. The pin- holes allow for alignment of the film image independent of the dose distribution. This is critical for evaluating the accuracy of MLC leaf position. Failure to use fidu- cials could lead to missing a systematic error in either MLC leaf positioning or introduction of a position offset in the leaf sequencer compared with the intended field.
The depth in water equivalent material and source-to- surface distance should be verified for all measurements.
The measurement depth should be chosen based on the depth of primary clinical interest.
A characteristic curve to convert the film response to dose should be measured at the same time as IMRT fields are measured and all films should be processed at the same time after verification of processor perfor- mance. The characteristic curve should be measured over the range of expected doses. An unexposed film is required to determine the fog level of each film batch.
Because a characteristic curve must be acquired each time, methods have been published in the literature to record multiple levels on a single film to save time and film [43]. The over-response of film to low energies must be considered when evaluating individual field measure- ments. To more accurately evaluate regions outside the field, multiple sensitometric curves can be used [44].
Quality assurance of the film processor is important since the gray level is dependent on the temperature of the chemicals and the concentration of developer and fixer. Processor stability should be assessed regularly.
The temperature of the chemicals should be recorded.
Fluctuations in the temperature are an indicator that maintenance should be called. To evaluate the effect of the processor on film, a light sensitometer can be used. On a regular basis, an unexposed film should be exposed to the sensitometer in the dark room, pro- cessed, and the optical density should be determined using a densitometer. The processor stability and ranges of acceptable values can be determined by plotting the film response over time from the sensitometer [28]. The system should be assessed after running several unpro- cessed films through the processor. The amount of fixer and developer should be checked to make sure that addi- tional chemicals are not required while processing films for data analysis. Another concern about processor per- formance relates to the number of film processed on a regular basis. As many institutions replace their film verification program with EPIDs, the processor perfor- mance may become instable and adversely affect a film dosimetry program.
The next part of a film dosimetry program is the dig- itizer or scanner system. The digitizer response should
be evaluated regularly for spatial intensity, characteris- tic response, and noise when there are large changes in optical density. Data transfer should be evaluated for ac- curacy where the pixel size and dimensions are assessed.
There are additional concerns with the digitizer for ra- diochromic film analysis such as the light source of the digitizer [45].
Finally, the user should have a means of applying the measured characteristic curve to the IMRT field measurements through analysis software. The software should be evaluated for accuracy of transformations of the data when aligning, correct application of dosimetric curve, import of calculations (if applicable) and analysis tools such as dose difference displays, profile extrac- tion, gradient evaluation, and other tools for comparing measurements with calculations.
11.5.2 EPID
Film is used at many centers for IMRT dosimetry be- cause of its availability and flexibility of placement in a phantom. However, film is time-consuming to use, re- quires additional hardware, and involves a multi-step process to determine the results. Once an IMRT pro- gram has been commissioned and started with ion chamber and film measurements, it may be appropriate to use another device for individual beam verification measurements.
As mentioned earlier, electronic portal imaging de- vices (EPIDs) have been mounted on linear accelerators at many centers for verification of patient position. It is a logical extension of EPID technology to investigate applying such systems to IMRT dosimetry. Dosimetric applications have been investigated for charge-coupled camera devices (CCD), scanning liquid ionization cham- ber (SLIC) imagers, and active matrix flat panel imagers (AMFPI). In applying these systems for dosimetry, the systems may need to be operated in a mode different from that used clinically for patient position verifica- tion (radiographic or continuous acquisition modes).
Additional software may also be required which is not available commercially yet.
To use an EPID for dosimetric verification, the EPID response must be characterized for dose, dose rate, field size, and leaf speed (if DMLC delivery) dependence. Cor- rections are required and depend on the construction of the system. In addition, a portal dose prediction or portal dose image must be calculated to evaluate the measurements.
CCD EPIDs have been applied to DMLC dosi- metric measurements using a modified system that includes a 1 mm thick stainless steel slab in addition to the fluorescent layer [46]. Corrections are applied for the dark frame, the system’s non-linear response, non- uniform spatial response, and optical cross talk. The application of a convolution kernel to correct for differ-
ence in absolute dose and penumbra for small fields has improved the response of the system for fields less than 3× 3 cm2[47]. Agreement between the CCD-EPID and measurements was within 2% for large fields measured with an ion chamber and for small fields measured with a linear diode array.
Commercial SLIC EPIDs have also been character- ized for dosimetric measurements for SMLC-IMRT and DMLC-IMRT [48–50]. Different acquisition modes have been used for SMLC and DMLC delivery [48–50]. A prob- lem with using this device for dosimetry is the need for equilibrium in the iso-octane layer of the device which led to measurements of only one segment per minute in one experiment [48]. Multiple investigators have found the agreement within about 2% with ion chamber and film data except in steep gradient regions.
The final EPID systems that have recently been char- acterized for dosimetric applications are active matrix flat panel imagers (AMFPIs). Commercially, the systems have a fluorescent layer above the imager area which ad- versely affects the dosimetric response [51]. The imager response has been modeled with Monte Carlo, decon- volution, and empirical methods for SMLC and DMLC delivery and agreement of approximately 2% has been measured by multiple investigators [52–54]. Further ap- plication of EPIDs for individual IMRT field verification is expected to continue. Commercial systems, including software, are still under development. Once such sys- tems are in place, they offer great potential for saving time for verification of individual IMRT fields.
11.5.3 2D Arrays
Two-dimensional detector arrays are commercially available for individual IMRT field verification. In the currently available system, 445 n-type diodes are dis- tributed over an area of 22× 22 cm2 (Sun Nuclear, Melbourne, FL). The spatial resolution is 7.07 mm in the central 10× 10 cm2region of the detector array and 14.14 mm in the outside area and the array is designed solely for perpendicular beam measurements. Similar to EPIDs, such arrays should only be used after an IMRT program has been commissioned.
Before measurements, the diode array is calibrated at depth. Each detector has its own sensitivity factor with respect to the diode on the central axis. After mea- surement, the dose calibration factors are applied for comparison to calculations (or measurements with an- other detector). The response of the array has been characterized as a function of dose, dose rate, repro- ducibility, and temperature. Overall, the diode array was determined to have a reproducible and linear response (up to 295 cGy) and a dependence on temper- ature [55, 56]. Measurements of output factors agreed to within 2% for field size ranging from 2× 2 cm2 to 25× 25 cm2 [56]. Additional studies have found diode
arrays to be sensitive enough to measure the effect of segments that are dropped during IMRT delivery [57].
While the overall response is very good, the current diode arrays are limited by their size and coarse spatial resolution. These systems only verify a representative sample of delivered IMRT fields.
11.6 Methods of Dosimetry of Complete IMRT Treatments
While individual field checks are essential for determin- ing the cause of errors in IMRT delivery, verification of the complete IMRT treatment allows for assessment of plan delivery at the proper gantry, collimator, and couch angles that are used for patient treatment. The patient’s IMRT plan is transferred to a phantom and calculations are done at the points or depths of interest. The phantom can be cylindrical, cubic, or anthropomorphic. Cylindri- cal and cubic phantoms of water equivalent material are often used because the phantoms and detectors can be set up reproducibly. Ion chamber measurements are of- ten used to verify the absolute dose at a single point.
Some phantoms have been designed that accommodate multiple detectors such as film, ionization chambers, TLDs, and MOSFETs [58, 59]. The advantage of such phantoms is that absolute dose can be verified at multiple points in both high and low dose regions.
11.6.1 2D Film
Complete IMRT treatments can be verified in two di- mensions using film placed between water equivalent material or in an anthropomorphic phantom [28]. In the US, EDR film, or less commonly, radiochromic film can be used for such measurements. The issues with a film dosimetry program were discussed in Section 11.5.1.
The dose response curve should be measured at the same time as the phantom measurement. To properly verify a plan, the calculated monitor units that are to be deliv- ered for the patient treatment should be used. Monitor unit scaling or a change in dose rate can result in small differences in delivery of the fields. When doing com- posite film dosimetry, it can be difficult to determine the cause of differences between the measurement and calculation. Therefore, individual field measurements (with film and/or ionization chamber) may be required if unacceptable results are obtained with a 2D composite film.
11.6.2 3D Gel
Methods of 1- and 2D dosimetry provide an incom- plete evaluation of accuracy of delivery for a complete
