23 X-IMRT
Simeon Nill, Ralf Hinderer, and Uwe Oelfke
S. Nill, PhD; R. Hinderer, PhD; U. Oelfke, PhD
Abteilung Medizinische Physik in der Strahlentherapie, Deutsches Krebsforschungszentrum, Im Neuenheimer Feld 280, 69120 Heidelberg, Germany
23.1
Introduction
The IMRT dose delivery techniques are the second cornerstone of X-IMRT in addition to the concepts of inverse treatment planning and optimization (dis- cussed in Chap. 17). The result of an inverse treat- ment plan usually consists of ideal photon-fl uence distributions for each selected beam angle. In this chapter we discuss on a very fundamental level the main concepts and strategies for the actual delivery of these fl uence patterns to a patient. This very brief summary of some of the interesting techniques can- not be complete and very detailed. The interested reader who aims to explore the touched topics in more depth is referred to the standard literature, e.g., Webb (2001) or (Palta and Mackie 2003).
The main focus in this chapter is on the discussion of the most prominent dose delivery techniques with linac-integrated multi-leaf collimators (MLCs). The
basic concepts employed for the two standard meth- ods – the “step-and-shoot” approach and the “dy- namic” dose delivery – are briefl y reviewed before the new concept of “helical tomography” is introduced.
Finally, a short section describes the dose delivery technique with individually designed compensators.
23.2
MLC-based IMRT Delivery
Most modern linear accelerators are equipped with MLCs, which were originally designed to replace the use of lead blocks to shield normal tissue during the treatment (Biggs et al. 1991; Boesecke et al. 1991;
Jordan and Williams 1994). In the next section we fi rst describe the main IMRT relevant characteristics of a typical MLC including its dosimetric properties.
Then, various dose delivery techniques are briefl y de- scribed. Based on the MLC computer-control system, two main dose delivery methods are distinguished:
(a) the “step-and-shoot” approach; and (b) the “dy- namic” dose delivery concept. A very detailed de- scription of both techniques with references to the original papers can be found in the excellent book by Webb (2001). Besides these two basic IMRT delivery concepts with linac-integrated MLCs, we also briefl y discuss some examples of “high-resolution” IMRT with add-on mini- or micro-MLCs.
23.2.1 MLC Design
23.2.1.1
IMRT-Relevant MLC Data
Multi-leaf collimators and their design and func- tion for 3D conformal radiotherapy (3D CRT) are described in detail in Chap. 20, together with their geometrical and mechanical characteristics, see also Galvin et al. (19939.
CONTENTS
23.1 Introduction 289
23.2 MLC-based IMRT Delivery 289 23.2.1 MLC Design 289
23.2.1.1 IMRT-Relevant MLC Data 289 23.2.2 Step-and-Shoot Dose Delivery 290 23.2.2.1 The Step-and-Shoot Technique 290 23.2.2.2 Leaf-Sequencing Algorithms 290 23.2.2.3 Hardware Constraints: Matchlines 292 23.2.3 Dynamic Dose Delivery 292
23.2.4 Improved Spatial Resolution: Add-on MLCs 294 23.3 Helical Tomotherapy Delivery 295
23.4 Compensator-Based IMRT Delivery 296 References 297
spatial resolution of the fl uence in the direction of the leaves’ movement is only restricted by the positioning accuracy of the MLC. For the delivery of IMRT fi elds a possible large overtravel range of the different leaves (10–15 cm) is often required to irradiate extended target volumes.
2. Maximum leaf speed. The leaf speed is especially important for the “dynamic” delivery of intensity- modulated fi elds. A typical leaf speed is of the order of 2–4 cm/s (Webb 2001; Hug 2002).
3. The maximum fi eld size of the MLC. For IMRT applications, the maximum fi eld size is not the same as the maximum open fi eld which can be achieved, but it is mainly defi ned by the maximum overtravel.
4. Maximum overtravel. Maximum overtravel (see Chap. 20.2.1) is the distance of how far a leaf can move over the midline of the MLC.
5. Leakage and transmission. The leakage and transmission (see Chaps. 20.2.2.2, 20.2.2.3) play an important role specifi cally for the delivery of IMRT treatments because the dose delivery often requires a substantial number of monitor units (MU); these are applied where most areas of the treatment fi eld are covered with closed leaves, i.e., the leakage radiation received by these areas is signifi cantly enhanced.
6. Leaf-positioning accuracy. One of the most impor- tant properties of an MLC with respect to IMRT is the leaf-positioning accuracy. For IMRT deliv- ery the leaf-positioning accuracy is even more important than for most conventional treatment techniques due to the large number of small fi eld components for an average IMRT treatment. For a typical fl uence grid with a resolution of 10 mm in both directions a leaf-positioning error of 1 mm has to be considered as a large error. This is due to the characteristic output factor curve with a very steep gradient for small fi eld sizes. An error of 1 mm for the leaf position can cause a dose error of up to approximately 10% irradiating a 10×10-mm fi eld and therefore the maximum leaf-positioning error should not be larger than 0.5 mm. It can also be concluded that the required leaf-positioning accuracy depends on the fl uence grid resolution of the fl uence maps.
be converted into fi eld segments to be delivered with the MLC, a process that is called the “sequencing” of the fl uence. In this section we shortly describe the
“step-and-shoot” dose delivery method and discuss briefl y the two basic sequencing techniques.
23.2.2.1
The Step-and-Shoot Technique
The “step-and-shoot” technique of IMRT dose de- livery (Bortfeld et al. 1994) is a straightforward extension of the conventional multiple-fi eld irradia- tion technique. The “step-and-shoot” approach su- perimposes the dose delivered by a number of ir- regularly shaped and partially overlapping treatment fi elds, often called subfi elds or segments. For each segment a well-defi ned number of monitor units is delivered. Then, the beam is turned off while the leaves of the MLC move to the positions required by the next IMRT segment. After the verifi cation and record system (V&R) has validated the new leaf posi- tions, the beam is turned on and the dose is delivered for this segment. This process is repeated for all seg- ments per incident beam angle and all beam direc- tions (see Fig. 23.1).
23.2.2.2
Leaf-Sequencing Algorithms
In order to describe the basic features of leaf-sequenc- ing algorithms we introduce the following terms: a fl uence map is defi ned on a 2D fi xed grid covering the respective beam aperture. It is usually divided into discrete, quadratic elements called “bixels.” The fl uence map assigns one beam intensity to each bixel.
A 1D line of intensities for all bixels created by a single leaf pair is called a channel. The width of each channel coincides with the leaf width of the MLC.
The goal of the sequencing process is to decompose the fl uence maps into a number of fi eld components or subfi elds.
The fi rst step of the sequencing process is the
stratifi cation of the continuous fl uence profi les pro-
vided by the inverse planning process, i.e., each line
of intensity for a specifi c channel is forced to take on
only a few discrete fl uence levels (Fig. 23.2). For each
fl uence level the MLC leaves shape and deliver a beam segment. In general, not all levels may be deliverable without an additional step. One problem is the appear- ance of spatial “holes” for a fl uence pattern generated by the optimization process (e.g., two pyramids next to each other with zero fl uence in between). For these cases each level has to be divided into separately de- liverable subfi elds. This is one constraint which, in addition to other MLC hardware constraints, must be taken into account during the process of calculating the required leaf positions.
The two most prominent concepts are the “close- in” and ”sweep” technique. In Fig. 23.3 we show an example of the “close-in” technique. For this example the fl uence map was divided into three levels. The typical numbers of levels for clinical cases is in the range of fi ve to ten. In the fi rst step the left leaf is moved towards the fi rst positive gradient of the fl u- ence pattern while the right leaf is moving towards the fi rst negative fl uence gradient. This defi nes the fi rst segment which is then delivered. Next, the two leaves move to the next positive and negative gradi- ents of the respective fl uence map to defi ne the next subfi eld followed by its delivery. This procedure is re- peated until the whole fl uence map is delivered.
We briefl y describe the basic algorithm of the
“sweep” technique for the 1D example of a fl uence distribution given in Fig. 23.4. For this fl uence map,
Fig. 23.1. The basic idea of the “step-and-shoot” approach is to deliver an intensity-modulated beam as a superposition of a set of irregularly shaped, partially overlapping fi eld components
Fig. 23.2. Stratifi cation of a continuous fl uence distribution into a fl uence map with a number of discrete levels
Fig. 23.4. Decomposition of a one-dimensional fl uence map into deliverable segments using the “sweep” technique Fig. 23.3. Decomposition of a one-dimensional fl uence map into deliverable segments using the “close-in” technique
stop at the respective positive and negative fl uence gradients. With this technique the treatment time is reduced compared with the “close-in” technique since the leaves are always moving only in one direction.
Most inverse treatment planning programs have a build-in leaf-sequencing algorithm. The total number of segments depends on the complexity of the fl uence maps, the number of beams, and other technical fac- tors. Since the total treatment time depends linearly on the number of segments, a signifi cant effort is put into optimizing the sequencing algorithm to fi nd the best solution in terms of treatment time. Up to now no optimal sequencer has been found which creates the best possible solutions for all clinical cases (Xia and Verhey 1998; Siochi 1999).
23.2.2.3
Hardware Constraints: Matchlines
For real 2D fl uence distributions the sequencer must not only calculate the leaf positions but also must take into account the geometric and dosimetric properties of the MLC. For example, the leaf-end design and the focusing properties, which can lead to signifi cant dose artifacts if not taken care of, must be taken into account. Two of these artifacts are described below.
The fi rst dose artifact is called the tongue- and-groove effect. To reduce the interleaf leakage some MLCs are using a tongue-and-groove design (Fig. 23.5; see also Fig. 20.8 in Chap. 20). If a large fi eld perpendicular to the leaf motion direction is divided into two subfi elds, an underdosage at the matchline of the two treatment fi elds is observed.
artifact (Kamath et al. 2004).
The second dose artifact can be caused by trans- versal matchlines. If two segments have a joint bor- derline perpendicular to the leaf motion direction, stripes of underdosage can occur. These underdos- ages are an effect of an incomplete compensation for the penumbras of the two fi elds. Possible reasons are the fi nite thickness of the leaves, the extended radia- tion source, and scatter effects. This effect leads to the same order of underdosage as the tongue-and-groove effect and can be compensated for by adjusting the leaf positions of the involved segments.
The main differences between conventional tech- niques and the IMRT “step-and-shoot” approach are the increased number of fi elds, a longer treatment time, and that very small fi elds are delivered. In ad- dition, the number of monitor units per fi eld is low compared with conventional treatment techniques and a higher total number of monitor units are deliv- ered per fraction. Especially the low number of moni- tor units per segment is of great importance. It must be validated that the linear accelerator is capable of delivering such small monitor units so that 10×3 MU equals the dose delivered with 30 MU. If this is not the case, a large additional error is introduced.
23.2.3
Dynamic Dose Delivery
For the “dynamic” delivery technique (DMLC;
Fig. 23.6) the intensity modulation is achieved by an individual variation of the velocities of the moving
Fig. 23.5. The tongue-and-groove un- derdose effect is caused by the tongue- and-groove leaf design to suppress in- terleaf leakage
leaves, i.e., the treatment can be realized without inter- rupting the treatment beam. The clinical application of this technique was pioneered at the Memorial Sloan Kettering Center in New York (LoSasso et al. 1998;
Chui et al. 2001; Zelefsky et al. 2002). The “dynamic”
MLC sequencing problem has been solved in various ways (see Webb 1997, and references therein).
One way to illustrate its basic features is to con- sider the “dynamic” mode as an extension of the
“sweep” mode of the static dose delivery as illustrated in Fig. 23.7. Firstly, the intensity profi le in Fig. 23.7a is converted into positive intensity differences that are equal to the intensities to be delivered (see Fig. 23.7b).
It can be easily seen that the differences of the upper and lower profi le in Fig. 23.7b equals the intensity profi le in Fig. 23.7a. If the discrete bixels in Fig. 23.7b are made smaller and smaller as shown in Fig. 23.7c and d, the representation of the continuous intensity
Fig. 23.6 Principle of dynamic IMRT delivery
Fig. 23.7a–f Transition from leaf-sweep “step-and-shoot”
approach to the “dynamic”
approach (if the number of intensity levels converges to infi nity)
tory of the “leading” leaf and the upper profi le corre- sponds to the trajectory of the “trailing” leaf, i.e., the inverse of the derivative of these profi les represent the required velocity profi le of the leaves. For example, at the beginning of the irradiation with closed leaves, the leading leave has to move with an “infi nite” velocity to the position of the fi rst intensity maximum before the trailing leaf shapes the required intensity profi le. As a fi nal step, the trajectories are adjusted to the maximal available speed of the leaves. With this approach the leaf trajectories can be changed in such a way that still the same fl uence profi les are delivered (Spirou and Chui 1994; Stein et al. 1994; M
Aet al. 1998; Convery and Webb 1998; Boyer and Yu 1999).
The treatment time to deliver calculated leaf tra- jectories is approximately the time the leaves need to move from the leftmost side to the rightmost side plus the delivery time to irradiate the leaf sweep decom- position. The irradiation time is defi ned mostly by the complexity of the fl uence patterns and therefore given by the treatment time of the intensity channel which takes the longest time to be delivered. The to- tal treatment time for “dynamic” technique is mostly shorter than the treatment time to deliver the same fl uence pattern with any “step-and-shoot” approach (Ma et al. 1999).
In addition to the technical properties required for an MLC to be used in static mode, the “dynamic” de- livery technique requires that the leaf speed be con- trollable with a very high accuracy and that the cali- bration process for leaf speed and leaf position must be easily possible (LoSasso 1998). The leaf-position accuracy is not only important to create the correct aperture for small fi elds but any leaf-positioning error may lead to a dose error even for large fi elds (Zygmanski et al. 2003); therefore, strict quality as- surance for both parameters is needed.
For the “dynamic” delivery process matchlines perpendicular to the direction of the leaf movement do not pose a problem. The tongue-and-groove ef- fect, on the other hand, basically leads to the same problem as for the static delivery process. Using a method called leaf synchronization the effect could eliminated from the delivery process, but this could lead to a longer treatment time (Webb et al. 1996; Van Stanvoort et al. 1996).
cal point of view the “dynamic” approach is more complex than the “step-and-shoot” approach. For the
“dynamic” process the leaves are moving while the beam is turned on and therefore a very accurate con- trol of the leaf positions, leaf speed, and dose rate at the same time must be achieved. On the other hand, a shorter delivery time is the main advantage of this technique. The static approach allows an easier veri- fi cation process and is viewed as a natural extension of established conventional dose delivery techniques.
Plan comparisons of static and DMLC optimized plans have shown that the differences for the achiev- able dose distribution for the target volume are less then a few percent (Palta and Mackie 2003). Up to now both techniques have been successfully imple- mented clinically (Zelefsky et al. 2002; Eisbruch 2001; Lee et al. 2002; Münter et al. 2002; Thilmann et al. 2002; Thilmann et al. 2004) and both tech- niques are still improving (Chui et al. 2001).
23.2.4
Improved Spatial Resolution: Add-on MLCs The spatial resolution currently achievable with in- ternal MLCs is in the range of 5–10 mm. It was shown by Bortfeld et al. (2000) that for a 6-MV photon beam the optimal leaf width according to basic phys- ics is in the range of 1.5–2 mm. A number of MLCs with smaller leaf widths in the range of 1.6–5.5 mm were developed to generate more conformal dose distributions (Chap. 20.3.2; Cosgrove et al. 1999;
Meeks et al. 1999; Xia et al. 1999; Hartmann and Föhlisch 2002). For intensity-modulated radiation therapy (IMRT) the effect of the leaf width on the physical dose distribution for a MLC with 5- and 10- mm leaf width was recently evaluated by Fiveash et al (2002) and for even smaller leaves by Nill et al.
(2005). The result of these studies was that for most
clinical cases the spatial resolution of the internal
MLCs is good enough to create acceptable 3D dose
distributions, but for very complex geometries of the
anatomy where for instance the organ at risk is in
close proximity to the target volume, the 3D dose
distribution achievable with an improved spatial
resolution is signifi cantly better. The disadvantages
with add-On MLC approach are the reduced clear- ance between the MLC and the patient, the limited fi eld size and, due to the increased spatial resolution, the increased number of total monitor units to be delivered.
23.3
Helical Tomotherapy Delivery
The basic idea of tomotherapy is that the fl uence modulation of a radiation beam could be achieved by leaves, which move rapidly in and out of a fan beam;
therefore, a binary MLC was developed to temporally modulate a fan beam. To achieve a dose distribution highly conformal to the target, an irradiation from many directions is needed. One possible solution is a continuously rotating linear accelerator with the bi- nary MLC attached. To cover the whole target volume, the radiation must be delivered in a slice-by-slice fashion. This technique is analogous to a CT scanner.
The combination of the word tomography and ra- diation therapy leads to the expression tomotherapy (Mackie et al. 1993).
One realization of the principle of tomotherapy is helical tomotherapy. Here, a linear accelerator is mounted on a ring gantry (see Fig. 23.8). The binary MLC is attached downstream to the linear accelera- tor. Technically, this is the fusion of a helical CT scan-
ner and a linear accelerator. During the continuous rotation of the linear accelerator around the patient the couch is moved continuously in longitudinal direction through the bore of the gantry. From the patient’s point of view the radiation is delivered in a helical fashion (Mackie et al. 1993; Mackie et al.
1999; Olivera et al. 1999). This is the main difference between helical tomotherapy and another implemen- tation of tomotherapy, sequential tomotherapy, as it is employed by the Peacock system (Carol 1996).
In sequential tomotherapy the patient couch is in a fi xed position during the rotation of the linear accel- erator. After each revolution, the couch is indexed in longitudinal direction by the width of the fan beam.
Compared with helical tomotherapy, this procedure leads to a prolonged treatment time.
The binary MLC of the helical tomotherapy ma- chine consists of 64 interdigitated leaves with 32 leaves on each side. The motion of the leaves is per- formed by a pneumatic system. Due to the high air pressure, the leaves can open or close the radiation fi eld within only 40 ms. The leaves are made of an alloy consisting of 95% tungsten. A single leaf has a height of about 10 cm and projects to a width of 0.625 cm at isocenter. A tongue-and-groove design is employed to reduce the interleaf leakage. The modu- lation of the beam fl uence is achieved by varying the leaf opening time (Palta and Mackie 2003).
The inverse treatment planning for helical tomo- therapy is done by approximating the rotational de-
Fig. 23.8 Helical tomotherapy machine and its components mounted on a ring gantry.
Some of the components, such as the control computer, high- voltage power supply, and data acquisition system are required for the operation of the detector rather than the beam delivery system. (Courtesy T.R. Mackie, University of Wisconsin, and G.H. Olivera, TomoTherapy Inc.)
and couch positions are determined. Typical values for the pitch are in the range of 0.2–0.5 with a typical slice thickness between 2.5 and 5.0 cm. A common ro- tation time is 20 s. Consequently, the total treatment time is equal to 20 s multiplied by the number of rota- tions required to treat the whole target volume.
23.4
Compensator-Based IMRT Delivery
Not all linear accelerators are equipped with an MLC.
An alternative to an MLC is the use physical fl uence attenuators called compensators (Ellis et al. 1959;
Mageras et al. 1991; Jiang 1998; Chang et al. 2004).
A physical compensator is made of a material which absorbs radiation and is mounted in the accessory tray of the linear accelerator. The compensators con- sist of bixels which are fi lled with different thick- ness of a photon absorbing alloy, which determines the fl uence modulation. The thickness map over the beam aperture is calculated by a computer program based on the result of the optimization process (see Fig. 23.9). To mount and manufacture compensa- tors special equipment, such as a furnace, to melt the suitable compensator material, a milling machine to produce the moulds (see Fig. 23.10) and a special holder to mount the compensator to the accessory tray are needed. A typical material for compensator is the alloy MCP96.
Since each compensator represents one individ- ual fl uence distribution, a new compensator must be manufactured for each fi eld of a patient after the treatment planning process. Another important issue is the time needed to replace the compensators dur- ing the daily fi eld by fi eld treatment. For each fi eld a technician has to walk into the treatment room and change the compensator in the accessory tray.
Another problem of the compensator approach is the achievable range of fl uence modulations (Chang et al. 2004). Despite these obvious disadvantages, there are a number of important advantages of compensa- tor-based IMRT. As already mentioned with compen- sators, IMRT can be performed on even older linacs and the spatial resolution of the fl uence modulation
Fig. 23.9 Physical compensators offer a very simple way to cre- ate IMFs by means of individually tailored absorbers
Fig. 23.10 A mould of a compensator mould processed with a milling machine. The material is a plastic called “obomodu- lan.” The indicated cavities in the mould will be fi lled with a photon attenuation alloy
can be very high, e.g., <5 mm. Another important advantage of compensators is that the total moni- tor units delivered for a complete IMRT plan is only slightly increased compared with the monitor units used for the delivery of a conventional treatment plan. This might be an important fact in the ongo- ing discussion about secondary radiation-induced tumors due to the higher exposure of normal tissue with low doses for MLC-based IMRT delivery (Hall and Wuu 2003).
In conclusion, IMRT with compensators is feasible, but the effort to produce compensators can be very high and special equipment is needed; however, there seems to be advantages which can justify the applica- tion of compensators (Chang 2004).
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