IMRT for Malignancies of the Upper Abdomen and Retroperitoneum
Jerome C. Landry, Christopher G. Willett, Natia Esiashvili, Mary Koshy
8
Contents
8.1 General Introduction . . . 383 8.2 Unique Anatomic Challenges . . . 383 8.3 Target Volume Delineation
and Organs at Risk Definition . . . 384 8.4 Planning Dose Prescription
and Optimization Strategies . . . 385 8.5 Clinical Experience and Trials
to Define the Role of IMRT . . . 386 8.6 Future Directions . . . 389 References . . . 389
8.1 General Introduction
Radiation therapy is indicated for different upper ab- dominal tumors, but is often associated with substantial gastrointestinal or genitourinary side-effects. With con- ventional radiotherapy, the most commonly applied techniques are two or four fields with custom-made blocks to exclude portions of normal structures. Even then, patients are still at risk for both acute and long-term side effects. Clinical applications of IMRT for treatment of upper abdominal tumors may allow the minimizing of serious acute and long-term com- plications while increasing cure probability by dose escalation.
Despite advancements in diagnostic tools, under- standing cellular mechanisms, and invention of novel systemic therapies, pancreatic cancer remains one of the greatest challenges for clinicians. Pancreatic cancer is the fourth leading cause of cancer-related death with less then 5% of patients surviving at five years after di- agnosis [1]. Improved radiation treatment delivery can have great impact on this disease, as local regional failure is the one of the most predominant patterns of progres- sion for pancreatic tumors with local recurrence in up to 30–86% of patients after surgery alone [2–4]. Efforts to escalate radiation dose in pancreatic cancer patients
are hampered by the proximity of dose-sensitive adja- cent normal structures and increased risk for serious complications [5, 6]. Combined chemotherapy and ra- diation further increases the risk of significant acute and long-term complications [7–10].
Retroperitoneal sarcomas account for 14% of all soft tissue sarcomas and 0.7% of all cancers diagnosed in the United States [11]. Surgical resection has been and re- mains the only curative modality for this disease [12].
Because of the large tumor size at presentation and in- timate involvement with adjacent organs, it is difficult to obtain resection with negative margins. Historically, rates of complete surgical resectability have varied from 38 to 65% with local recurrence rates as high as 70–90%
[13–15]. Local recurrence in retroperitoneal sarcoma is the primary cause of mortality in patients with this dis- ease. Clearly, this is a disease in which improvements in local control have the potential to significantly im- pact survival [16,17]. Retroperitoneal sarcoma has been responsive to radiation dose escalation [18–22], yet ef- forts to achieve this with external beam radiation alone (EBRT) have been hampered by OAR (organs at risk) toxicity. IMRT can be used as a means to minimize dose to OAR and concurrently maximize tumor dose coverage.
The introduction of 3D-CRT (3D conformal radio- therapy) has led to progress in objective evaluation of dose-to-target volumes and critical structures. IMRT, as the most advanced form of conformal therapy, can improve even further the dose conformity to the tu- mor targets and minimize the dose to organs at risk.
This strategy may give the oncologist and physicist more freedom to push the radiation dose to the tumor itself.
8.2 Unique Anatomic Challenges
There are several serious challenges in the treatment of abdominal tumors. The upper abdomen is one of the most complex anatomical sites in terms of number and proximity of normal structures with the lowest ra- diation dose tolerance. Liver, kidneys, and spinal cord
surround the pancreas and its regional lymph nodes in almost all directions and limit the radiation field design and optimal dose delivery to the target. Even with 3D- CRT, it is difficult to achieve good dose conformity. By employing inverse radiation treatment planning tech- niques and setting dose constraints to normal organs, IMRT can potentially achieve the most optimal beam angles and shape and give us the best conformal dose distribution.
The most critical step in designing IMRT plans for upper gastrointestinal malignancies is the precise iden- tification of the tumor volume and normal structures. In some cases, collapsed hollow organs, such as segment of small bowel or stomach, can be difficult to differentiate from tumor extension or adenopathy. Another consider- ation is to evaluate organ motion in the upper abdomen.
Based on data from Massachusetts Hospital, the pan- creas can move during different phases of respiration up to 5–7 mm anterior to posterior, 2–3 mm medial to lateral, and < 5 mm cranio-caudal direction [23]. Other investigators have reported movement with the respira- tory cycle as much as 23.7± 15.9 mm [24]. Obtaining CT images in static exhalation phase may limit this mo- tion [25]. In one study, when patients were tested at static exhalation, which would represent differential gas- trointestinal distention, CTV (clinical tumor volume) expansion for the pancreatic head was required to be at least 4 mm in right-left, and 3.5 mm in antero-posterior and cranio-caudal directions to compensate for organ motion [26]. Accurate delineation of regional lymph nodes is also crucial and requires good knowledge of CT anatomy. We are evaluating pancreatic motion by scanning patients with our 4D CT simulator allowing an evaluation of tumor motion during all phases of the respiratory cycle.
Accurate delineation of regional lymph nodes in the treatment of pancreatic cancer is crucial. There is substantial evidence of disease spread to the celiac axis, porta hepatis, and pancreato-duodenal and splenic hilum for tail and body lesions according to surgical pathology and patterns of failure data [27–30]. The celiac axis is typically located at T11–T12 and one can often visualize the celiac trunk. During contouring, the celiac axis should be drawn on approximately three to five CT slices. The porta hepatis is located at the level of the hep- atic duct bifurcation. A study at our institution indicated that if one uses a reference point derived as 4.5 cm to the right of and 4.7 cm cephalic to the inferior end plate of L-1 vertebral body, and constructs a 10×10-cm portal that is 6 cm superior, 4 cm inferior, 4 cm to right lateral, and 6 cm to left lateral direction, the porta hepatic nodes will be covered in about 80% of cases [19].
The pancreato-duodenal lymph nodes are difficult to visualize on CT. Data from the Mayo clinic show that there is a change in location of celiac axis, porta hepatic, and superior mesenteric artery after Whipple resection, with only minimal anterior-posterior variation in the
celiac axis (median 2 mm), and more prominent change in the portal vein, up to 2 cm in the lateral-medial di- rection and 1.9 cm in the anterior-posterior direction [32]. Physicians must keep this variability in mind while designing the margins around the treatment volumes.
Treatment volumes must be individualized based on the volume and location of the primary tumor mass.
Treatment of retroperitoneal sarcomas with radia- tion also has been limited due to the close proximity of these tumors to small bowel, liver, and kidney. To avoid critically overdosing these organs at risk, the total dose delivered to the tumor is often compromised and consequently, the risk of local recurrence is increased.
Although the kidneys and liver are dose-limiting struc- tures when treating retroperitoneal sarcomas, the small bowel as an OAR poses the greatest challenge. Radia- tion doses beyond 45–50 Gy have been associated with small bowel obstruction; this is often the rate-limiting factor in dose escalation to a variety of tumors in the abdominal region [5]. Historically, these tumors have been treated with a 3–5-cm margin around the gross tumor volume (GTV) to include the anatomy of the in- volved tissues [20, 21, 33]. To treat with tighter margins than previously described in order to achieve dose esca- lation may potentially underdose the peritoneal cavity, where the risk of local recurrence is the greatest. We be- lieve that the use of IMRT and intent of dose escalation does not give one a mandate to compromise the mar- gin that would normally be employed in the treatment of retroperitoneal sarcoma. The use of IMRT through- out treatment, from the beginning, allows for optimal dose minimization to OAR and maximization to tumor volume.
8.3 Target Volume Delineation and Organs at Risk Definition
IMRT planning starts with good simulation techniques.
To assure accurate visualization of the small bowel, all patients are given three to four glasses of radiopaque gastrograffin oral contrast and placed supine in a rigid foam cradle. Approximately 30 min after drinking the oral contrast, treatment planning computer tomogra- phy (CT) scans of the abdomen and pelvis are obtained.
The planning volume is scanned with 3-mm increments.
The next step is to create a treatment plan based on CT images, with precise outlining of all the volumes of in- terest. The gross tumor volume is defined as all known gross disease determined from comparing the diagnos- tic with the treatment planning CT. Both GTV and lymph node groups are included in the clinical tumor volume (CTV) for pancreatic cancers. The Planning Target Vol- ume (PTV) in non-resected pancreatic cancers provides 2–3-cm margins in all directions around the CTV to compensate for set-up and organ motion. In some cases,
the PTV at the vertebral column and|or skin may be too generous, in which case PTV can be modified. In- corporation of functional image fusion techniques in radiation treatment planning potentially can help radi- ation oncologists in modifying CTV. Additionally, newer techniques for definition of organ motions such as on- board imaging and gating will give us an opportunity to farther minimize the margins around CTV. The PTV for retroperitoneal sarcoma ultimately included the GTV plus a 5-cm margin in the superior and inferior dimen- sions and a 2-cm margin in the anterior|posterior and medial/lateral dimensions. Because the main advantage of IMRT is sparing of the normal organs adjacent to the CTV from receiving excessive doses of radiation, IMRT planning routinely includes outlining the nor- mal organs, including kidneys, liver, small intestines, and spinal cord. The literature supports the fact that small bowel obstruction increases when radiation doses above 45 to 50 Gy are delivered. IMRT allows one to de- crease the dose to the small bowel and other critical structures.
8.4 Planning Dose Prescription and Optimization Strategies
Target volumes and contours are transferred to the 3D treatment planning computer station and used to generate 3D conformal and IMRT plans (Fig. 1). A four-field conventional arrangement was utilized for the 3D planning process. Usually six to ten non- opposing beams with 0.25×0.5 cm minimum beam resolution are employed for IMRT. The IMRT plans are generated using inverse treatment planning tech- niques. In our institution originally we used CAD plan (Helios) and later Eclipse (Varian Medical Systems) treatment planning software for generation of IMRT plans (Fig. 2).
The PTV is specified to receive uniformly 100% of the dose and no more than 110% inhomogeneity within the
Fig. 1. Axial CT images demonstrating GTV (gross tumor vol- ume), PTV (planning target volume) and organs at risk (kidneys, liver)
Fig. 2. CT axial image demonstrates multiple IMRT beam angles and dose-distribution for pancreatic tumor
target volume. Inverse planning (optimization process) may generate larger (than we are accustomed to) dose gradients across the PTV; Generally, high degrees of dose conformity and constraints on critical organs will cause large dose gradients within the PTV. The PTV of both plans is designed to receive 100% uniformity of dose with the 95% isodose line encompassing the CTV + 2.5 cm and no more than + 110% inhomogeneity within the target volume.
In respect to pancreatic cancer, after a dose of 45 Gy, the treatment margins are reduced to include GTV with 1.5–2-cm margins in all dimensions except at the in- terface of the small bowel and GTV. For IMRT plans for retroperitoneal sarcomas, after 45 Gy the treatment margins are also reduced to 2 cm around the GTV in all dimensions. If there are MLC (multi-leaf collima- tor) restrictions and field widths larger than 15 cm, it is necessary to employ the technique of “beam splitting”
[34, 35].
The GTV and OAR were all assigned an optimal dose, constraints, and priority. Tables 1 and 2 illustrate the various dose volume constraints and priorities for IMRT plans for pancreatic and retroperitoneal tumors. The PTV and GTV are usually assigned a constraint of 90% or greater while small bowel and other OAR were assigned a priority of 80% or greater. Isodose distributions, field arrangements, and DVHs (dose volume histogram) are calculated.
According to the treatment planning method in- troduced by the Emory group, clinical tumor volume as well as the nodal and soft tissue volumes are de- fined by the 3D outlining process and designated as the volume at risk approach, or VaRA. The descrip- tion of this technique has been reported [36]. Instead of defining conventional field borders, the physician de- mands that the VaRA receive a certain minimum isodose coverage, in most cases 98% or greater. In employing the VaRA approach, the boost field margin at the in-
Table 1.IMRT inverse treatment planning algorithm constraint template for pancreatic cancer
Structure Volume(%) IMRT Constraint criteria (Gy) Planning treatment
volume (PTV)
100 Prescription dose:
50.4
Minimum dose: 45 Priority: 90%
Gross tumor volume (GTV)
100 Prescription dose:
61.2
Minimum dose:
59.4 Priority: 90%
Small bowel 100 Maximum dose: 45
75 Maximum dose: 48
50 Maximum dose: 50
25 Maximum dose: 55
Priority: 80%
Table 2.IMRT inverse treatment planning algorithm constraint template for retroperitoneal sarcoma
Structure Volume(%) IMRT
Constraint(Gy) Planning treatment
volume
100 Prescription dose:
45–50.4
Minimum dose: 45 Priority: 90%
Gross tumor volume (GTV)
100 Prescription dose:
50.4
Minimum dose: 45 Priority: 90%
Small bowel 100 Maximum dose: 45
75 Maximum dose: 48
50 Maximum dose: 50
25 Maximum dose: 55
Priority: 80%
Kidney 100 Maximum dose: 12
50 Maximum dose: 15
Priority: 80%
Liver 100 Maximum dose: 30
50 Maximum dose: 40
Priority: 80%
terface between the small bowel and PTV is defined as the 95% or greater isodose level. This strategy al- lows a decrease in the volume of small bowel that is treated.
8.5 Clinical Experience and Trials to Define the Role of IMRT
After introduction of 3D-CRT and IMRT, radiation on- cologists looked for ways to further minimize the doses to organs at risk (OAR) and improve dose conformity to the tumor and regional lymph nodes [37]. Clinical applications of IMRT strategies may allow us to mini- mize serious acute and long-term complications while increasing cure probability in cancer patients. Although anatomical variations must be taken into account, there are data to demonstrate superior dose conformity with IMRT plans for delivering dose to PTV and also less dose to the normal organs. A study from Emory Univer- sity has considered several parameters when comparing treatment plans for IMRT and 3D conformal radiation in ten randomly selected patients treated for pancreatic cancer [36]. There was superior outcome in minimiz- ing the dose to the small bowel and right kidney when employing the IMRT technique. The average dose de- livered to small bowel was lower with the IMRT plan compared to 3D-CRT. Using Lyman-Kutcher models, normal tissue complication probability (NTCP) was 9.3
± 6% with IMRT compared to 24.4 ± 18.9% with 3D- CRT (P=0. 021) (Table 3). The median volume of small bowel that received greater than either 50 or 60 Gy was also reduced with IMRT. The median volume of small bowel that exceeded 50 Gy was 19.2± 11.2% (range 3–45%) compared to 31.4 ± 21.3% (range 7–70%) for 3D-CRT (P=0. 048). The median volume of small bowel that received greater than 60 Gy was 12.5± 4.8%
from IMRT compared to 19.8 ± 18.9% for 3D-CRT (P=0. 034). A comparison of DVHs between 3D-CRT and IMRT is presented in Fig. 3. The Emory group also reported treatment related toxicities from utiliza-
Table 3. Analysis of DVHs for small bowel comparing IMRT and 3D-CRT treatment plans for ten patients with adenocarcinoma of the pancreatic head
IMRT Mean±S D (range)
3D-CRT Mean± SD (range)
P-value
Percent of SBV > 50 Gy
19.2± 11.2 (3.0–45.0)
31.4± 21.3 (7.0– 70)
0.048
Percent of SBV > 60 Gy
12.5± 4.8 (0.0–17.0)
19.8± 18.6 (4.0–62.0)
0.034
Dose to 1/3 of SB (Gy)
30.0± 12.9 (5.0–50.0)
38.5± 14.2 (8.0–56.0)
0.006
Percent of SB NTCP
9.3± 6.0 (0.3–23.2)
24.4± 18.0 (3.8–68.0)
0.021
SBV = small bowel volume 7 SB = small bowel
NTCP = normal tissue complication probability
Fig. 3. Comparison of small bowel DVHs of IMRT and 3D-CRT plans
tion of IMRT for pancreatic cancer in a separate study.
Most patients were treated with preoperative combined regimen with concomitant continuous infusion 5-FU.
Dosimetric parameters were favorable for sparing nor- mal organs. Based on the RTOG toxicity scale, most patients experienced only grade I or II gastrointestinal symptoms [38].
Bai et al. from China reported on dose-escalation with IMRT and concurrent chemotherapy for locally advanced pancreatic cancer [39]. Tolerable dose with their dose-fractionation regimen (60 Gy in 25 fractions) achieved good palliative effect. All patients had less then grade II gastrointestinal toxicity. There were no late gastrointestinal complications.
A phase I study of gemcitabine dose escalation in conjunction with hypofractionated radiotherapy for un- resectable pancreatic tumors was tested by Crane and colleagues from MD Anderson Cancer Center [40]. Ra- diotherapy was started at 33 Gy in 11 fractions treating the primary tumor and lymphatics with an IMRT tech- nique that included escalating the dose by 3 Gy. Patients also received concurrent gemcitabine starting at a dose of 350 mg|m2. Because of dose-limiting toxicity due to myelosuppression and upper gastrointestinal symp- toms, the regimen did not permit either radiation dose or gemcitabine dose escalation.
Ringash et al. from Princess Margaret Hospital in Canada had retrospectively re-planned 20 cases of gas- tric cancer and had physicians compare them with 3D-CRT plans [41]. IMRT plans were preferred in most cases based on better dose-volume histogram (DVH) data, which showed better target volume coverage and sparing of critical organs. Chen et al. [23] also retrospec- tively designed their study to compare 3D-CRT plans with IMRT for patients treated for hepato-cellular carci- noma. The IMRT plan was superior in limiting the dose
to the spinal cord, but it had diverse dosimetric effects on the liver itself with reduction of normal tissue com- plication probability based on their model, but increase in mean dose as compared with 3D-CRT.
Hong et al. recently reported the use of IMRT for whole abdomen radiation and found bone marrow dose reduction and improved tumor coverage when com- pared to traditional whole abdomen treatment [34]. A five-field arrangement was used and the volume of pelvic bones receiving a dose > 21 Gy was reduced by 60% and tumor coverage improved by 11.8% with the use of IMRT.
Clearly, the use of large fields, sometimes necessary for retroperitoneal sarcoma does not preclude employment of IMRT. The Emory University group reported their institutional experience with IMRT in the treatment of retroperitoneal sarcoma. They analyzed the benefits of IMRT with respect to the reduction of dose to critical OAR and enhanced tumor coverage: three patients pre- sented with tumors < 10 cm, seven patients had tumors between 10 and 20 cm, and one patient had a tumor
> 20 cm. Eight of the patients had primary tumors while the remaining three presented with recurrence of dis- ease. Of the 11 patients, 2 had pelvic involvement and 9 of the 11 patients were treated with preoperative radi- ation followed by resection. Two patients were treated postoperatively.
Dose-volume histograms of patients planned and treated with IMRT to 50.4 Gy were compared with 3D- CRT treatment plans to the same dose. For all 11 patients, the IMRT plans with the VaRA approach were generated and compared with 3D-CRT. Tumor coverage, tumor dose received, and OAR toxicity are further illustrated in comparative DVHs in Figs. 4 and 5. For the same dose constraints assigned to liver, small bowel, kidney, and PTV, IMRT resulted in improved coverage of the PTV and reduced dose to critical organs at risk. The dif-
Fig. 4. Composite dose volume histogram (DVH) for 3D-CRT for retroperitoneal sarcoma for patient no. 8
Fig. 5. Composite DVH of IMRT for retroperitoneal sarcoma for patient no. 8
ference was statistically significant for dose received to the small bowel and for the maximum and minimum dose received to the tumor volume. For the prescription dose to 50.4 Gy, both the maximum and minimum doses delivered to the PTV were significantly increased by 6 and 22%, respectively (P=0. 011, P=0. 055) resulting in better dose distribution within the tumor volume. In ad- dition, tumor coverage as measured by the V95 (volume receiving 95% of the dose) was improved from 95.3%
with conventional treatment to 98.6% with IMRT, al- though this value did not reach statistical significance.
The mean average dose to the small bowel decreased from 36 Gy with conventional 3D conformal treatment to 27 Gy using IMRT. Furthermore, the mean dose to left kidney, liver, and spinal cord were all decreased with the use of IMRT. Although the difference in mean dose to the left kidney, liver, and spinal cord structures was not statistically significant due to the small sample size and large standard deviation, the overall trend favors IMRT. The doses received by clinically significant vol- umes of small bowel, liver, and kidney with both IMRT and 3D-CRT were also analyzed (Table 4). The volume of small bowel receiving >30 Gy was significantly de- creased from 63.5± 25.2% (range 20–92%) to 43.1 ± 20.6% (range 20–92%) with IMRT compared with con-
Table 4. Analysis of DVHs for small bowel, left kidney, and liver comparing IMRT and 3D-CRT treatment plans for patients with retroperitoneal sarcoma
IMRT Mean± SD (range)
3D-CRT Mean± SD (range)
P-value
Percent of small bowel
> 30 Gy
43.1± 20.6 (20–92)
63.5± 25.2 (20–97)
0.043
Percent of small bowel
> 50 Gy
8.8± 12.1 (0–31)
23.5± 34.4 (0–85)
0.073
Dose to 33%
of small bowel
31.3± 7.9 (2–48)
40.6± 11.5 (2–54)
0.098
Percent of left kidney
> 15 Gy
50.3± 43.9 (1–100)
55.1± 39.3 (3–100)
0.422
Percent of left kidney
> 25 Gy
37.0± 40.6 (0–97)
49.0± 41.9 (0–100)
0.312
Dose to 33%
of eft kidney
27.0± 19.0 (2–47)
28.7± 18.6 (2–47)
0.442
Percent of liver > 30 Gy
33.3± 26.3 (1–60)
49.6± 37.5 (13–100)
0.201
Percent of liver > 40 Gy
26.8± 23.1 (0–50)
46.0± 38.1 (11–99)
0.158
Dose to 33%
of liver
27.0± 19.0 (10–48)
33.3± 19.2 (11–55)
0.289
ventional treatment (P=0. 043). In addition, the median volume of small bowel that received a dose greater than 50 Gy and the dose delivered to one-third of the bowel volume was reduced with IMRT. The median volume of small bowel that received a dose greater than 50 Gy was 8.8± 12.1% with IMRT compared to 23.5 ± 34.4% for 3D-CRT (P=0. 073). The volume of left kidney that re- ceived a dose greater than 25 Gy decreased from 49 to 37% with the use of IMRT.
For patients with recurrent disease, recurrence varied from three to six years and on average was 4.3 years. The majority of the resected tumors were liposarcoma and most patients presented with Stage III disease. Only two patients did not present with Stage III disease; one had Stage I, and one had Stage II tumor. All 11 patients had complete excision of gross tumor. On review of patho- logic specimens, four patients had microscopic positive margins and the remaining seven patients had negative margins. A total of eight patients required some element of organ removal (defined as removal of the kidney, spleen, pancreas, adrenals, or colon) with nephrectomy the most common. RTOG scoring was used to measure both acute and chronic toxicities for all patients. The most common symptoms were nausea and vomiting and less frequently diarrhea. Seven patients developed grade 2 nausea, three developed grade 2 diarrhea, and one pa-
tient with primary groin involvement experienced grade 2 skin toxicity. One patient, who had extensive liver involvement and received 3D-CRT, developed grade 3 liver toxicity six months after his radiation and was hos- pitalized for management of ascites. This patient had approximately 85% of his liver involved with gross tu- mor and consequently 67% of the whole liver received 30 Gy while 60% received 40 Gy with 3D-CRT. Currently, his ascites and hepatitis resolved and he remains free of disease recurrence. Other than this patient, there have been no other delayed toxicities related to radiation. No genitourinary or wound toxicities were observed and no treatment breaks were necessary. At a median follow-up of 58 weeks, there were no local recurrences and only one patient developed disease progression with distant metastasis in the liver.
At the University of Alabama, 14 patients with retroperitoneal sarcomas were treated with preoperative IMRT with PTV (GTV +1 to 1.5 cm), initially receiv- ing 45 Gy in 25 fractions [42]. The tumor volume that was judged to be at highest risk for positive margins at surgical resection received a higher “boost” dose of 57.5 Gy. Of the 12 patients undergoing surgery, 11 had negative margins at resection and only one patient experienced grade III or greater toxicity. At a median follow up of 48 weeks there were no late toxicities and although 3 of the 11 patients developed disease progres- sion, only 1 of these patients had a local recurrence.
From a treatment planning perspective, the boost dose could theoretically have been escalated to 75.2–82.8 Gy while continuing to respect the OAR tolerance. These data as well as data from Princess Margaret on the use of IMRT in retroperitoneal sarcoma have shown encourag- ing clinical results and demonstrated feasibility of dose escalation [42, 43].
8.6 Future Directions
Additional studies with a larger number of patients and longer follow-up may be necessary to clearly demon- strate the benefit of IMRT for upper abdominal and retroperitoneal tumors in respect to superior tumor cov- erage and lowering treatment toxicity, as well as the potential for dose escalation for certain tumors. Dose escalation is a potential area of investigation for GI tu- mors as well as retroperitoneal sarcomas. Differential dose rate delivery with altered fractionation is another potential area that can be explored in the future. Studies evaluating IMRT in combination with novel chemother- apeutic and molecular agents are warranted because of the potential for IMRT to reduce GI toxicity and allow es- calation of both the chemotherapy and radiation dosage.
Organ motion studies along with incorporation of gated radiotherapy techniques may find have an important role in IMRT delivery for abdominal tumors.
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CTOS Annual Meeting Posters. Radiation Oncology 2001
