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Connective Tissue Oncology Society |
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| 2001 CTOS Annual Meeting Posters— Radiation Oncology HIGH RESOLUTION,
INTENSITY MODULATED RADIATION THERAPY (IMRT) FOR RETROPERITONEAL
SOFT TISSUE SARCOMA (RPS) OBJECTIVE: RPS are large tumors that are often positioned between critical radiosensitive structures, and which present challenging treatment planning scenarios. Bowel may be adequately excluded from the high-dose radiation volume with pre-operative conformal therapy, but liver, kidney and cord within or adjacent to the clinical target volume (CTV) often compromise radiation delivery. This study compares IMRT to conventional conformal techniques with regard to the radiation dose profile to the CTV and adjacent critical structures. METHODS: 9 patients were referred for pre-operative radiotherapy of RPS. 6 presented with right-sided tumors, 3 with left-sided tumors. 5 right-sided tumors were located in both the upper and lower quadrants of the abdomen, 1 in the lower quadrant only. All left-sided tumors were located in the lower quadrant. The gross tumor volume (GTV), CTV, liver, cord, bowel and the non-involved kidney were contoured throughout the region of interest. The CTV encompassed the GTV with a 1.5 to 5 cm margin, as determined by the clinical situation. Patients were prescribed 45 Gy in 25 fractions over 5 weeks. A forward conformal plan and an inverse IMRT plan were generated for each patient. Co-planar conformal distributions employed 2 to 4 beams with 6 or 25MV photons. IMRT beam fluence was optimized by the Helios inverse planning system for 10MV photons, using 6 to 9 co-planar beams and a Varian 120-leaf dynamic multi-leaf collimator. The conformity index (CI), and dose-volume histograms (DVH) of conformal and IMRT plans were compared for each patient with regard to CTV, liver, cord, bowel, and the contralateral kidney. The statistical significance of differences in DVH were determined with the two-tailed t-test. RESULTS: Mean volumes of the GTV and CTV were 2042cc (range 93 - 4668cc) and 4042cc (range 478 – 6532cc) respectively. Dose homogeneity is comparable for both treatment systems. Mean variance within the CTV for conformal plans was +6% to –9% and was +7 to –7% for IMRT plans. The CI of the IMRT plans was significantly smaller (p <0.05). Mean for IMRT was 1.1 (range 1.0 – 1.2), and 2.1 (range 1.2 – 5.4) for conformal plans. IMRT significantly reduced mean %bowel volume within the 40 Gy isodose (17% vs 30%, p<0.05) and reduced the maximum dose given to the 25% volume of bowel (30Gy vs 41Gy, p <0.05). IMRT significantly reduced the maximum dose to the liver for right upper quadrant tumors (p<0.05). CONCLUSION: Patients with RPS presented for pre-operative RT with huge tumors adjacent to critical uninvolved structures. The improved CI with IMRT reduced the dose to uninvolved liver and bowel. This may result in overall reduced toxicity and for some patients with right upper quadrant tumors, IMRT represented the only opportunity for safe radiation delivery to an adequate CTV. Pre-operative IMRT for RPS offers an opportunity to improve the CTV coverage and/or increase the dose to 50 Gy or higher, while meeting the dose constraints of adjacent normal structures. This may result in less toxicity, improved local control, and improved disease-free survival.
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