Intensity-modulated radiation therapy (IMRT): Differences in target volumes and improvement in clinically relevant doses to small bowel in rectal carcinoma.

586 Background: In the treatment of rectal cancer, a strong dose-volume relationship exists between the amount of small bowel receiving low- to intermediate-doses of radiation and the rates of acute, severe gastrointestinal toxicity. Highly conformal treatment approaches, such as intensity-modulated radiation therapy (IMRT), may reduce dose to adjacent organs-at-risk (OAR). We performed a dosimetric evaluation of IMRT compared to 3-dimensional conformal radiation therapy (3DCRT) in standard, preoperative treatment for rectal cancer. Methods: Using RTOG consensus contouring atlas, treatment volumes were generated for ten patients treated preoperatively, with IMRT plans compared to 3DCRT plans derived from classic anatomic landmarks, as well as modified 3DCRT plans treating the RTOG consensus volume. The patients were all T3, were node-negative (N=1) or node–positive (N=9), and were planned to a total dose of 45-Gy. Bowel displacement was achieved using a carbon-fiber bellyboard apparatus with prone positioning. Results: IMRT plans had superior PTV coverage, dose homogeneity, and conformality in treatment of the gross disease and at-risk nodal volume, in comparison to 3DCRT. Additionally, in comparison to the modified 3DCRT plans, IMRT achieved a concomitant reduction in doses to the bowel, bladder, pelvic bones, and femoral heads, with an improvement in absolute volumes of small bowel receiving dose levels known to induce clinically-relevant acute toxicity. In the six patients with the highest volume of small bowel (range: 209-537-cc), the volume of bowel receiving 15-Gy was reduced from a median of 224-cc in the modified 3DCRT plans to 185-cc with IMRT. Also, the IMRT volumes were typically larger than that covered by classic 3DCRT fields, without incurring penalty with respect to adjacent OAR. Conclusions: For rectal carcinoma, IMRT, compared to 3DCRT, yielded plans with superior target coverage, homogeneity, and conformality, while lowering dose to adjacent OAR. This is despite treating larger volumes, raising the possibility of a clinically-relevant improvement in the therapeutic ratio through the use of IMRT with a belly-board apparatus. No significant financial relationships to disclose.