SU-E-J-206: Adaptive Radiotherapy for Gynecological Malignancies with MRIGuided Cobolt-60 Radiotherapy.

PURPOSE: Even in the IMRT era, bowel toxicity and bone marrow irradiation remain concerns with pelvic irradiation. We examine the potential gain from an adaptive radiotherapy workflow for post-operative gynecological patients treated to pelvic targets including lymph nodes using MRI-guided Co-60 radiation therapy.

METHODS: An adaptive workflow was developed with the intent of minimizing time overhead of adaptive planning. A pilot study was performed using retrospectively analyzed images from one patient's treatment. The patient's treated plan was created using conventional PTV margins. Adaptive treatment was simulated on the patient's first three fractions. The daily PTV was created by removing non-target tissue, including bone, muscle and bowel, from the initial PTV based on the daily MRI. The number of beams, beam angles, and optimization parameters were kept constant, and the plan was re-optimized. Normal tissue contours were not adjusted for the re-optimization, but were adjusted for evaluation of plan quality. Plan quality was evaluated based on PTV coverage and normal tissue DVH points per treatment protocol. Bowel was contoured as the entire bowel bag per protocol at our institution. Pelvic bone marrow was contoured per RTOG protocol 1203.

RESULTS: For the clinically treated plan, the volume of bowel receiving 45 Gy was 380 cc, 53% of the rectum received 30 Gy, 35% of the bladder received 45 Gy, and 28% of the pelvic bone marrow received 40 Gy. For the adaptive plans, the volume of bowel receiving 45 Gy was 175-201 cc, 55-62% of the rectum received 30 Gy, 21- 27% of the bladder received 45 Gy, and 13-17% of the pelvic bone marrow received 40 Gy.

CONCLUSION: Adaptive planning led to a large reduction of bowel and bone marrow dose in this pilot study. Further study of on-line adaptive techniques for the radiotherapy of pelvic lymph nodes is warranted. Dr. Low is a member of the scientific advisory board of ViewRay, Inc.