The Impact of Radiation Dose to Bowel on the Risk of Developing Gastrointestinal Toxicities When Treating Abdominopelvic Soft-Tissue Sarcomas With Preoperative Radiation Therapy.

Purpose: Abdominopelvic soft-tissue sarcomas (AP-STS) are selectively treated with radiation therapy (RT) followed by surgery. We investigated dosimetric factors predictive of acute and long-term gastrointestinal (GI) toxicities in patients treated with preoperative RT for AP-STS. Methods and Materials: We performed a retrospective cohort study of patients treated for nonmetastatic AP-STS with preoperative RT and surgery from 2005 to 2020. Individual loops of small and large bowel, as well as a "Bowel Bag" space, were autocontoured using a clinically validated deep learning-based approach (nnU-Net) and then manually reviewed. Chi-square or Fisher's exact test was used to assess how bowel dosimetry related to the development of GI toxicities. In particular, the effect on GI toxicity of exceeding bowel constraints as defined in the consensus guidelines was evaluated. Recurrence outcomes were analyzed using the Kaplan-Meier method. Results: We evaluated 76 patients with a median follow-up of 46 months (IQR, 42-50). Approximately half of the tumors were located in the retroperitoneal space (n = 39, 51%). A total of 60 patients (79%) presented with de novo disease, and 21% (n = 16) were recurrent presentations that had not received prior RT. Fourteen patients (18%) had a local recurrence alone, 23 (30%) a distant recurrence alone, and 3 (4%) combined local and distant recurrence. As a result of RT, 23 patients (30%) had acute grade 1 to 2 diarrhea, and 14 (18%) had grade 1 nausea. There were no acute grade 3 toxicities. Six patients (8%) had any long-term RT-related toxicity, including 1 small bowel obstruction. Patients were more likely to have an acute GI toxicity if bowel bag V45>195 cm3 (P = .05). Conclusions: Traditional RT volumetric bowel dose constraints are frequently exceeded, given the large size of AP-STS at presentation and the corresponding large RT volumes. Exceeding the conventional bowel dosimetric constraints should not dissuade the use of RT, but instead signal the need for aggressive prophylactic management of potential toxicities.