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Bowel Bag Dosimetric Parameters for Ultrahypofractionated Pelvic Elective Nodal Irradiation in Prostate Cancer
Wee Loon Ong1, Melanie Davidson2, Matt Wronski2
1Department of Radiation Oncology, Sunnybrook Odette Cancer Centre, University of Toronto, Toronto, Ontario, Canada; Alfred Health Radiation Oncology, School of Translational Medicine, Monash University, Melbourne, Victoria, Australia.
Purpose:
Most prostate SABR trials with pelvic elective nodal irradiation (ENI) applied dose constraints to individual bowel loops. There is no evidence-based data to guide bowel bag dose constraints in ultrahypofractionated ENI for prostate cancer. We aimed to correlate bowel bag dosimetric parameters with gastrointestinal toxicities and bowel quality of life (QOL).
Methods And Materials:
We included 59 men enrolled in the SATURN and 5STAR trials. The prescribed dose of ENI was 25 Gy in 5 fractions delivered in weekly fractionation. The bowel bag was retrospectively contoured as per the Radiation Therapy Oncology Group guidelines. Gastrointestinal toxicities and bowel QOL data were prospectively collected up to 5 years after the trial using the National Cancer Institute Common Terminology Criteria for Adverse Events version 4 and the Expanded Prostate Cancer Index questionnaire. Dosimetric parameters for the bowel bag based on delivered treatment plans were extracted. Logistic regressions were performed to identify dosimetric parameters associated with gastrointestinal toxicities and bowel QOL.
Results:
Of the 59 men included in this analysis, 7 of 59 men (12%) and 12 of 59 men (20%) had acute and late grade 2 gastrointestinal toxicities, respectively, with no grade 3+ gastrointestinal toxicities. There were 21 of 55 men (38%) and 10 of 55 men (18%) who had minimal clinically important changes in acute and late bowel QOL. No bowel bag dosimetric parameters were identified to be associated with acute/late grade 2+ gastrointestinal toxicities or minimal clinically important changes in bowel QOL. Based on bowel bag dosimetry of the delivered treatment plans, we propose bowel bag dose constraints for ultrahypofractionated ENI, applying the median and IQR upper limit as optimal and mandatory constraints: V20 Gy (optimal <320 cc; mandatory <380 cc), V24 Gy (optimal <150 cc; mandatory <200 cc), V25 Gy (optimal <80 cc; mandatory <100 cc), and maximum point dose (optimal <27 Gy; mandatory <35 Gy).
Conclusions:
Using individual patient data from prospective trials, we did not identify bowel bag dosimetric parameters associated with gastrointestinal toxicities and bowel QOL; however, this provided us with data that suggested dose constraints for future practice.
