In the RAPIDO trial of short course RT and TNT for rectal cancer, radiation planning was physician preference with no mandatory delineation of OARs nor specified dose constraints. This study developed NTCP models for diarrhea using data from RAPIDO. It included 588 patients from the TNT and CRT arms of RAPIDO, and the bowel was contoured for each patient using both the RTOG (entire abdominal and pelvic space, aka bowelbag) and EMBRACE (outer borders of small and large bowel, aka bowelloop) guidelines. Among various bowel dose parameters, RTOG bowel bag V15 and V20 were associated with acute grade 2+ diarrhea after short course RT. V10, V15, and V20 using both guidelines were associated with late grade 1+ diarrhea. In the CRT arm, all parameters were significant, but V15, V30, and V40 were particularly predictive of acute diarrhea. No specific thresholds were reported, but the plots show pretty linear increase in diarrhea risk with increasing VX, meaning the planning goal should be to reduce bowel dose at these levels as much as achievable. | Tanaka, Int J Radiat Oncol Biol Phys 2026
