Better late than never.

Compared to what most folks use in practice these days, the dose constraints for moderately hypofractionated prostate radiation in RTOG 0415 were pretty lenient. While many of these constraints are designed to reduce acute toxicity, this study explores RTOG 0415 dosimetric predictors of late GI and GU toxicity. As a reminder, patients in the hypofractionated arm received 2.5 Gy x 28 daily fractions to a total of 70 Gy. Late toxicity modeling included dose-volume cut-points, age, acute toxicity, and treatment technique. The three most important predictors of late toxicity were 1) experiencing acute GI toxicity, 2) absolute volume of rectum receiving 35Gy, and 3) D5% to the rectum. On multivariable modeling, the rectal D5% in the presence of acute GI toxicity was the main predictor of late toxicity. The overall rate of late GI toxicity was 19%, correlated with a D5% of >65 Gy. They hypothesize this rate could be reduced to 10% with a D5% of <62 Gy, meaning we may need to be even more strict than the V65 < 10% commonly used in practice.  TBL: Rectum D5% is a good predictor of late GI toxicity for men receiving hypofractionated prostate radiation. | Thor, Radiother Oncol 2019