Searching for optimal dose-volume constraints to reduce rectal toxicity after hypofractionated radiotherapy for...

Late rectal toxicity is a major concern for prostate cancer patients treated with radiotherapy. Rectal dose-volume constraints, set as guidelines to reduce its incidence, vary among institutions. From a group of patients uniformly treated with hypofractionated radiotherapy, we correlated the incidence of late rectal toxicity with rectal dose-volume rectal constraints as described in three randomised trials for prostate cancer.

Favourable-risk prostate cancer patients received a dose of 66 Gy in 22 fractions without hormonal therapy. Toxicity was prospectively assessed using Common Toxicity Criteria v3. The whole or part of the rectum and rectal wall were contoured as an organ at risk for all patients. The rectal constraints of the RTOG 0126, RTOG 0415 and the PROFIT trials were used to correlate with late rectal toxicity.

The median follow-up time was 58 months. Late rectal toxicity was 62, 20 and 18% for grades 0, 1 and 2/3, respectively. No statistically significant correlation was found between late rectal toxicity and the rectal constraints used in the three trials. The number of patients violating the recommended constraints was similar for the group with grade 2/3 toxicity and the group without any toxicity. Analysis derived from the actual dose-volume histogram dose parameters of this group of patients did not show a relationship between dose to volume of the rectum and late rectal toxicity that could generate a guideline of dose constraints.

For this group of patients, despite the use of recognised dose-volume constraint guidelines of three trials, we were unable to establish a relationship between these constraints and the late rectal toxicity registered. Further studies on the correlation of dosimetric parameters with rectal toxicity, particularly for hypofractionated regimens, are required. Non-dosimetric factors may also be involved in the risk of late rectal toxicity.