Clinical and dosimetric predictors of acute toxicity after a 4-week hypofractionated external beam radiotherapy regimen for prostate cancer: results from a multicentric prospective trial.

Purpose: To investigate predictors for gastrointestinal (GI) and genitourinary (GU) acute toxicity after a short-course hypofractionated radiotherapy regimen for prostate cancer.

Materials And Methods: Three institutions included 102 patients with T1-T3N0M0 prostate cancer in a Phase II study. Patients were treated with 56 Gy in 16 fractions over 4 weeks. Acute toxicity was scored weekly during treatment and 1 and 2 months after treatment using the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer criteria extended with additional symptoms and the International Prostate Symptom Index (IPSS). Correlation with a number of clinical and dosimetric parameters was assessed by univariate and multivariate analyses.

Results: No Grade 3 or 4 GI side effects were observed. Grades 1 and 2 rectal GI toxicity occurred in 36%, and 38%, respectively. Corresponding figures for Grades 1 and 2 GU toxicity were 42% and 39%, respectively. Grade 3 or higher GU toxicity was detected in 4% of patients. In multivariate analysis, percent rectal volumes higher than 8% receiving doses >/=53 Gy (V(53)) were statistically correlated to Grade 2 acute rectal reaction (p = 0.006). For GU morbidity, only the IPSS pretreatment score was independently associated (p = 0.0036) with an increase in GU acute effects.

Conclusions: Acute GU and GI toxicity were comparable with other series. Our data show that increased incidence and intensity of acute toxicity is a transient effect related to shorter overall treatment time rather than a larger effect in biological equivalent dose with respect to a conventional fractionation regime.