Acute toxicity and its dosimetric correlates for high-risk prostate cancer treated with moderately hypofractionated radiotherapy

Aims: To report the acute toxicity and the dosimetric correlates after moderately hypofractionated radiotherapy for localized prostate cancer.

Methods: A total of 101 patients with localized prostate cancer were treated with image-guided intensity-modulated radiation therapy. Patients were treated to 65Gy/25Fr/5 weeks (n = 18), or 60Gy/20Fr/4 weeks (n = 83). Most (82.2%) had high-risk or pelvic node-positive disease. Acute toxicity was assessed using Radiation Therapy Oncology Group (RTOG) acute morbidity scoring criteria. Dose thresholds for acute rectal and bladder toxicity were identified.

Results: The incidence of acute grade 2 GI toxicity was 20.8%, and grade 2 genitourinary (GU) toxicity was 6.9%. No Grade 3 to 4 toxicity occurred. Small bowel toxicity was uncommon (Gr 2 = 4%). The 2Gy equivalent doses (EQD2) to the rectum and bladder (α/β = 3) calculated showed that the absolute doses were more consistent predictors of acute toxicities than the relative volumes. Those with grade 2 or more GI symptoms had significantly higher V_EQD2-60Gy (13.2 vs 9.9cc, p = 0.007) and V_EQD2-50Gy (20.6 vs 15.4cc, p = 0.005). Those with grade 2 or more GU symptoms had significantly higher V_EQD2-70Gy (30.4 vs 18.4cc, p = 0.001) and V_EQD2-65Gy (44.0 vs 28.8cc, p = 0.001). The optimal cutoff value for predicting grade 2 acute proctitis, for V_EQD2-60Gy was 9.7cc and for V_EQD2-50Gy was 15.9cc. For grade 2 GU symptoms, the threshold values were 23.6cc for V_EQD2-70Gy and 38.1cc for V_EQD2-65Gy.

Conclusions: Hypofractionated radiotherapy for prostate cancer is well tolerated and associated with manageable acute side effects. The absolute dose-volume parameters of rectum and bladder predict for acute toxicities.