Exceptionally High Incidence of Grade 2-3 Late Rectal Toxicity in Patients with Prostate Cancer Receiving Hypofractionated (2.2 Gy) Soft Tissue-matched Image-guided Intensity-modulated Radiotherapy

Aim: To evaluate the incidence of rectal toxicity in patients undergoing hypofractionated (2.2 Gy) image-guided intensity-modulated radiotherapy (IG-IMRT) for prostate cancer. Patients and Methods: We examined 117 consecutive patients with prostate cancer who underwent IG-IMRT from June 2007 to July 2009. The median follow-up time was 32 months (range 20-42 months). The clinical target volume (CTV) consisted of the prostate and seminal vesicles, and the planning target volume (PTV) consisted of the CTV plus a 5-mm expansion, not avoiding the rectum. The PTV received a dose of 72.6-74.8 Gy in 33-34 fractions (2.2 Gy/fraction). Megavoltage computed tomographic (MVCT) scans were performed before each treatment and corrected to the registered position for planning CT scans using prostate soft tissue matching. Results: Late rectal bleeding of grades I, 2, and 3 (Common Terminology Criteria for Adverse Events v3.0) occurred in 19 (16%), five (4%), and four (3%) patients, respectively. Late urinary toxicities of grades 1 and 2 occurred in five (4.3%) and eight (6.8%) patients, respectively. We found a paradoxically increased risk of rectal bleeding with more accurate irradiation of the rectum using soft tissue matching, whereas only a small percentage was reported in other IMRT series. Conclusion: IG-IMRT using daily MVCT scans allowed for exact dose delivery, which resulted in an increased rectal dose and exceptionally high incidence of rectal toxicity. Therefore, careful PTV contouring and dose schedule settings are important for safe administration of IG-IMRT.