Interfractional Rectal Displacement Requiring Repeated Precaution Did Not Correlate to Biochemical Control and Rectal Toxicity in Patients with Prostate Cancer Treated with Image-guided Intensity-modulated Radiation Therapy

Aim: To investigate the correlation between frequency of action level of interfractional rectal displacement requiring repeated precaution in patients with prostate cancer and late toxicity from image-guided intensity-modulated radiation therapy (IG-IMRT) using helical tomotherapy. Patients and Methods: We examined 264 patients who underwent IG-IMRT during 2007-2011. Megavoltage computed tomographic (MVCT) images were acquired before radiation therapy and was examined with soft-tissue matching by comparing treatment planning images within 9,345 fractions. Displacement of the anterior rectal region larger than 5 mm, requiring repeated precaution, was defined as the level of rectal displacement requiring action (ARD). Results: ARD was identified in 815 (7.7%) out of 9,345 fractions and at least once in 82% (216/264) of patients. The highest incidence of ARD (11%) was found during the initial week of treatment (first five and next five fractions), after which the incidence decreased to 6% (p<0.0001). Patients with lean body (lower body mass index (BMI) tended to have a higher incidence of ARD. We identified 16 (6%) cases of gastrointestinal toxicity and 12 (4.5%) genitourinary toxicities as a late adverse reaction (3 months or later after IG-IMRT). There was no correlation between ARD and late toxicity. Prostate-specific antigen (PSA) control was also similar (p=0.12) between those with ARD (96% at 5 year) and those without ARD (88%). Conclusion: ARD occurred predominantly in lean patients, during the initial week of treatment and became less likely over time. ARD was not correlated to late toxicity and PSA control, therefore, IG-IMRT technique was able to adequately control error due to interfractional prostate and rectal motion.