Frequency and predisposing factors for interfractional rectal displacement requiring repeated precaution in prostate cancer patients treated with image-guided intensity-modulated radiation therapy.

Aim: To investigate the frequency and characteristics of interfractional rectal displacement in patients with prostate cancer treated with image-guided intensity-modulated radiation therapy (IG-IMRT) using helical tomotherapy.

Patients And Methods: Data for a total of 256 patients were analyzed. Megavoltage computed tomography (MVCT) images were acquired before radiation therapy and interfractional rectal displacement was assessed with soft-tissue matching by comparing treatment planning images within 9,445 fractions. Anterior rectal region displacement larger than 5 mm, requiring repeated precaution, was defined as the action level of rectal displacement (ARD).

Results: ARD was identified in 676 (7.2%) out of 9,445 fractions and at least once in 75% (190/256) of patients. Univariate analysis identified three predisposing factors for ARD: body mass index (BMI), rectal volume and prostate volume. Multivariate logistic regression analysis revealed that lower BMI and large rectal volume were statistically significant predictors of ARD. The highest incidence of ARD (13.6% and 9.1%) was found during the initial two weeks of treatment (first five and next five fractions), after which the incidence decreased to 5.96% (p<0.0001).

Conclusion: ARD was identified in 7.9% of fractions and in 74.8% of patients and was most likely to occur in patients with a low BMI and/or large rectal volume. ARD occurred predominantly during the initial two weeks of treatment and became less likely over time.