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Purpose: To evaluate the performance of a rectum model in predicting late rectal toxicity of prostate patients undergoing 3D conformal radiation therapy while following a dietary protocol combined with image guidance. Methods: A linear accelerator equipped with a Cone Beam Computed Tomography (CBCT) system was used to treat 20 patients who were following a dietary protocol. The set-up was verified by co-registering CBCT scans with the planning CT scan (pCT). A mean dose volume histogram () as the arithmetical mean of the rectum DVHs from each CBCT scan was obtained. A suitably defined 3D rectum model (Average Rectum, AR) was defined and its DVH (DVHAR) was calculated. DVHs were also evaluated for the first five CBCT scans using both methods (5 > and DVHAR5). The Lyman-Kutcher-Burman NTCP model with QUANTEC parameters was used to compare the calculated DVHs. The QUANTEC dose values were used to describe the time behaviour of the relative volumes using the Gamma Distribution for the frequency of the relative rectum volumes at each QUANTEC dose value. Results: No statistically significant differences between NTCPAR5 and NTCPAR and between NTCP and NTCP were found. The best agreement with the observed toxicity rate (0%) was obtained form DVHAR. The Gamma Distributions of the rectum volumes at the QUANTEC dose levels were found to be highly variable among the patients. Conclusions: Both dietary protocol and image guidance were found effective in limiting late rectal toxicity. AR was a better predictor for late rectal toxicity and better described the rectum volume during the treatment course. Finally, from the Gamma distributions, and from our toxicity data, we can suggest V75 as the best predictor of late rectal toxicity.

Construction of a Simple Rectum Model Using Image Guidance in Prostate Patients Treated with 3D Conformal Radiotherapy