Preliminary experience of a predictive model to define rectal volume and rectal dose during the treatment of prostate cancer.

OBJECTIVES: The aim of this study was to define a method to evaluate the total dose delivered to the rectum during the whole treatment course in six patients undergoing irradiation for prostate cancer using an offline definition of organ motion with images from a cone beam CT (CBCT) scanner available on a commercial linear accelerator.
METHODS: Patient set-up was verified using a volumetric three-dimensional CBCT scanner; 9-14 CBCT scans were obtained for each patient. Images were transferred to a commercial treatment planning system for offline organ motion analysis. The shape of the rectums were used to obtain a mean dose-volume histogram (<DVH>), which was the average of the DVHs of the rectums as they appeared in each verification CBCT. A geometric model of an average rectum (AR) was produced using the rectal contours delineated on the CBCT scans (DVH(AR)). To check whether the first week of treatment was representative of the whole treatment course, we evaluated the DVHs related to only the first five CBCT scans (<DVH5> and DVH(AR5)). Finally, the influence of a dietary protocol on the goodness of our results was considered.
RESULTS: In all six patients the original rectal DVH for the planning CT scan showed higher values than all DVHs.
CONCLUSION: Although the application of the model to a larger set of patients is necessary to confirm this trend, reconstruction of a representative volume of the rectum throughout the entire treatment course seems feasible.