Influence of rectal volume changes during radiotherapy for prostate cancer: a predictive model for mild-to-moderate late rectal toxicity.

Purpose: To assess the rectal volume changes during radiotherapy for prostate cancer, to estimate an average rectal dose distribution profile during treatment, and to correlate these parameters with mild-to-moderate late rectal toxicity.

Materials And Methods: Nine patients with localized prostate cancer underwent virtual CT simulation using a six-field conformal 18-MV photon technique. During treatment, patients underwent weekly pelvic CT scans under simulation conditions. Dosimetries were run with each CT data set using the same beam parameters as in the initial treatment plan. The influence of weekly rectal volume changes on the dose-volume histogram (DVH) profiles was studied. A polynomial function correlating the initial rectal volume with the mean percentage of change in the rectal volume during treatment was used to define a correction factor for rectal DVHs. The model was validated using data from 100 patients treated with 74 Gy according to the same technique. Areas under the curve of the initial rectal DVHs were correlated with toxicity (Radiation Therapy Oncology Group Grade 0 vs. 1-2, Student's t test), with or without the use of the above correction factor.

Results: A trend for enlargement of the rectal volume during treatment was observed for most patients in the study with small rectal volumes (<75 cm(3)) at simulation, resulting in an increase in the integral rectal dose by a factor ranging from 1.3 to 2.1. Corrected, but not uncorrected, rectal DVH profiles were strongly predictive of Grade 0 vs. 1-2 late rectal morbidity.

Conclusions: Correcting the area under the curve of the rectal DVH at simulation by a factor that takes into account the projected volume changes during treatment correlates significantly with the probability of mild-to-moderate late rectal toxicity (Grade 1-2). This reliable predictor for mild-to-moderate late rectal morbidity may also be a practical tool for treatment planning.