A volumetric trend analysis of the prostate and seminal vesicles during a course of intensity-modulated radiation therapy

Purpose: The purpose of this study was to evaluate the volumetric changes of the prostate and seminal vesicles (SV) during a definitive course of intensity-modulated radiation therapy using a combined computed tomography (CT)-linear accelerator system. Methods and Materials: Fifteen patients were enrolled in this IRB-approved prospective study. The treatment plan was designed to deliver a total dose of 75.6 Gy over 42 fractions to 98% of the planning target volume. In-room CT scans were acquired using a CT-on-rails system 3 times weekly just before daily treatment throughout the course of radiation therapy. Rates of volume change over time for the prostate and SV were calculated using linear regression analysis. Results: Fifteen patients had a total of 369 CT scans during the study. For 53% of patients (N = 8), there was a statistically significant decrease in prostate volume over time (range, 0.05-0.47 mL/d or 0.7%-3.5% per week). The median volume reduction was -11.5% (4.9 mL) at the end of the treatment. There was no significant change in the volume of the SV during the course of treatment. The decrease in prostate volume was significantly correlated with the initial volume of the prostate, with larger glands shrinking more during treatment (P = 0.001). Conclusion: Patients with enlarged prostates may experience volumetric reduction in their gland size during a protracted course of radiation therapy. However, the magnitude of the volume reduction is relatively small. In the current era of dose escalation, future studies with adaptive radiation therapy strategy may be appropriate to minimize radiation exposure to normal tissues.