The effects of volumetric changes on radiation doses of the rectum and bladder during radiotherapy in patients with prostate cancer

Background: In prostate radiotherapy, changes in the volume of the bladder and rectum can influence dose delivery. In this study, cone-beam computerised tomography (CBCT) imaging was used to assess volumetric, and corresponding radiation dosimetric changes, for the bladder and rectum in patients with prostate cancer treated using VMAT. Materials and Methods: Treatment planning computed tomography (simCT) and CBCT images were retrospectively evaluated in 22 patients with prostate cancer. Bladder and rectal volumes were recontoured in 176 CBCT images. CBCT images were used for VMAT treatment plan recalculation and to obtain bladder and rectum radiation doses. Results: Mean rectal volumes measured by CBCT were significantly larger than those estimated by simCT (P=0.001). A 14% increase in rectum volume resulted in a 9% increase in mean rectum doses. The percent volumes (Vx) of the rectum receiving 40, 50, 60 and 70 Gy doses based on CBCT results were significantly larger than those based on simCT results (P=0.002, P=0.001, P=0.001, P 0.003, respectively). Mean bladder volumes measured by CBCT were significantly smaller than those estimated by simCT (P=0.001). A 13% decrease in bladder volume resulted in a 8% increase in mean bladder doses. Mean bladder V65 and V70 values based on CBCT results were significantly higher than those based on simCT results (P<0.001, P=0.002, respectively). Conclusion: Results during prostate radiotherapy, daily changes in bladder and rectal volumes can result in larger actual doses to these organs than the planned dose.