The Effects of Laxative Use on Rectum Volume and Doses During Radiotherapy in Patients With Prostate Cancer

Objective: Changes in rectum volume can affect dose delivery during prostate radiotherapy. The aim of this study was to evaluate how laxative use affected rectum volumes and doses in patients with prostate cancer treated with Volumetric Modulated Arc Therapy (VMAT). Material and Methods: Treatment planning computed tomography (simCT) and cone-beam computed tomography (CBCT) images collected from 20 patients with prostate cancer were retrospectively evaluated. These patients were divided into two groups as those who either used or did not use laxatives during radiotherapy. Rectum volumes were re-contoured on 160 CBCT images, and VMAT treatment plans were re-calculated to determine rectum doses. Results: In all patients, CBCT images showed increased mean rectum volumes and doses relative to simCT images. Furthermore, the percent volume (Vx) of the rectum receiving 40, 50, 60, and 70 Gy doses based on CBCT results were larger than those based on simCT. The Dmean values in the treatment plans for the group with laxative use were 39 Gy on simCT images and 43 Gy on CBCT images (p = 0.009). Alternatively, in the group without laxatives, the Dmean values in the treatment plans were 40 Gy on simCT images and 44 Gy on CBCT images (p = 0.047). Conclusion: In patients undergoing prostate radiotherapy, rectum volume and doses increase regardless of laxative use. Although laxatives can limit volume expansion of the rectum, they do not have the expected effect on rectum doses and actually significantly increase all dosimetric parameters.