Strategies to evaluate the impact of rectal volume on prostate motion during three-dimensional conformal radiotherapy for prostate cancer

Objective: To evaluate the rectal volume influence on prostatemotion during three-dimensional conformal radiotherapy (3D-CRT) for prostatecancer. Materials and Methods: Fifty-one patients with prostatecancer underwent a series of three computed tomography scans including an initial planning scan and two subsequent scans during 3D-CRT. The organs of interest were outlined. The prostatecontour was compared with the initial CT images considering the anterior, posterior, superior, inferior and lateral edges of the organ. Variations in the anterior limits and volume of the rectum were assessed and correlated with prostatemotion in the anteroposterior direction. Results: The maximum range of prostatemotion was observed in the superoinferior direction, followed by the anteroposterior direction. A significant correlation was observed between prostatemotion and rectal volume variation ( p = 0.037). A baseline rectal volume superior to 70 cm3 had a significant influence on the prostatemotion in the anteroposterior direction ( p = 0.045). Conclusion: The present study showed a significant interfraction motion of the prostateduring 3D-CRT with greatest variations in the superoinferior and anteroposterior directions, and that a large rectal volume influences the prostatemotion with a cutoff value of 70 cm3. Therefore, the treatment of patients with a rectal volume > 70 cm3 should be re-planned with appropriate rectal preparation.