Daily Rectal Dose-volume Histogram Variation in Prostate Intensity-modulated Radiation Therapy: Is It Clinically Significant in the Era of Image Guidance?

Abstract

Aim: Because of the steep dose gradient associated with dose-escalated intensity-modulated radiation therapy, interfraction motion and variation in rectal volume may result in the rectum receiving a larger dose than predicted at treatment planning. This study aims to quantify the variation in daily rectal dose-volume histograms (DVHs) from the treatment plan and to discuss the potential clinical significance of this variation.

Materials and methods: Daily cone beam computed tomography scans of nine patients treated with definitive prostate intensity-modulated radiation therapy were collected. The daily dose distribution to the rectum was calculated retrospectively. The variation between the planned and on-treatment rectal DVHs was determined using Friedman tests with post hoc analysis and Wilcoxon matched-pairs tests. The on-treatment DVHs were compared with dose-volume constraints (DVCs) to assess the potential clinical significance of this variation using Wilcoxon signed-rank tests.

Results: Significant variation (P < .05) was observed between the planned and on-treatment DVHs. The DVCs for the volume receiving 50 Gy (V50), V60, and V65 were adhered to. The mean V70 and V75 values were above the DVC; however, this variation was not statistically significant.

Conclusion: The initial treatment plan does not accurately represent the dose received by the rectum on treatment. Investigation into the most effective rectal protocol is recommended to reduce the likelihood of these variations occurring on a daily basis.

Keywords: IMRT; Rectal DVH; prostate; rectal variation.