Is rectal preparation necessary in contemporary image-guided prostate radiotherapy?

Rectal preparation during prostate radiotherapy (RT) was established in the era of two-dimensional bone-based IGRT, but its necessity with contemporary three-dimensional IGRT is uncertain. We examined the impact of omitting rectal preparation on prostate inter- and intrafraction motion, rectal volume, and their relationship.

Two datasets were analysed. Dataset one included patients receiving RT for de novo prostate cancer on a C-arm linear accelerator, stratified by use of micro-enema or no rectal preparation. Interfraction prostate motion was calculated from daily CBCT, and compared between preparation groups, with further stratification by rectal volume on planning CT (pCT) < 90 or ≥ 90 cm³. Correlation between rectal volume and prostate motion was assessed.Dataset two comprised 40 patients with localised prostate cancer treated without rectal preparation. Inter- and intrafraction prostate motion were calculated, with intrafraction motion assessed using post-treatment CBCT acquired at five RT fractions. The rectum was contoured on pre-treatment CBCT, and correlation between rectal volume and intrafraction motion evaluated.

Dataset one included 255 patients: 96 with and 159 without rectal preparation. Median rectal volume on pCT (proportion with rectal volume ≥ 90 cm³) was 64 cm³ (18 %) in the preparation group and 68 cm³ (28 %) in the no-preparation group. Interfraction motion differences were small; however, patients without preparation and a pCT rectal volume ≥ 90 cm³ demonstrated increased posterior motion.In dataset two, median rectal volume on pCT was 75 cm³, 33 % had a rectal volume ≥ 90 cm³. Intrafraction motion was modest and not influenced by pCT rectal volume. No correlation was observed between rectal volume during treatment and intrafraction motion.

Rectal preparation reduced the proportion of patients with a large rectum on pCT by only 10-15 %. A large rectum modestly increased interfraction motion but did not affect intrafraction motion. Findings suggest rectal preparation is not universally required with contemporary IGRT.