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UK radiotherapy guidance recommends instigating a repeat planning CT (pCT) scan for prostate cancer (PCa) patients with a rectal diameter (RD) >4cm[1]. Additional bowel preparation, diet or voiding instruction are typically given for this. But are these diameter restrictions achievable on pCT and during radiotherapy?

From March-May 2021 inclusive, the number of radical PCa radiotherapy patients requiring a rescan was audited. All patients used micro-enemas 2-days and 1.5-hours prior to pCT. Rescan (on another day with additional micro-enemas) was initiated if RD, measured on an 8-slice short series (SS) pCT over the pubic symphysis (ps), was >4.5cm anterior-posterior (AP) and 3cm left-right (LR).

An experienced RTT retrospectively measured RD for the rescan group plus a matched control group (patients not requiring rescan). Maximum RD, AP and LR, was measured on the SS, pCT and cone-beam CT (CBCT) images from all fractions. Measurements were taken at 3 levels: superior aspect, inferior aspect and mid-point of ps.

RD differences between groups and for different schedules was compared.

175 PCa patients underwent a pCT during this time, 22 (13%) required a rescan. 18/22 patients had CBCT imaging at every fraction (#), these were reviewed further. In the rescan group, four different fractionations were prescribed; 36Gy in 6#’s (n=8), 60Gy in 20#’s (n=8), 57Gy in 19#’s (n=1) and 36.25Gy in 5#’s (n=1). Control group patients were matched by radiotherapy schedule.

Mean RD was larger and more variable in the rescan group (Figure 1). Difference in mean RD in the rescan group (across all measurement points) was AP/LR respectively: -0.36/-0.36cm from SS to pCT, -0.24/-0.14cm from pCT to CBCT and -0.59/-0.50cm from SS to CBCT. The control group RD difference AP/LR was smaller: +0.15/+0.16cm from SS to pCT, -0.03/0.08cm from pCT to CBCT and +0.13/+0.07cm from SS to CBCT.

Rectum visualisation was clearest at the superior-ps position, change in RD for 6 and 20# schedules was compared as this point (Figure 2).

6# rescan group: Mean RD was larger than the control for all timepoints and >4cm in at least 1 direction for all bar #1. CBCT RD was larger than pCT for 4#’s AP and 5#’s LR and was closer to SS diameter for 2 #’s AP and 4#’s LR.

20# rescan group: Mean RD was larger than the control (both directions) for 15 timepoints (SS, pCT and CBCTs) and >4cm in at least 1 direction for 12. CBCT RD was more similar to pCT than SS for all bar 1# AP and 3#’s LR.

For patients breaching RD thresholds on SS, mean RD was reduced on rescan pCT, however reduction (including all measuring points) was <4mm in both directions.  A trend for these patients to sustain a larger rectum throughout treatment was seen, suggesting rescanning may be of limited use. Especially so for 6# patients where mean RD on CBCT was more akin to the rejected SS. This finding may be indicative of the 6# patient group, they are often older with more co-morbidities, both factors known to influence rectal filling.