ESTRO 2023 - Abstract Book

S247

Saturday 13 May

ESTRO 2023

Kingdom; 3 The Royal Marsden NHS Foundation Trust, Radiotherapy, Sutton, United Kingdom; 4 The Royal Marsden NHS Foundation Trust/ The Institute of Cancer Research, Uro-Oncology, Sutton, United Kingdom Purpose or Objective 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? Materials and Methods 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. Results 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.