ESTRO 2023 - Abstract Book

S400

Sunday 14 May 2023

ESTRO 2023

S. Alexander 1 , A.C. Tree 2 , R. Westley 3 , U. Oelfke 4 , H.A. McNair 5 1 The Royal Marsden NHS Foundation Trust / The Institute of Cancer Research, Radiotherapy , Sutton, United Kingdom; 2 The Royal Marsden NHS Foundation Trust / The Institute of Cancer Research, Uro-Oncology, Sutton, United Kingdom; 3 The Royal Marsden NHS Foundation Trust/ The Institute of Cancer Research, Uro-Oncology, Sutton, United Kingdom; 4 The Royal Marsden NHS Foundation Trust/ The Institute of Cancer Research , Joint Department of Physics, Sutton, United Kingdom; 5 The Royal Marsden NHS Foundation Trust/ The Institute of Cancer Research, Radiotherapy, Sutton, United Kingdom Purpose or Objective Biochemical failure rates following radical radiotherapy to localised prostate cancer (PCa) are reported as higher in patients with larger rectal volumes on pre-radiotherapy scans[1-3]. The literature posits that distended rectums promote excessive prostate motion (PM) during radiotherapy, causing target underdosing[1-3]. Data is largely from patients treated with 2D bone based IGRT, with motion not quantified. Volumetric IGRT, aligning to prostate is now commonplace and provides opportunity to test the theory; Do large rectal volumes on pre-radiotherapy scans predict greater PM during treatment? Materials and Methods Rectal volume, inter- and intrafraction motion were retrospectively calculated for 40 patients who received PCa radiotherapy: 20 prescribed 36.25Gy in 5#’s on Unity MR-linac (MRL) (Elekta, Sweden) and 20 prescribed 60Gy in 20#’s on TrueBeam c-arm linac (CLIN) (Varian, USA). The rectum was contoured on T2-weighted diagnostic MR (dMRI) and planning CT (pCT) images (RayStation, RaySearch, Sweden). Rectal preparation was given prior to pCT using micro enemas, but not for dMRI. Interfraction motion data, registering to prostate, was retrieved. MRL Intrafraction PM was taken as the difference in prostate position from pre- to post-treatment T2-weighted MRI. CLIN data was the deviation in prostate position on post treatment CBCT from planned, after pre-treatment motion correction. Data was analysed using R-Studio (RStudio, USA). Total patient specific, inter- and intrafraction median (range) motion was calculated in each plane. Correlation between dMRI and pCT rectal volume and motion was examined using Spearman’s roe (significant p<0.01). Patients were then grouped by dMRI or pCT rectal volume ≤ or > median volume and < or ≥ 90cm3 (literature volume cut-off[1]). Difference in inter- and intrafraction motion was compared between groups using a Mann Whitney U Test (significant p<0.01). Results All patients had a pCT, 39/40 had a dMRI, the patient without dMRI was removed from analysis. Volume and motion data was not normally distributed. Inter- and Intrafraction motion was calculated for 474 and 430 fractions respectively. Median (range) rectal volume on dMRI and pCT was 74.47cm3 (31-246) and 63.99cm3 (33-162) respectively, not significantly different (p=0.1311). A weak positive correlation between dMRI and pCT rectal volume was found, 0.4558 (p<0.01). No correlation between rectal volume on dMRI or pCT and inter- or intrafraction motion presented. Median rectal volume on dMRI and pCT combined was 66cm3, 26/39 patients had a rectal volume >66cm3 (9 dMRI, 4 pCT, 13 both), 13/39 had a rectal volume ≥ 90cm3 (10 dMRI, 2 pCT, 1 both). No significant difference in motion between patients with rectal volume ≤ or > median volume presented. Intrafraction superior-inferior median motion was significantly greater in the <90 cm3 volume group (Table 1).

Conclusion For this patient cohort, larger rectal volumes on pre-radiotherapy imaging did not result in greater target motion during radiotherapy.