ESTRO 2024 - Abstract Book

S2566

Clinical - Urology

ESTRO 2024

2762

Digital Poster

Is there an association between inter-fraction motion during prostate SABR and toxicity?

Orla A Houlihan 1,2 , Niamh Clarke 3 , Christina Agnew 3 , Owen McLaughlin 1,3 , Ciara Lyons 1,2 , Aidan Cole 1,2 , Joe O'Sullivan 1,2 , Kevin M Prise 1 , Alan R Hounsell 3 , Darren M Mitchell 2 , Conor McGarry 3 , Suneil Jain 1,2 1 Queen’s University Belfast, Patrick G. Johnston Centre for Cancer Research, Belfast, United Kingdom. 2 Northern Ireland Cancer Centre, Belfast City Hospital, Clinical Oncology, Belfast, United Kingdom. 3 Northern Ireland Cancer Centre, Belfast City Hospital, Radiotherapy Medical Physics, Belfast, United Kingdom

Purpose/Objective:

The rectum and bladder are mobile organs at risk (OARs). This study explored whether pre-fraction CBCTs provided a better indication of gastrointestinal (GI) and genitourinary (GU) toxicity rates than the planning CT for patients in a randomised trial of SABR ± elective nodal irradiation in high-risk prostate cancer (SPORT trial).

Material/Methods:

In the SPORT trial, 30 men were randomised 1:1 to SABR to prostate-only (P-SABR) or to prostate plus pelvic lymph nodes (PPN-SABR). P-SABR patients received 36.25Gy/5 fractions/29 days. PPN-SABR patients also received 25Gy/5 fractions to pelvic nodes with the final cohort receiving a boost to the dominant intraprostatic lesion of 45-50Gy. A CBCT was performed prior to each weekly fraction. Retrospectively the bladder and rectum were contoured on each pre-fraction CBCT. OAR dose/volume constraints (DVCs) were averaged across all five pre-fraction CBCTs and compared to their respective DVCs as determined by the planning CT. Acute GI and GU toxicities (CTCAE v4.03) were measured from commencement of treatment to 90 days post-completion of SABR. Late GI and GU toxicities (RTOG) were measured from 90 days to 36 months post-completion of SABR. DVCs for rectum were V18.1 Gy < 50%, V29 Gy < 20% and V36 Gy < 1 cc. DVCs for bladder were V18.1 Gy < 40% and V37 Gy < 10 cc. Correlation between dose/volume constraints and toxicities were examined using Spearman’s rank correlation coefficient (significant p < 0.05).

Results:

40.0% (12/30) of patients developed ≥ grade 1 acute and 36.7% (8/30) developed ≥ grade 1 late GI toxicity. 3.3% (1/30) developed ≥ grade 2 acute and 6.7% (2/30) developed ≥ grade 2 late GI toxicity. The average change in rectal volume between the averaged CBCT data versus the planning CT was + 2.6 cc (± 8.3 cc). The strength of correlations between the V18.1 Gy (%), V29 Gy (%) and V36 Gy (cc) for the averaged CBCT data with GI toxicity were stronger than the same correlations for the planning CT data, particularly V29 Gy (r = 0.3198, p = 0.0850 versus r = 0.2328, p = 0.2157 respectively for acute GI toxicity) and V36 Gy (r = 0.2332, p = 0.2148 versus r = 0.1494, p = 0.4307 for acute GI toxicity and r = 0.2434, p = 0.1949 versus r = 0.1681, p = 0.3744 for late GI toxicity respectively) (Fig. 1).

80% (24/30) developed ≥ grade 1 acute and 73.3% (22/30) developed ≥ grade 1 late GU toxicity. 13.3% (4/30) developed ≥ grade 2 acute and 20% (6/30) developed ≥ grade 2 late GU toxicity. Average change in bladder volume