ESTRO 2024 - Abstract Book

S5978

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

James Iddenden 1,2 , Debra Howard 1 , Eleanor Hudson 3 , Mark Teo 4 , Patricia Diez 1 , Elizabeth Miles 1 , Louise Turtle 2 , Rushil Patel 1 , Ane Appelt 5,6 , Simon Gollins 7 1 The Radiotherapy Trials Quality Assurance (RTTQA) Group, Mount Vernon Cancer Centre, London, United Kingdom. 2 The Clatterbridge Cancer Centre NHS Foundation Trust, Radiation Oncology, Liverpool, United Kingdom. 3 Leeds Institute of Medical Research, Clinical Trials Research Unit, Leeds, United Kingdom. 4 St James's University Hospital, Leeds Cancer Centre, Leeds, United Kingdom. 5 Leeds Institute of Medical Research at St James's, University of Leeds, Leeds, United Kingdom. 6 Department of Medical Physics, Leeds Teaching Hoispitals NHS Trust, Leeds, United Kingdom. 7 The Shrewsbury and Telford Hospital NHS Trust, Lingen Davies Centre, The Royal Shrewsbury Hospital, Shrewsbury, United Kingdom APHRODITE (ISRCTN16158514), funded by Yorkshire Cancer Research, is a phase II randomised controlled trial comparing radical (chemo)radiotherapy (CRT) alone versus dose-escalated CRT with a simultaneous integrated boost (SIB). Patients with early stage rectal cancer, who are considered by their multidisciplinary team as unsuitable for radical total mesorectum excision or have a strong preference for organ preservation, will all receive 50.4Gy in 28 fractions to a small mesorectal-only planning target volume (PTV). Those in the experimental dose-escalation arm also receive up to 62Gy SIB to the primary tumour volume (PTVp). The trial is currently in active recruitment with a target sample size of 104 patients. Few studies exist detailing dose-volume constraints applied in this setting and none which examine the frequency to which they are achieved¹. Anonymised trial plans were retrospectively reviewed to determine if the optimal organ at risk (OAR) dose-volume constraints as set out in the trial protocol are achievable. The conformity of the target volumes coverage was also assessed. Purpose/Objective:

Material/Methods:

All centres completed the pre-trial radiotherapy quality assurance programme prior to recruiting. Radiotherapy planning data was requested for all patients. To date, 8 centres have recruited patients, with plan data for 46 out of 73 trial patients available at the time of analysis.

Radiotherapy was planned according to their randomisation following APHRODITE dose-volume constraints.

All plans were retrospectively reviewed on Velocity version 4.1 (Varian Medical Systems, Inc.) and dose-volume constraints extracted from DVH data. Conformity indices, as defined by RTOG (95% isodose volume/volume of PTV), were calculated for all PTVs. The standard deviation was calculated for optimal OAR dose-volume constraints and target volume conformity. Mann-Whitney U tests (two-tailed) were performed to test differences between the standard and dose-escalated arms.

Results: