UK RADIOTHERAPY PRACTICE FOR OLIGODENDROGLIOMA –DATA FROM THE APPROACH TRIAL (ISRCTN:1339049) RADIOTHERAPY QUALITY ASSURANCE (RTQA) PROGRAMME

Abstract AIMS A comprehensive RTQA programme for the APPROACH trial (Analysis of Proton vs Photon Radiotherapy in Oligo- dendroglioma and Assessments of Cognitive Health) includes a pre-accrual facility questionnaire. Information on equipment, software, planning and outlining techniques was captured to assess variations in practice for UK centres enrolled in the trial. METHODS Between December 2022 to November 2024 the national Radiotherapy Trials Quality Assurance (RTTQA) Group circulated a questionnaire to 21 photon and 2 proton centres. Data from 21 photon centres is presented here. RESULTS Each centre routinely uses thermoplastic shells for immobilisation and acquires CT planning scans with 1-3mm slice thickness. Some centres co-register the post-operative MRI, whilst others acquire dedicated planning MRI scans. MRI slice thickness ranges from 0.8-3mm. T1+ contrast and T2/FLAIR MRI sequences are co-registered. All 21 centres routinely use CTV margins of 10-15mm, PTV margins of 3-5mm and radiotherapy doses consistent with the APPROACH trial protocol. All centres use Volumetric Arc Therapy (VMAT). All centres routinely contour brainstem, chiasm, optic nerve and eyes, with 95% (20/21) also contouring lens and spinal cord. Cochleas were contoured by 81% (17/21). There is variability between centres in the frequency of lacrimal gland, pituitary, optic tracts, brain, retina, cornea and parotid contouring. Hippocampi are routinely contoured by 19% (4/21). No centres routinely contour the hypothalamus. Auto-contouring for organs-at-risk (OAR) is used by 62% of centres (13/21) and in 52% of centres OARs may be contoured by a multi-professional team member. CONCLUSION The facility questionnaire data highlights a consistent approach to immobilisation, volume expansion, dose and fractionation using VMAT delivery with variation between centres in pre-treatment imaging processes and OAR contouring. The APPROACH trial aims to improve consistency in practice for oligodendroglioma in the UK, to standardise the use of a dedicated planning MRI and thin slice image acquisition, and introduce hippocampal and hypothalamus contouring.