S1464
Interdisciplinary - Other
ESTRO 2026
radiotherapy centres. Material/Methods: An electronic survey, based on the 2018 version and refined for clarity, was distributed to all 68 UK radiotherapy centres between November 2024 and January 2025. The survey covered MRI access, clinical sites, sequences, QA, staffing, safety, and future developments. Responses were consolidated per institution. Results were analysed descriptively, with comparisons made to 2018 data to evaluate national trends and guideline impact. Results: Fifty-nine responses were received, of which 54 were complete. This corresponds to a complete response rate of 79% (54/68): 88% (52/59) NHS and 22% (2/9) non-NHS centres. All reported MRI use for EBRT planning, via either direct scanner access (64%), PACS import (97%), or both. Only five centres had dedicated MRI scanners for radiotherapy. MRI use was most common for brain (83%), spine/bone SABR (73%), and prostate (68%) treatments. The proportion of UK radiotherapy patients receiving planning MRI approximately doubled since 2018, from 6% to 13%. Most centres used 1.5 T scanners, with increasing use of flat-top couches, coil bridges, and external lasers compared with 2018.Compliance with 2021 IPEM MRI guidance remained variable, sequence optimisation compliance was 61% for centres with direct MRI access versus 44% for centres with PACs only access to MRI. Routine QA was reported by 57% of centres, most commonly those with dedicated radiotherapy sessions. MRI-to-CT registration remained predominantly rigid (91%), with QA mainly qualitative. Principal barriers to wider adoption were MRI access (78%), capital investment, and lack of reimbursement. Despite advances, significant variation in imaging protocols, QA, and staff training remain. Financial and workforce limitations are the main constraints on further expansion. Conclusion: MRI use in UK EBRT planning has expanded significantly since 2018, with approximately double the patients scans (up to 13%), broader site coverage, improved infrastructure, and better guideline adherence. However, national provision remains uneven, and QA practices and staff training require further standardisation. Persistent financial and workforce constraints limit progress. Sustained investment and updated multidisciplinary guidance are essential to achieve equitable, safe, and efficient MRI integration across UK radiotherapy services. References: Speight, R., et al. (2019). IPEM Topical Report: A 2018 IPEM audit of MRI use for external beam radiotherapy treatment planning in the UK. Phys Med Biol, 64, 175021. https://doi.org/10.1088/1361- 6560/ab2c7c Speight, R., et al. (2021). IPEM
topical report: Guidance on the use of MRI for external beam radiotherapy treatment planning. Phys Med Biol, 66, 055025. https://doi.org/10.1088/1361-
6560/abdc30 Keywords: MRI, audit
Digital Poster 4904 Inter-observer variations in prostate cancer radiotherapy delineation Gracinda Johansson 1,2 , Emil Fredén 1 , Pelin Sen 1 , Ali Alkhiat 3 , Tim Melhus 4 , Carlos Carrascal Gordillo 5 , Victoria Agudelo Guerra 6 1 Medical Physics, Development and technology, Södersjukhuset, Stockholm, Sweden. 2 Department of Clinical Sciences and Education, Karolinska Institutet, Stockholm, Sweden. 3 Medical Diagnostics Center, Karolinska University Hospital, Stockholm, Sweden. 4 Medical diagnostics Center, Karolinska University Hospital, Stockholm, Sweden. 5 Oncology, Karolinska University Hospital, Stockholm, Sweden. 6 Department of Oncology, Södersjukhuset, Stockholm, Sweden Purpose/Objective: This study aims to evaluate inter-observer variations among radiation oncologists (ROs) in the delineation of target volumes and organs of interest (OI) for prostate cancer (pCa) radiotherapy, with the objective of enhancing consistency and harmonizing contouring practices between two radiotherapy centers in
Stockholm, Sweden. Material/Methods:
Two pCa patients previously treated with radiotherapy were included (Patient-1 from Södersjukhuset (SÖS), Patient-2 from Karolinska University Hospital (KS)). Patient-1 received 72.5 Gy to the prostate (CTVT1) and seminal vesicles (CTVT2), and 52.2 Gy to the pelvic lymph nodes (CTVN), (29 fractions). Patient-2 received 42.7 Gy to the prostate and the base of the seminal vesicles (7 fractions). Reference structures were jointly delineated by a ROs and a radiologist to include CTVT1, CTVT2, CTVN, rectum, bladder and bowel bag (patient-1) and CTVT, rectum and bladder (patient-2). Twelve ROs independently delineated these structures, according to their institutional guidelines. All structures were delineated on co-registered MR images, expect bowel bag for all ROs and CTVN for the SÖS ROs. The agreement between the reference and the dummy-run structures was quantified using the Dice similarity coefficient (DSC). The impact of delineation variability on dose-volume metrics for the target and OIs was assessed on the clinical plan for patient-1, selected due to its relative anatomical and planning complexity. Results: For patient-1, median DSC values of 82%, 78% and
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