ESTRO 2025 - Abstract Book

S4393

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Keywords: Internal pudendal arteries, prostate SBR, MR-Linac

3900

Digital Poster Analysis of Feasibility and Robustness of Proton Beam Therapy for Treating Vulva Cancer using Cone beam computed tomography: A planning study Vossco Nguyen 1 , Molly Munro 1 , Deepali Purohit 1 , Asma Sarwar 2 , Wiwatchai Sittiwong 2 1 Radiotherapy Physics, UCLH, London, United Kingdom. 2 Clinical Oncology, UCLH, London, United Kingdom Purpose/Objective: Vulvar cancer is rare, accounting for <1% of all cancers. Surgical excision is the mainstay of treatment. Adjuvant radiotherapy is indicated in patients with risk factors for recurrence. Properties of proton beam therapy (PBT) may allow a reduction in treated volumes and hence toxicities. However, the robustness of PBT plans in this context is unknown. In this study we assessed the feasibility and robustness of PBT plans using daily Cone Beam computed tomography (CBCTs) from treatment in a retrospective analysis of five patients who had vulval and nodal disease irradiated Material/Methods: Five patients treated with adjuvant VMAT radiotherapy to unilateral or bilateral groins and vulva were selected for retrospective planning with PBT. Treatment doses ranged from 45-66Gy in 25-30 fractions. All had minimum weekly CBCT data. Three were planned with single-field optimization (SFO) and two with multiple-field optimization (MFO). All CBCTs were assessed and matches showing significant visual deviation (external separation gains/losses, bladder, rectal and bowel filling) from the planning scan were selected for recalculation. Deformable registrations (DIR) on CBCTs with considerable (separation changes beyond robustness margins and/or gross changes in filling of rectum, bladder or bowel) separation changes, bony rotation and internal target motion were conducted using Velocity V4.0. Clinical input was sourced for plan evaluations and recalculations to assess the target coverage, OAR tolerances and accuracy of deformation. Treatment planning was conducted using Eclipse V16 for both photon and proton modalities. Results: A total of 45 CBCTs were reviewed, only 4 images requiring recalculations. Of the four, three were the SFO plans, and one was the MFO plan. Vulvar swelling (beyond 7mm) was the most prominent trigger for recalculation in all scans. All recalculations were reviewed with a Clinical Oncologist and were established as clinically acceptable. No replans would have been triggered based on the image reviews for any case, in a clinical PBT scenario.

Recalculations showed, CTV coverage was more likely to be cooler than planned whilst OARS such bladder and bowel tended to increase in dose.

Conclusion: