S1750
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
References: [1] Nils Peters N et al. Consensus guide on CT-based prediction of stopping-power ratio using a Hounsfield look-up table for proton therapy. Radiother Oncol. 2023 Apr 19;184:109675. doi: 10.1016/j.radonc.2023.109675 Keywords: Photon Counting CT, Proton Therapy, Stopping Power Digital Poster 3055 Dosimetric comparison of Bone Marrow and Integral Dose between PBT and VMAT in extended- field Radiotherapy for locally advanced Cervical Cancer Molly Munro 1 , Vossco Nguyen Nguyen 1 , Deepali Purohit 1 , Courtney Reynolds 1 , Asma Sarwar 2 , Wiwitchai Sittiwong 3 1 Radiotherapy Physics, University College London Hospital, London, United Kingdom. 2 Radiotherapy Consultant, University College London Hospital, London, United Kingdom. 3 Radiotherapy Fellow, University College London Hospital, London, United Kingdom Purpose/Objective: Cervical cancer is the fourth most common cancer in women globally, with approximately 3,300 new cases diagnosed each year in the UK [1]. Extended-field external beam radiotherapy (EF-EBRT) is indicated for patients with extensive pelvic, common iliac, or para- aortic nodal involvement. Despite excellent target conformity, VMAT exposes large volumes of bone marrow (BM) to radiation, increasing the risk of haematologic toxicity and subsequent treatment delays [2]. Proton beam therapy (PBT) may reduce BM dose. The primary objective of this study was to compare BM dose exposure from VMAT and PBT in EF- EBRT for cervical cancer. Material/Methods: Ten patients previously treated with EF-EBRT for cervical cancer using VMAT were retrospectively replanned for PBT. All patients received a prescribed dose of 45 Gy in 25 fractions to CTV1 and CTV2, with a simultaneous integrated boost of 55 Gy to involved nodes.Using Varian Eclipse v16.1.0, VMAT plans were generated according to departmental guidelines with two full arcs. PBT plans were created using a four-field multi-field optimisation (MFO) technique comprising two posterior oblique and two lateral posterior oblique beams, selected to minimise dose to OARs, particularly bowel, and reduce setup uncertainty.The exterior contours of bony structures were retrospectively delineated as a surrogate for BM. For analysis, BM was divided into pelvic BM (inferior to L5) and spinal BM (superior to L5). No dose constraints
were applied to BM, and it was not used as an optimisation structure. Dosimetric evaluation compared BM dose between VMAT and PBT plans using the parameters V10Gy<90%, V20Gy ≤ 75%, V40Gy<37%, and mean total BM dose <30.3Gy. Results: Both PBT and VMAT plans achieved comparable target D98% coverage and met OAR dose objectives. Pelvic BM doses were consistently lower with PBT, showing statistical significance for V20Gy (p = 1.66 × 10 ⁻ ⁶ ) and V10Gy (p = 3.06 × 10 ⁻ ⁵ ). Spinal BM doses were slightly higher with PBT due to the posterior beam arrangement (V20Gy p = 0.77; V10Gy p = 0.79) (Fig 2). The mean total BM dose was <30.3Gy=28.34Gy for VMAT, constraint met in 80% of cases and <30.3Gy=26.60Gy for PBT, met the constraint in 90% of cases. The mean integral dose was 48.9% lower in PBT plans (Fig 1).
Conclusion: PBT for EF-EBRT in cervical cancer has the potential to reduce BM dose. PBT substantially reduced low-dose
Made with FlippingBook - Share PDF online