ESTRO 2025 - Abstract Book

S3490

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTRO 2025

Conclusion: Information about high LET dose deposited by individual beams is diluted when evaluating composite plan LETd distributions. A field-by-field assessment of LETd is a more informative approach to understanding the contribution of LETd to normal tissue effects.

Keywords: Proton treatment, LETd

References: 1. McIntyre M, Wilson P, Gorayski P, Bezak E. A Systematic Review of LET-Guided Treatment Plan Optimisation in Proton Therapy: Identifying the Current State and Future Needs. Cancers. 2023;15(17). 4268. 2. Souris K, Lee JA, Sterpin E. Fast multipurpose Monte Carlo simulation for proton therapy using multi- and many core CPU architectures. Med Phys. 2016;43(4):1700-12.

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Mini-Oral Dose deposited by high LET particles contributes to image changes in pediatric craniopharyngioma Thomas E Merchant 1 , Vadim P Moskvin 1 , Fakhriddin Pirlepesov 1 , Chris J Beltran 2 1 Radiation Oncology, 1. St. Jude Children's Research Hospital, Memphis, USA. 2 Radiation Oncology, 2. Mayo Clinic, Jacksonville, USA Purpose/Objective: Post-irradiation MR imaging abnormalities are precursors to irreversible adverse complications. The contribution of dose and linear energy transfer (LET) to imaging abnormalities following proton radiotherapy is poorly understood. Conventional analyses of dose and LET effects typically use the total proton therapy plan, thereby overlooking the LET distribution of individual fields. We hypothesized that a field-by-field analysis of dose and LET would more accurately correlate with imaging changes observed in children and adolescents with craniopharyngioma treated with pencil beam scanning proton radiotherapy. Material/Methods: We evaluated 102 pediatric patients treated with two-field proton radiotherapy plans for post-treatment imaging abnormalities beginning one year after treatment. Imaging abnormalities were contoured using BraTumIA 2.0.0.5 software [1] and compared to baseline. Dose and dose-weighted LET (LETd) were calculated using the MCsquare Monte Carlo code [2]. Field-specific high LETd areas were defined as ROILET1 (immediate distal edge of SOBP), ROILET2 (low dose end of the distal edge), and ROILET3 (lateral field edges). Mean dose, LETd, and distance to high LET volumes from the centers of imaging abnormalities were analyzed for each field. Results: A single treatment plan was used for 51 patients and 51 patients had multiple plans. Amongst the single plan cohort, the first field was delivered from the left in 45 patients. Post-treatment imaging changes greater than 1 cm³ were observed in areas of high LETd in 52.9% of cases. Imaging changes were also noted in areas of high dose deposited by low LET particles. Progressive imaging changes, relative to baseline, were observed both within and outside the irradiated volume. The largest volumetric imaging changes, approximately 12 cm³, and the shortest distances from the centers of imaging abnormalities (mean: 5 mm) were observed for ROILET3 (Figure 1). The mean distances for ROILET1 and ROILET2 were 15 mm and 30 mm, respectively. The first treated beam deposited several Gy from high LET particles into the imaging changes for ROILET1, ROILET2, and ROILET3, followed by a high dose of low LET particles (Figure 2).