ESTRO 2025 - Abstract Book

S3535

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

ESTRO 2025

3457

Mini-Oral Reducing stopping-power ratio uncertainty for pediatric proton therapy through dedicated patient-size conversion curves Kyriakos Fotiou, Lasse Bassermann, Maria F Jensen, Stine E Petersen, Pia Randers, Cecilia B Trinh, Yasmin A Lassen, Ludvig P Murren, Vicki T Taasti Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark Purpose/Objective: The impact of beam hardening on the stopping-power ratio (SPR) uncertainty is typically estimated from CT number variations of tissue-equivalent inserts scanned within adult-sized body and head phantoms. This uncertainty is then applied universally across all patients, regardless of size, which could pose concerns for pediatric patients. The large size difference used in the head-body method potentially overestimates uncertainty, especially for younger pediatric patients, with less pronounced beam hardening, and for intermediate-sized patients who fall between adult head and body sizes. This study aimed to develop conversion curves for pediatric size ranges, enabling more accurate estimation and reduction of beam hardening induced SPR uncertainty. Material/Methods: The study utilized a custom, modular ring phantom (Sun Nuclear) with diameters of 10, 20, 30, and 40 cm. Tissue equivalent inserts were CT scanned at both central and peripheral positions within each phantom size, using a twin beam dual-energy CT scanner (Somatom Definition Edge, Siemens Healthineers). Virtual monoenergetic images were generated across energy levels from 40 to 190 keV. To address inter-patient size variations, three conversion curves were created for patients within size ranges of 10-20 cm, 20-30 cm, and 30-40 cm. The average insert CT numbers were calculated from two adjacent phantom sizes and used to construct conversion curves that reflect patient sizes within the span defined by the two phantoms. Uncertainties due to phantom size and insert position were independently estimated for three tissue groups (lung, soft tissue, and bone) [1]. Composite SPR uncertainty was evaluated by applying tissue-specific weighting factors [2]. For the 10-20 cm size range, brain patients were evaluated, while for larger ranges, pelvic patients were used. Comparative analysis of beam hardening induced SPR uncertainty was conducted between the head-body approach and the new, size-range conversion curves. Results: Size-range specific conversion curves notably reduced SPR uncertainty due to beam hardening (Figure 1). For brain patients, the head-body approach yielded a minimum uncertainty of 1.0%, which dropped to 0.1% with the conversion for the 10-20 cm range. For pelvic patients, the head-body approach produced a minimum uncertainty of 1.0%, while size-range curves reduced this to 0.3% for the 20-30 cm range and 0.5% for the 30-40 cm range.