S1665
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
beams, there are very few experimental studies dedicated to comparing out-of-field doses for different primary ions [1]. This is of particular relevance for pediatric patients, since lower out-of-field doses decrease the secondary cancer risk. This work aimed at assessing the out-of-field doses during proton, helium and carbon ion beam therapy for a pediatric brain tumor. Material/Methods: The measurements were conducted at the Heidelberg Ion Beam Therapy Center (HIT) using a phantom representing a 10-year-old child. The planning target volume in the head was defined as a 6 cm diameter sphere, similar to previous studies [2]. The treatment plans were computed to optimize the radiobiological effectiveness (RBE) with a fraction target dose of 4 Gy(RBE). Different types of dosimeters, including thermoluminescent detectors (TLDs), radiophotoluminescent detectors (RPLs), poly-allyl- diglycol-carbonate (PADC) nuclear track detectors and Fluorescent Nuclear Track Detectors (FNTDs), were used to measure the out-of-field doses inside several organs of the phantom, including the thyroid, lungs, and breasts, prostate and liver. Monte Carlo (MC) simulations were used to complement the experimental measurement. Results: The measurements yielded maximum out-of-field doses in the thyroid. For primary helium ions, the non- neutron dose was around 0.2 mGy/4Gy(RBE). For primary protons, the neutron dose was around 0.4 mSv/4Gy(RBE). The findings of the MC simulations followed the trends of the experimental values, with systematic overestimation. For the thyroid, the MC out-of-field dose was less than 1.5 mSv/4Gy(RBE) for primary protons. These discrepancies are probably due to the detection limits of each dosimeter type and the limitations of the MC for describing out-of-field doses. Despite this, considering the worst-case scenario for a 15-fraction treatment, the out-of-field dose in the thyroid will not exceed 23 mSv. Both experiments and simulations suggest that primary carbon ions lead to the lowest out-of-field organ doses. Nevertheless, further implications of different RBE models remain to be studied. Conclusion: The out-of-field doses of the same target treated with three different primary ions, namely protons, helium and carbon ions, were quantified in a phantom representing a pediatric patient. Based on this information, the treatment leading to the lowest out- of-field dose can be selected. References: [1] Vedelago J, Schmidt S, Stengl C, Karger CP, Jäkel C. Secondary neutrons in proton and light ion beam therapy: a review of current status, needs and potential solutions. Radiat Meas 2024;176:107214.
https://doi.org/10.1016/j.radmeas.2024.107214[2] Knežević Ž, Stolarczyk L, Ambrožová I, Caballero- Pacheco MA, Davídková M, De Saint-Hubert M, et al. Out-of-Field Doses Produced by a Proton Scanning Beam Inside Pediatric Anthropomorphic Phantoms and Their Comparison With Different Photon Modalities. Front Oncol 2022;12:904563. https://doi.org/10.3389/fonc.2022.904563 Keywords: proton and ion beam therapy, out-of-field doses Verification of Out-of-Field Radiotherapy Dose Estimation at Cardiac Device Depths Using Micro- Silica Bead Thermoluminescent Dosimeters Shakardokht M Jafari 1 , Komal Soni 2 , Sarah Muscat 1 , Antony L Palmer 1 1 Radiotherapy Physics, Portsmouth Hospitals University NHS Trust, Portsmouth, United Kingdom. 2 Medical Physics, University of Surrey, Guildford, United Kingdom Digital Poster 3292
Purpose/Objective: To evaluate the use of micro-silica bead
thermoluminescent dosimeters (TLDs) for quantifying out-of-field radiotherapy doses at depths relevant to cardiac implantable electronic devices (CIEDs). To compare results to ion chamber measurements and treatment planning system (TPS) values for a range of treatment plans. Potential for use in estimating CIED dose levels via phantom measurement and assessing risk to CIED [[i]]. Material/Methods: Trueinvivo silica bead TLDs were evaluated in two phases. Phase 1 validated beads against a calibrated ion chamber (IC) and TPS under reference conditions. Phase 2 extended assessment to out-of-field (OOF) geometries at 2 cm depth in a solid-water phantom, representative of typical CIED placement. VMAT (6 MV, 2 Gy × 30 fractions) and SABR (10 MV FFF, 10 Gy × 3 fractions) plans were delivered. Silica bead arrays, CC04 IC, and TPS calculations were compared (Fig. 1).
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