S1773
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
dose calculation algorithms. Australas Phys Eng Sci Med. 2019 Sep 15;42(3):853–62. 2. Shirey RJ, Wu HT. Quantifying the effect of air gap, depth, and range shifter thickness on TPS dosimetric accuracy in superficial PBS proton therapy. J Appl Clin Med Phys. 2018 Jan 1;19(1):164–73. Keywords: Proton therapy, range shifter, air gap
compared CRS and MRS plans with a prescribed dose of 50 Gy through line-dose profiles and dose–volume histograms (DVHs). Results: The spot size in air increased proportionally with the air gap. In the phantom, the spot size also increased with depth, and higher energies produced a larger lateral penumbra due to cumulative MCS effects. Similar trends were observed in the TPS, FLUKA MC, and experimental measurements, confirming the consistency of results across all methods. The spot size characteristics in air and in the phantom for different air gaps are shown in Figure 1. Material investigation identified acrylic, with a density of 1.16 g/cm ³ and an RLSP of 1.147, as the most suitable material for the contacted range shifter. This value closely matched the RLSP of the MRS (1.145). In the clinical simulation using the CIRS head phantom, the line-dose profiles showed that the lateral penumbra was narrower with the CRS than with the MRS, particularly for superficial targets. The dose–volume histogram (DVH) comparison, shown in Table 1, indicated reduced doses to organs of interest with the CRS, confirming its dosimetric advantage and clinical feasibility.
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In silico validation of a universal bolus-based proton Bragg Peak FLASH setup for mammary tumours in small animal breast cancer models Rutger J.C. de Koster 1,2 , Jeremy P. Gunawan 1,3 , Lara Barazzuol 1,3 , Marc-Jan van Goethem 1,2 , Rob P. Coppes 1,3 , Alexander Gerbershagen 1,2 , Stefan Both 1,2 1 Particle Therapy Research Center (PARTREC), University Medical Center Groningen, Groningen, Netherlands. 2 Radiation Oncology, University Medical Center Groningen, Groningen, Netherlands. 3 Biomedical Sciences, University Medical Center Groningen, Groningen, Netherlands Purpose/Objective: This study aimed to validate a novel small animal irradiation setup for single spot Pencil Beam Scanning (PBS) Ultra-High Dose-Rate (UHDR) proton Bragg Peak FLASH radiotherapy using a real mouse µCT/PET dataset. The proposed configuration is designed to replicate clinically relevant single fraction breast cancer treatment geometries and enable high- throughput preclinical investigations into FLASH- induced normal tissue sparing effects in the skin and other organs-of-interest, as it eliminates the need of ridge filters and similar complex beam modifiers. Material/Methods: A computational simulation framework was developed using Python and the TOPAS Monte Carlo dose engine to evaluate a beamline setup comprising a copper degrader, lead collimator, and a 3D-printed universal bolus. µCT/PET images from 10 C57BL/6 mice bearing orthotopic mammary tumours (144–262 mm ³ ) were utilised. PET-defined tumour contours were refined to correct for resolution artefacts and skin structures were delineated. Individual boluses were generated for each subject, and a universal bolus was created by averaging these ten personalised boluses. Dose distributions were normalised to meet clinical goals derived from the ABLATIVE protocol: D99% ≥ 19.0 Gy and heterogeneity index (HI) ≤ 1.30. FLASH dose-rate metrics (DRmin,skin > 40 Gy/s) were assessed within a maximum beam current of 500nA. Robustness was evaluated using 28 perturbed scenarios per mouse, simulating ±3% proton range uncertainties and 0.35 mm setup uncertainties.
Conclusion: This study confirmed the theoretical relationship between air gap, proton energy, and spot size, showing consistent results across simulations, calculations, and measurements. The findings support further development of a helmet-type contacted range shifter to minimize air-gap effects, improve dose conformity, and reduce dose to organs of interest in head-and-neck proton therapy. References: 1. Rana S, Samuel EJJ. Measurements of in-air spot size of pencil proton beam for various air gaps in conjunction with a range shifter on a ProteusPLUS PBS dedicated machine and comparison to the proton
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