S1637
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Department of Medical Physics, Maria Sklodowska- Curie National Research Institute of Oncology, Warsaw, Poland Purpose/Objective: 3D printing makes individualized boluses a reality in radiotherapy. Their dosimetric properties depend on multiple factors including printing material, pattern, thickness and infill density. These variables make it challenging to select an optimal combination of parameters that ensures the desired surface dose modification. The objective of this study was to identify a single unifying parameter that could integrate these independent variables into one framework, thereby simplifying the process of 3D- printed bolus design and selection. Material/Methods: Four materials (PLA, ABS, TPU93A, and TPU40D) were 3D-printed into 10x10 cm² samples with nominal physical thicknesses of 5 and 10 mm, and varying infill densities. PDD curves were measured for each sample using Markus TM23343 ionization chamber in water- equivalent phantom, with Gerbi's correction applied. Measurements were performed for 6 MV photon beams: flattened (FF) and flattening filter-free (FFF), using 5x5 cm2 field. Radiological thickness (drad) of each sample was determined from CT within TPS (Eclipse v.16) and compared with dose measured beneath the bolus (D0).D0 was extracted from the measured PDD curves. Linear regression analyses were performed between dradand D0. Coefficient of determination (R²) was used to evaluate the strength of this relationship, with the aim of assessing radiological thickness as a parameter characterizing the dosimetric properties of the 3D-printed bolus. Results: A strong positive relationship was observed between drad and D0 for both 6 MV FF (R2 = 0.860) and 6 MV FFF beams (R2 = 0.844) (Figure 1 & 2). These results demonstrate that drad serves as a reliable predictor of D0 modification, outperforming physical thickness or material type alone. Samples with identical physical thickness but different materials (e.g., PLA vs. ABS) or infill densities exhibited different drad values, which directly corresponded to differences in D0
For 6 MV FFF beam, a clinically desired surface dose of 95% was achieved by PLA with 60% infill (drad=8 mm), ABS with 60% infill (drad=7 mm), and both TPU93A and TPU40D with 10 mm thickness and 60% infill (drad=8 mm). For the 6 MV FF beam, only TPU93A and TPU40D samples with 60% infill reached the 95% dose level. Notably, FFF beams achieved higher surface doses at lower drad values compared to FF beams, reflecting their steeper build-up characteristics. Conclusion: Radiological thickness was validated as a robust and practical predictor for dosimetric performance for 3D- printed boluses. Its use enables straightforward optimization of bolus design and predictable achievement of clinically desired D0 levels. Keywords: 3d printing, bolus, radiological thickness Mini-Oral 1424 From bench to bedsite: real-time and time- resolved in-vivo dose measurements in the “Skin- Flash I” trial Riccardo Dal Bello 1 , Serena Psoroulas 1 , Dominik Flückiger 1 , Jerome Krayenbühl 1 , Arvid Kemper 2 , Rafael Kranzer 2 , Benjamin Côté 3 , Jens von der Grün 1 , Panagiotis Balermpas 1 , Matthias Guckenberger 1 , Stephanie Tanadini-Lang 1 1 Department of Radiation Oncology, University Hospital Zurich and University of Zurich, Zurich,
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