S1669
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Purpose/Objective: Modern radiotherapy techniques require quality assurance of small field deliveries. Gafchromic film is a valuable dosimeter for these procedures due to its high spatial resolution. Quality assurance requires independent traceability of film from the treatment planning system (TPS) and an established film dosimetric uncertainty. Marroquin et al. (2016) quantified dosimetric uncertainty in a 0 - 120 Gy dose range for an EBT3 radiochromic film dosimetry system. Scanner response uniformity was identified as contributing to total uncertainty. Our work presents a strategy for quantifying scanner dosimetric uncertainty independently from film uncertainty using transmission filters with uniform optical density. Interscan and intrascan standard deviation in pixel value (PV) has been quantified, converted to apparent dose using a nominal calibration and propagated to dose uncertainty. Material/Methods: Optical transmission through uniform density filters was assigned an apparent dose using a nominal third order polynomial EBT4 calibration function of net optical density (netOD) to dose. Where the netOD function is the log of the ratio of unirradiated to irradiated film pixel value. Using these function’s derivatives, standard deviation in pixel value was propagated to an uncertainty as a percentage of dose.Interscan variabilityFour uniform neutral density filters (ND2, ND4, ND8, ND16) (Neewer, USA), Figure 1a, were consistently positioned and scanned (n = 88) using an Epson Expression 12000XL flat-bed scanner (Epson, Suwa, Japan). Mean pixel value for each RGB channel was measured in a central ROI for each filter (PVROI), Figure 1b. Standard deviation in RGB PVROI was calculated and propagated to dose uncertainty (%). Intrascan variabilityA ND6 filter (Rosco, USA) was cut into three 33 x 44 cm pieces, Figure 2a, and scanned (n ≥ 10) in single, double and triple layers. Average images were calculated for each RGB channel and layer combination, Figure 2b. Standard deviation in image pixel value was calculated and propagated to dose uncertainty (%). Results: Dose uncertainties resulting from interscan variability and intrascan variability are presented in Figures 1c and 2c.
calibrated in air against an NRC-traceable FC65G ion- chamber. To simulate mouse irradiation conditions, three solid water phantoms (2×2×8cm³) were fabricated to accommodate the film, microMOSFET, and microDiamond detectors during exposure. Each phantom consisted of two layers stacked together, with the detectors placed at the center of the phantoms, 1cm below the surface. Treatment plans were generated from CBCT scans of each individual phantom using the SmartART planning system. Measurements were performed using 5mm and 8mm circular fields, as well as a 40×40mm² square field, both in air and in a solid water phantom. The mobileMOSFET dosimetry system was used for microMOSFET measurements, while an Epson flatbed scanner was employed for film readout. The scanned films were analyzed using RIT-software. Results: The dose measured in air showed good agreement among the film, microMOSFET, and microDiamond detectors. A difference of 2.0% was observed between the film and microDiamond detector, and 2.37% between the film and microMOSFET. For in-phantom measurements, agreements of 2.4±0.32% and 2.85±2.45% were observed between the film and microDiamond, and between the film and microMOSFET, respectively. The agreement between the SmartART treatment plan and film measurements was 5.15±3.07%. Comparatively larger deviations were observed for the microDiamond (11.25±2.04%) and microMOSFET (9.79±2.51%). All detectors under- responded compared to the SmartART treatment plan. Conclusion: Gafchromic film is commonly recognized as an optimal detector for small-field dosimetry due to its high spatial resolution and near–tissue-equivalent response. Our results indicate that the microMOSFET detector is also suitable for small-field measurements because of its real-time readout capability, small sensitive volume, and ease of use. Similarly, the microDiamond detector offers real-time readout and near–tissue-equivalent characteristics, making it a reliable choice for small-field dosimetry applications. Keywords: Small-field, dosimetry, detector
Digital Poster 3665
Quantification of scanner dosimetric uncertainty using transmission filters with uniform optical density Katherine Collins, Sabeena Beveridge, Sarah Judd, Andrew Alves, Kate Francis, Rhonda Brown Australian Clinical Dosimetry Service, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Australia
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