S1687
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
by more than 20% for the highest pulse doses (Dpp) (see Figure 1). If this correction is not considered in the calculation of the OFs, the values are underestimated by up to 5% for the highest Dpp configurations (range [-0.2% – 5.69%]).
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Evaluating a high-resolution complementary metal oxide semiconductor detector as a beam profiler for small fields Beth Gledhill 1 , Chris Stepanek 1 , Louise Charlton 1 , Jack Aylward 1,2 1 Department of Medical Physics and Bioengineering, Bristol Haemotology & Oncology Centre, Bristol, United Kingdom. 2 Medical Physics, School of Applied Sciences, University of the West of England, Bristol, United Kingdom
Purpose/Objective: The objective of this study was to evaluate a
complementary metal oxide semiconductor (CMOS) as a practical and accurate QC tool for characterising photon beam profiles of field sizes 5.0 x 5.0 cm2 and smaller. Material/Methods: Central axis (CAX) inline and crossline profiles were measured using a CMOS detector (myQA SRS, IBA Dosimetry) and three single-channel detectors (IBA Razor chamber, IBA Razor diode, and PTW MicroDiamond) for three beam qualities (6 MV, 10 MV and 6 MV FFF) and for a range of square field sizes (side length 0.5 cm, 1.0 cm, 2.0 cm, 3.0 cm and 5.0 cm). Reference measurements were acquired in a scanning water phantom (IBA Blue Phantom2). The CMOS measurements were carried out by stacking water- equivalent RW3 plates above and below the detector to replicate water phantom scatter conditions. Modelled profiles were also calculated using the Raystation treatment planning system (RaySearch Laboratories) for comparison. Analysis was performed using myQA Accept software (IBA Dosimetry). In addition to visual inspection, analysis was based on full width half maximum (FWHM), penumbra width and symmetry. Results: CMOS, MicroDiamond and diode measured profiles were found to be closely comparable in terms of FWHM and penumbra width (Table 1). The greatest difference between the CMOS detector and MicroDiamond FWHM was 0.7 mm with average differences of +1.5% (6 MV), -0.2% (10 MV) and -1.0% (6 MV FFF). Deviations between CMOS detector and diode were of similar magnitude: -1.3% (6 MV), +1.5% (10 MV) and +0.7% (6 MV FFF). The ion chamber was found to consistently overestimate FWHM compared to the other detectors due to penumbra broadening; mean FWHM differences compared to the CMOS detector were +2.1% (6 MV), +5.7% (10 MV) and +5.4% (6 MV FFF). This became less apparent with increasing field size, with no significant difference between detector measurements for field sizes equal to or larger than 3.0 x 3.0 cm (Figure
Once corrected for ksat, the OFs estimated with the ionisation chamber and the solid-state detector are very similar, and comparable to those of the MC model (see Table 1): the differences between measurements and the model are less than 2%, except for the 4 cm applicator and 12 MeV energy, where they reach 6% with the solid-state detector and 3.5% with the chamber. The maximum difference between the two detectors is 3.7% for the same applicator, 6 MeV energy.
Conclusion: The output factors of the LIAC HWL have been correctly determined using both a solid-state detector and a small-volume ionisation chamber. Due to the high Dpp obtained in this equipment, the OFs determined with the chamber may be underestimated by up to 5% if the measurements are not properly corrected by ksat. Therefore, it is recommended to use an ionisation chamber with a smaller electrode separation, or a solid-state detector, to introduce less uncertainty in the characterisation of the OFs of this
equipment. References:
[1] Laitano, R. F., Guerra, A. S., Pimpinella, M., Caporali, C., & Petrucci, A. (2006). Charge collection efficiency in ionization chambers exposed to electron beams with high dose per pulse. Physics in Medicine & Biology, 51(24), 6419. Keywords: Intraoperative radiotherapy, Ionization chambers
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