S1661
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Digital Poster 3069 The impact of linear accelerator aging on the small field dosimetry of megavoltage photon beams Mohammed Ghazal 1,2 , Aseel Aziz 3 , Hamza Benmakhlouf 4,2 1 Nuclear Medicine and Medical Physics, Karolinska University Hospital, Stockholm, Sweden. 2 Oncology- Pathology, Karolinska Institutet, Stockholm, Sweden. 3 Radiology, Danderyd Hospital, Stockholm, Sweden. 4 Radiation Oncology, Karolinska University Hospital, Stockholm, Sweden
target degradation. Overall, OFs increased over five years, with two heavily used linacs exhibiting deviations up to 5%, suggesting usage-dependent aging effects (Figure 2).
Purpose/Objective: Small megavoltage photon fields (<2 cm) are
increasingly employed in stereotactic and dynamic radiotherapy. Accurate determination of output factors (OFs) in these fields is essential for beam modeling and treatment planning but remains challenging due to detector response, collimation accuracy, and beam focal spot characteristics. The IAEA TRS-483 report provides a code of practice with corresponding output correction factors (OCFs)1. Although small-field dosimetry has been extensively studied, the impact of linear accelerator (linac) aging on small-field OFs remains unexplored. This study aims to: (i) assess the reproducibility of small-field OFs in newly installed versus five-year-old linacs; (ii) evaluate the intra-linac applicability of TRS-483 OCFs considering aging; and (iii) analyze inter-linac OF variation over a five-year period. Material/Methods: Measurements were conducted at installation (year 0) and after five years (year 5) on six beam-matched Varian TrueBeam linacs using four small-field detectors. Detector readings were acquired for 0.5-3 cm square fields, normalized to a 10 cm reference field. OCFs from TRS-483 were applied to determine OFs. Lateral dose profiles were measured for 0.5 cm fields to derive full-width at half maximum (FWHM) values, and for 10 cm fields to calculate penumbra widths as surrogates for effective field size and focal spot size, respectively. Results: Detector-reading reproducibility at 0.5 cm declined over time, with uncertainties increasing from 0.2% to 1%, indicating reduced collimator precision. The applicability of TRS-483 OCFs was unaffected by linac aging, maintaining OF agreement within 1% for fields ≥0.8 cm (Figure 1). For 0.5 cm fields, applying OCFs improved OF agreement from 4-5% to 2-4%. Inter-linac variation at 0.5 cm² increased by 2–3 percentage points in year 5, while fields ≥1 cm² remained within 1%. No correlation was observed between OF and FWHM for small fields; however, penumbra widths of large fields correlated inversely with small-field OFs, suggesting a reduced effective focal spot size due to
Conclusion: Linac aging reduces reproducibility and inter-linac consistency of small-field OFs, particularly for 0.5 cm² fields, while larger fields remain stable. The findings suggest aging-related decreases in focal spot and collimator precision, emphasizing the need to account for linac aging when establishing OF tables for fields <0.8 cm and when performing OF-matching across linacs for fields <1 cm. Continuous and rigorous
Made with FlippingBook - Share PDF online