ESTRO 2025 - Abstract Book

S2604

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

References: [1] Vuik, F.E.R., Nieuwenburg, S.A.V., Bardou, M., et al. (2019). Increasing incidence of colorectal cancer in young adults in Europe over the last 25 years. Gut , 68(10), pp.1820-1826.

2277

Digital Poster Air kerma energy response of Farmer type chambers

Christopher P Oliver, Duncan J Butler PSDL, ARPANSA, Melbourne, Australia

Purpose/Objective: Farmer type chambers are routinely calibrated against air kerma primary standards in Co-60 and orthovoltage x rays beams with peak energies up to 300 kVp. To determine an air kerma calibration coefficient for a photon beam quality between these energies such as Cs-137 or Ir-192, linear interpolation is often used. This study investigated the error introduced by this interpolation. Material/Methods: The EGSnrc suite of software was used for this study [1]. Mass energy transfer coefficients were calculated for photons with energies up to 1.5 MeV in air using user code g . The three most popular farmer type chambers including their buildup caps were then modelled in the software using manufacturer supplied and open source information. These were the PTW 30013, IBA FC65-G and NE2571 chambers. The dose to the cavity of each of these chambers was calculated using user code cavity for monoenergetic photon beams in steps of 25 keV or less from 0 up to 1.5 MeV to model their response. User code egs_kerma was then used to determine the total air kerma in the monoenergetic photon beams in the absence of the chambers, which allowed a detailed energy response curve to be plotted for each chamber. Results: The mono energetic energy response curves for the Farmer chambers show that linear interpolation between Co-60 and a high energy orthovoltage x-ray calibration for air kerma will introduce errors for intermediate energies. For Cs-137 the error is of the order of 2 %. This error was confirmed by calibrating the Farmer chambers against a plane parallel pancake chamber in Cs-137 which is the primary standard for air kerma in Cs-137 and Co-60 in Australia. The result was also compared to to the primary standard of air kerma for Cs-137 of Taiwan showing a similar result. Conclusion: Care should be taken when interpolating air kerma calibration coefficients between orthovoltage x-ray and Co-60 energies to an intermediate energy using a Farmer chamber. Errors of up to 2 % can be introduced if using linear interpolation. This may be acceptable for radiation protection measurements but may be an issue if being implemented in a clinical environment.

Keywords: air kerma, energy response References: [1] Kawrakow I, Rogers DWO, Mainegra-Hing E, Tessier F, Townson RW, Walters BRB. EGSnrc toolkit for Monte Carlo simulation of ionizing radiation transport, doi:10.4224/40001303 (2000).