ESTRO 2020 Abstract book

S738 ESTRO 2020

model that linearizes response of XR-QA2 GafChromic TM film model at low energy photon beams used in diagnostic radiology. Material and Methods Pieces of XR-QA2 film were irradiated to 9 different (up to 10 cGy) air kerma in air values (Fig.1.a), following reference calibration (using AAPM TG-61 protocol) at 5 beam qualities ranging in HVLs from 4.03 to 8.25 mm Al (effective energy range from 38.8 keV to 56.3 keV), corresponding to beam qualities commonly used in CT scanners. Film pieces were scanned with 127 dpi (0.2 mm/pixel) 24-hours post irradiation and only these images were used for pixel value sampling, over a region of interest 2 mm x 2 mm (10x10 pixels) in size (Fig.1.b). For each beam quality, we tested new functional form: K air air = A {(PV 0 /PV) n -1}, where A and n are fitting parameters, and PV 0 and PV represent pixel values of unexposed and exposed film pieces respectively. Results Figure 1.c represents calibration curves for measured beam qualities showing linear response in investigated air- kerma range. Table summarizes resulting fitting parameters for the beam qualities investigated in this work. The inset in the figure suggests nearly linear change of fitting parameter n with effective photon energies investigated. Parameter n must be determined for a given beam quality, following film irradiations described in Fig.1.a.

Centre, Edinburgh, United Kingdom ; 6 Northern Ireland Cancer Centre, Radiotherapy Physics, Belfast, United Kingdom ; 7 University College London, Department of Medical Physics & Biomedical Engineering, London, United Kingdom Purpose or Objective The 1990 code of practice (COP), produced by the IPSM (now the Institute of Physics and Engineering in Medicine, IPEM) and the UK National Physical Laboratory (NPL), gave instructions for determining absorbed dose to water for megavoltage photon (MV) radiotherapy beams (Lillicrap et al Phys. Med. Biol. 1990 35 1355–60). The simplicity and clarity of the 1990 COP led to widespread uptake across the region, and high levels of consistency in external dosimetry audits. An addendum was published in 2014 to include the non-conventional conditions in Tomotherapy units. However, the 1990 COP lacked detailed recommendations for calibration conditions, and the corresponding nomenclature, to account for the full range of modern treatment units with different reference fields, including small fields as described in IAEA TRS-483 (International Atomic Energy This updated COP recommends the irradiation geometries, the choice of ionisation chambers and appropriate correction factors, and the derivation of absorbed dose to water calibration coefficients, for carrying out reference dosimetry measurements on MV external beam radiotherapy machines. It also includes worked examples of application to different conditions. Results Strengths of the 1990 COP are retained: recommending the NPL2611 chamber type as secondary standard; the use of tissue phantom ratio (TPR) as the beam quality specifier; and standards lab-provided direct calibration coefficients for the user's chamber in a range of beam qualities similar to those in clinical use. Therefore generic correction factors for beam quality are not required. In addition, the formalism is now extended to units that cannot achieve the standard reference field size of 10cm x 10cm, and recommendations are given for measuring dose in non-reference conditions. This COP is designed around the service that the standards lab provides and thus it does not require the range of different options presented in TRS-483. This approach results in a significantly simpler, more concise, and easier to follow protocol. Conclusion The update to the 1990 IPEM/NPL COP retains the simplicity and ease of application for clinical users, but is also compatible with international guidance and non- conventional treatment units. PO-1308 Relative Dose Measurements in Diagnostic Radiology Beams Using the XR-QA2 GafChromicTM Film Model N. Tomic 1 , J. Seuntjens 1 , S. Devic 1 1 McGill University, Oncology, Montreal, Canada Purpose or Objective Most of the early work with radiochromic films used either optical density or reflectance as a response to radiation dose deposition. These response functions, although having a physical meaning, result in non-linear dose vs. response calibration curves, which may lead to significant measurement errors if one assumes that relative dose values can be obtained by simple division of such response functions (similar to ion chamber measurements in high- energy photon beams). Lately, a number of functional models have been proposed that linearize dose response function in both kilo-voltage [Tomic at al. Med. Phys. 41 (6), 062125 (2014)] and mega-voltage [Aldelaijan at al. Phys. Med. 49, 112-118 (2018)] photon beams. In this work, we report on new (further simplified) functional Agency, Vienna 2017). Material and Methods

Conclusion We described a new simplified functional form that linearizes dose response curve of the XR-QA2 film model based radiochromic film dosimetry system. Relative dosimetry can be conveniently performed using this dosimetry system without the need for establishing calibration curve. For the purpose of relative measurements only, the plot of chamber reading (corresponding to different mAs settings) as a function of {(PV 0 /PV) n -1} is sufficient for parameter n to be determined. However, if the reference radiochromic film dosimetry system is needed for measurements of absolute dose, reference irradiations with the same setup should be used to determine the fitting parameter A .