ESTRO 38 Abstract book

S974 ESTRO 38

2 RefleXion Medical, 25841 Industrial Blvd- Suite 275, Hayward- CA, USA Purpose or Objective The design of the new RefleXion biology-guided radiotherapy (BgRT) system restricts the maximum field size to 2 cm in the International Electrotechnical Commission (IEC) Y dimension at the source-to-axis distance (SAD) of 85 cm. The closest field size to the conventional reference field in this machine is 2×10 cm² at the isocenter. The energy of the beam is 6 MV and the beam is flattening filter free (FFF). The 2×10 cm² field size does not meet the lateral charged particle equilibrium condition of the machine-specific reference ( msr ) field introduced in the IAEA TRS-483 Code of Practice (CoP). Therefore the IAEA TRS-483 CoP cannot be directly used for the calibration of this machine. In this study, two methods of calibration are proposed for the reference dosimetry of the BgRT Unit. The BgRT Unit is calibrated using the two methodologies and the results are compared. Material and Methods The percent depth dose (PDD) and profile were measured in the BgRT unit using the Exradin A14SL chamber for the 2×10 cm² field size at SSD 85 cm. The BgRT system was modeled using the EGSnrc/BEAMnrc Monte Carlo (MC) code and the PDD and profile were calculated using EGSnrc/DOSXYZnrc. The beam model was tuned to achieve good agreement with the 2×10 cm² PDD and profile measurements. Two methods of calibration are suggested in this study. In the first method, the generic correction factors ($k_{Q_{F},Q_{0}}^{f_{F},f_{ref}}$) are calculated directly using MC for Exradin A14SL. In this study the "F" Field refers to both msr and non - msr fields. In the second method, the IAEA TRS-483 protocol is extended to fields as small as 2 cm. The beam quality specifier and equivalent square field (S) are calculated and used to determine the beam quality correction factor ($k_{Q,Q_{0}}$) using the analytical approach (IAEA TRS- 398 CoP). The volume averaging and water to air stopping power ratio corrections are also applied to the $k_{Q,Q_{0}}$ values to correct for the differences between the With Flattening Filter (WFF) and FFF beams. Results The measured and calculated %dd(10,S) are 57.15±0.40% and 57.06±0.07% respectively. When using the first method, the calculated $k_{Q_{F},Q_{0}}^{f_{F},f_{ref}}$ value for Exradin A14SL is found to be 0.9965±0.0011. The equivalent field size S for the 2×10 cm² field size is determined as 3.5 cm. The $k_{Q,Q_{0}}$ value corresponding to TPR$_{20,10}$(10,3.5) in the TRS-398 CoP corrected for the volume averaging and water to air stopping power ratios is 0.9951. Conclusion Good agreement is achieved between measured and MC calculated %dd(10,S) values (0.16%). The correction factors determined using the two proposed approaches are in very good agreement (0.14% for Exradin A14SL). While the results of this study are promising, further studies are required to confirm that these two methodologies can be used for other small ionization chambers used in the calibration of BgRT. EP-1799 Characterisation of a commercially available large-area IC for dosimetry of scanned proton beams S. Berke 1,2,3 , E. Almhagen 3,4 , L. Stolarczyk 3,5 1 The Clatterbridge Cancer Centre NHS Foundation Trust, Physics Department, Bebington, United Kingdom ; 2 The Royal Liverpool and Broadgreen University Hospitals NHS Trust, Department of Medical Physics and Clinical Engineering, Liverpool, United Kingdom ; 3 Skandionkliniken, Physics Department, Uppsala, Sweden ; 4 Uppsala University, Department of Immunology- Genetics and Pathology – Medical Radiation Science,

Uppsala, Sweden ; 5 Institute of Nuclear Physics PAN, Cyclotron Center Bronowice, Krakow, Poland Purpose or Objective Large-area plane-parallel ionisation chambers are used to measure integrated depth dose curves (IDDCs) of proton pencil beams [1]. They have also been proposed for reference dosimetry, using the dose-area product of a single pencil beam instead of the dose at the centre of a broad field [2]. The larger the IC’s diameter, the greater the part of the low-dose “halo” of proton pencil beams caused by scattered protons that is covered [1]. On the other hand, potential inhomogeneities in the chamber construction may influence measured doses and limit its usage in reference dosimetry [3]. A Stingray (IBA Dosimetry) IC with a diameter of 12cm has been characterised with respect to the measured proton range and IDDC shape compared to IDDCs measured with the 8cm Bragg Peak chamber (PTW 34070), and its response homogeneity. A method of chamber heterogeneity verification doable in clinical conditions has been proposed. Material and Methods IDDCs were acquired for pencil beams with energies between 60 and 226 MeV using the 12cm and 8cm diameter ICs and a BP² water phantom (IBA Dosimetry). Differences in range parameters (R90, R80, R50, R20) and the relative dose difference between IDDCs measured with the two chambers were evaluated. The chamber response homogeneity was examined by scanning the chamber across a 226MeV pencil beam in air and calculating the ratio between the measured signal at each position and the expected signal, assuming a 2D- Gaussian beam profile with σx=3.0mm and σy=3.1mm determined in air. Results The range parameters measured with the two chambers agreed within 0.5mm. Relative dose differences between IDDCs measured with the two chambers were up to 4%. Generally, the difference was greater for higher energies and the maximum difference was observed at approximately half the proton range. The IC homogeneity measurements revealed that differences between the expected and measured signal across the chamber area ranged from about -3% to +6% (ignoring points within 10mm of the chamber edge, where inaccurate assumptions about the beam profile might impact on the results; a relative response of unity was assumed there). The response variations were not symmetrical, as seen in the figure below (the stripes are a result of the scanning technique).

Conclusion The examined large-area plane-parallel ionisation chamber is suitable for its intended application of measuring IDDCs for routine QA.