Abstract
PurposeThe goal of the present work was to provide a large set of detector‐specific output correction factors for seven small volume ionization chambers on two linear accelerators in four megavoltage photon beams utilizing perpendicular and parallel orientation of ionization chambers in the beam for nominal field sizes ranging from 0.5 cm2 × 0.5 cm2 to 10 cm2 × 10 cm2. The present study is the second part of an extensive research conducted by our group.MethodsOutput correction factors kQclin,Qreffclin,fref were experimentally determined on two linacs, Elekta Versa HD and Varian TrueBeam for 6 and 10 MV beams with and without flattening filter for nine square fields ranging from 0.5 cm2 × 0.5 cm2 to 10 cm2 × 10 cm2, for seven mini and micro ionization chambers, IBA CC04, IBA Razor, PTW 31016 3D PinPoint, PTW 31021 3D Semiflex, PTW 31022 3D PinPoint, PTW 31023 PinPoint, and SI Exradin A16. An Exradin W1 plastic scintillator and EBT3 radiochromic films were used as the reference detectors.ResultsFor all ionization chambers, values of output correction factors kQclin,Qreffclin,fref were lower for parallel orientation compared to those obtained in the perpendicular orientation. Five ionization chambers from our study set, IBA Razor, PTW 31016 3D PinPoint, PTW 31022 3D PinPoint, PTW 31023 PinPoint, and SI Exradin A16, fulfill the requirement recommended in the TRS‐483 Code of Practice, that is, 0.95<kQclin,Qreffclin,fref<1.05, down to the field size 0.8 cm2 × 0.8 cm2, when they are positioned in parallel orientation; two of the ionization chambers, IBA Razor and PTW 31023 PinPoint, satisfy this condition down to the field size of 0.5 cm2 × 0.5 cm2.ConclusionsThe present paper provides experimental results of detector‐specific output correction factors for seven small volume ionization chambers. Output correction factors were determined in 6 and 10 MV photon beams with and without flattening filter down to the square field size of 0.5 cm2 × 0.5 cm2 for two orientations of ionization chambers — perpendicular and parallel. Our main finding is that output correction factors are smaller if they are determined in a parallel orientation compared to those obtained in a perpendicular orientation for all ionization chambers regardless of the photon beam energy, filtration, or linear accelerator being used. Based on our findings, we recommend using ionization chambers in parallel orientation, to minimize corrections in the experimental determination of field output factors. Latter holds even for field sizes below 1.0 cm2 × 1.0 cm2, whenever necessary corrections remain within 5%, which was the case for several ionization chambers from our set.TRS‐483 recommended perpendicular orientation of ionization chambers for the determination of field output factors. The present study presents results for both perpendicular and parallel orientation of ionization chambers. When validated by other researchers, the present results for parallel orientation can be considered as a complementary dataset to those given in TRS‐483.
Highlights
A new international Code of Practice (CoP) for reference and relative dosimetry in high energy small static photon fields, TRS-483, has recently been published jointly by the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM).[1]
Detector-specific output correction factors obtained in our study for the remaining three ionization chambers (IBA CC04, PTW 31016 PinPoint 3D, and SI Exradin A16) in perpendicular orientation, confirm the corresponding data published in the TRS-483 using the 95% confidence limits
In addition to the output correction factors determined in perpendicular orientation, we have provided a large set of kQfclcinlin;f;rQefref values for the same set of small volume ionization chambers in parallel orientation [Fig. 1(b)]
Summary
A new international Code of Practice (CoP) for reference and relative dosimetry in high energy small static photon fields, TRS-483, has recently been published jointly by the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM).[1]. A summary of this CoP has been published by Palmans et al.[2]. The formalism recommend in the CoP is a slightly modified version of the formalism proposed by Alfonso et al.[3]. Field output factor XfQclcinli;nf;rQefref for a particular clinical field fclin and reference field fref can be determined from the ratio of absorbed doses in both fields, and is given by. Dfwc;lQinclin Dfwre;Qf ref where Qclin and Qref denote beam quality in the clinical and reference fields, respectively.
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