A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams

Abstract

Purpose The goal of this work is to provide a large and consistent set of data for detector‐specific output correction factors, kQitalicclin,Qitalicref0.166667emfitalicclin,fref, for small static fields for seven solid‐state detectors and to determine field output factors, ΩQitalicclin,Qitalicreffclin,fref, using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators and four megavoltage photon beams. Consistent measurement conditions and recommendations given in the International Code of Practice TRS‐483 for small‐field dosimetry were followed throughout the study. Methods ΩQitalicclin,Qitalicreffclin,fref were determined on two linacs, Elekta Versa HD and Varian TrueBeam, for 6 and 10 MV beams with and without flattening filter and for nine fields ranging from 0.5 × 0.5 cm2 to 10 × 10 cm2. Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. Volume averaging correction factors, determined from two‐dimensional (2D) dose matrices obtained with EBT3 films and fitted to bivariate Gaussian function, were used to correct measured signals. kQitalicclin,Qitalicreffclin,fref were determined empirically for six diodes, IBA SFD, IBA Razor, PTW 60008 P, PTW 60012 E, PTW 60018 SRS, and SN EDGE, and a PTW 60019 microDiamond detector. Results Field output factors and detector‐specific kQitalicclin,Qitalicreffclin,fref are presented in the form of analytical functions as well as in the form of discrete values. It is found that in general, for a given linac, small‐field output factors need to be determined for every combination of beam energy and filtration (WFF or FFF) and field size as the differences between them can be statistically significant (P < 0.05). For different beam energies, the present data for kQitalicclin,Qitalicreffclin,fref are found to differ significantly (P < 0.05) from the corresponding data published in TRS‐483 mostly for the smallest fields (<1.5 cm). For the PTW microDiamond detector, statistically significant differences (P < 0.05) between kQitalicclin,Qitalicreffclin,fref values were found for all investigated beams on an Elekta Versa HD linac for field sizes 0.5 × 0.5 cm2 and 0.8 × 0.8 cm2. Significant differences in kQitalicclin,Qitalicreffclin,fref between beams of a given energy but with and without flattening filters are found for measurements made in small fields (<1.5 cm) at a given linac. Differences in kQitalicclin,Qitalicreffclin,fref are also found when measurements are made at different linacs using the same beam energy filtration combination; for the PTW microDiamond detector, these differences were found to be around 6% and were considered as significant. Conclusions Selection of two reference detectors, EBT3 films and W1 plastic scintillator, and use of an analytical function, is a novel approach for the determination of ΩQitalicclin,Qitalicreffclin,fref for small static fields in megavoltage photon beams. Large set of kQitalicclin,Qitalicreffclin,fref data for s...

Output correction factors for small static fields in megavoltage photon beams for seven ionization chambers in two orientations — perpendicular and parallel

“…The experimental equipment and methodology used in the present study are the same as those used in the first part of our study, where we gave detailed discussion about the determination of field output factors using plastic scintillator Exradin W1 (Standard Imaging, Middleton, WI, USA) and radiochromic films EBT3 (Ashland Inc., Wayne, NJ, USA), as reference detectors and output correction factors for six diodes and a microdiamond detector. 32,33 Therefore, only a brief discussion on the salient features of the methodology will be given here.…”

“…where A and B correspond to the radiation field width at half maximum (FWHM) in the in-line and cross-line directions, respectively, which were determined using EBT3 film measurements in the present work. 32,33 Full description of the methodology can be found in our previous work. 32 Medical Physics, 47 (1), January 2020…”

“…32,33 Full description of the methodology can be found in our previous work. 32 Medical Physics, 47 (1), January 2020…”

“…Film doses were calculated using the Multigaussian method implemented in Radiochromic.com v3.0 (Radiochromic SL, Benifai o, Spain). 33 Detailed methodology for the determination of field output factors using two reference detectors, EBT3 films and W1 plastic scintillator, is provided in the paper by Casar et al 32 For the functional presentation of output correction factors, discrete values of k…”

“…In the first part, we discussed the determination of field output factors and output correction factors for seven solid-state detectors using two reference detectors. 32 The same methodology from that part of the study was also followed in the present paper.…”