Abstract
The goal of this study was to investigate small field output factors (OFs) for flattening filter‐free (FFF) beams on a dedicated stereotactic linear accelerator‐based system. From this data, the collimator exchange effect was quantified, and detector‐specific correction factors were generated. Output factors for 16 jaw‐collimated small fields (from 0.5 to 2 cm) were measured using five different detectors including an ion chamber (CC01), a stereotactic field diode (SFD), a diode detector (Edge), Gafchromic film (EBT3), and a plastic scintillator detector (PSD, W1). Chamber, diodes, and PSD measurements were performed in a Wellhofer water tank, while films were irradiated in solid water at 100 cm source‐to‐surface distance and 10 cm depth. The collimator exchange effect was quantified for rectangular fields. Monte Carlo (MC) simulations of the measured configurations were also performed using the EGSnrc/DOSXYZnrc code. Output factors measured by the PSD and verified against film and MC calculations were chosen as the benchmark measurements. Compared with plastic scintillator detector (PSD), the small volume ion chamber (CC01) underestimated output factors by an average of ‐1.0%±4.9%(max.=‐11.7% for 0.5×0.5cm2 square field). The stereotactic diode (SFD) overestimated output factors by 2.5%±0.4%(max.=3.3% for 0.5×1cm2 rectangular field). The other diode detector (Edge) also overestimated the OFs by an average of 4.2%±0.9%(max.=6.0% for 1×1cm2 square field). Gafchromic film (EBT3) measurements and MC calculations agreed with the scintillator detector measurements within 0.6%±1.8% and 1.2%±1.5%, respectively. Across all the X and Y jaw combinations, the average collimator exchange effect was computed: 1.4%±1.1% (CC01), 5.8%±5.4% (SFD), 5.1%±4.8% (Edge diode), 3.5%±5.0% (Monte Carlo), 3.8%±4.7% (film), and 5.5%±5.1% (PSD). Small field detectors should be used with caution with a clear understanding of their behaviors, especially for FFF beams and small, elongated fields. The scintillator detector exhibited good agreement against Gafchromic film measurements and MC simulations over the range of field sizes studied. The collimator exchange effect was found to be important at these small field sizes. Detector‐specific correction factors were computed using the scintillator measurements as the benchmark.PACS number(s): 87.56.Fc
Highlights
380 Qin et al.: Detector-specific output factors for small fields the medical physics community.[3,4,5,6,7] Small field measurements require the use of appropriate detectors, and must be performed with utmost attention to the setup accuracy
For dosimetry in nonreference conditions such as small fields and composite intensitymodulated radiotherapy (IMRT) fields, the International Atomic Energy Agency (IAEA) and the American Association of Physicist in Medicine (AAPM) have recommended a new formalism,(10) as an extension to the existing dTeatsekctGorr-osuppec5i1fi.c(11e)ffAenctsadadt itthioesnealncoonrrreefcetrieonncefaccotonrdiktQficoclilninn,f,smQ.msrMsr watahseminatrtoicdaulclye,dktQfcoclilnina,f,mQcmscrsor uisntthfeorratthioe of output factor obtained by the reference versus that measured by the detector of interest.[10]
The CC01 measured much lower output factors (OFs) compared to the other detectors, especially at field sizes smaller than 1 cm, where the differences were up to 11.7% relative to the plastic scintillator detector (PSD)
Summary
380 Qin et al.: Detector-specific output factors for small fields the medical physics community.[3,4,5,6,7] Small field measurements require the use of appropriate detectors, and must be performed with utmost attention to the setup accuracy. Electronic equilibrium breaks down as the radiation field size drops below the range of the secondary electrons, invalidating the Bragg-Gray conditions.[8,9] Source occlusion takes place at small fields, resulting in a decrease in the dose deposited.[4] In addition, measurements at small fields are limited by the various detector-specific effects.[4,9,10]. These include the volume averaging around high-gradient dose distributions, the fluence/dose perturbations introduced by the different physical densities between detector and medium, and the uncertainty in detector geometric alignment. All the aforementioned methods were employed for estimating small field OFs; the scintillator detector was selected as the reference
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