Monte Carlo modelling of diode detectors for small field MV photon dosimetry: detector model simplification and the sensitivity of correction factors to source parameterization

Abstract

The goal of this work was to examine the use of simplified diode detector models within a recently proposed Monte Carlo (MC) based small field dosimetry formalism and to investigate the influence of electron source parameterization has on MC calculated correction factors. BEAMnrc was used to model Varian 6 MV jaw-collimated square field sizes down to 0.5 cm. The IBA stereotactic field diode (SFD), PTW T60016 (shielded) and PTW T60017 (un-shielded) diodes were modelled in DOSRZnrc and isocentric output ratios (OR(fclin)(detMC)) calculated at depths of d = 1.5, 5.0 and 10.0 cm. Simplified detector models were then tested by evaluating the percent difference in (OR(fclin)(detMC)) between the simplified and complete detector models. The influence of active volume dimension on simulated output ratio and response factor was also investigated. The sensitivity of each MC calculated replacement correction factor (k(fclin,fmsr)(Qclin,Qmsr)), as a function of electron FWHM between 0.100 and 0.150 cm and energy between 5.5 and 6.5 MeV, was investigated for the same set of small field sizes using the simplified detector models. The SFD diode can be approximated simply as a silicon chip in water, the T60016 shielded diode can be modelled as a chip in water plus the entire shielding geometry and the T60017 unshielded diode as a chip in water plus the filter plate located upstream. The detector-specific (k(fclin,fmsr)(Qclin,Qmsr)), required to correct measured output ratios using the SFD, T60016 and T60017 diode detectors are insensitive to incident electron energy between 5.5 and 6.5 MeV and spot size variation between FWHM = 0.100 and 0.150 cm. Three general conclusions come out of this work: (1) detector models can be simplified to produce OR(fclin)(detMC) to within 1.0% of those calculated using the complete geometry, where typically not only the silicon chip, but also any high density components close to the chip, such as scattering plates or shielding material is necessary to be included in the model, (2) diode detectors of smaller active radius require less of a correction and (3) (k(fclin,fmsr)(Qclin,Qmsr)) is insensitive to the incident the electron energy and spot size variations investigated. Therefore, simplified detector models can be used with acceptable accuracy within the recently proposed small field dosimetry formalism.

Output factor comparison of Monte Carlo and measurement for Varian TrueBeam 6 MV and 10 MV flattening filter‐free stereotactic radiosurgery system

“…The dimensional resolution of measurement is limited by the two‐dimensional sensitive size of the detectors perpendicular to the axis of the radiation beam. According to a study by Cranmer‐Sargison et al, (12) there is no significant volume averaging effect for a $5\times 5\hspace{0.17em}{\text{mm}}^2$ field size measured with detector of an active area of less than 1 mm 2 . For this study, two small diode detectors and a cylindrical ionization chamber were used: an Edge detector (Sun Nuclear, Melbourne, FL) with sensitive volume of 0.019 mm 3 and a sensitive area of $0.8\times 0.8\hspace{0.17em}{\text{mm}}^2$, a Small Field Detector (SFD, IBA Dosimetry) with sensitive volume of 0.017 mm 3 and a sensitive diameter of 0.6 mm, and an Exradin A16 cylindrical micro‐ionization chamber (Standard Imaging, Middleton, WI) with sensitive volume of 7 mm 3 and a sensitive length of 3.4 mm which was used as a reference comparison for large cones.…”

“…Many recent studies have applied Monte Carlo simulations to examine the output factors of small field sizes used for radiosurgery such as CyberKnife, 6 , 7 Gamma Knife, 8 , 9 and Brainlab (10) . In addition, numerous studies have used Monte Carlo simulation to examine the accuracy of diode detectors measurements for small field dosimetry 11 , 12 , 13 , 14 …”

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators

“…The overresponse in small radiation fields of silicon diodes, which incorporate silicon of density 2.3 g/cm 3 and possibly other high‐density material in their sensitive volumes and surroundings, has been well documented 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 38 , 39 , 40 , 41 . Diamond detectors, which have a sensitive volume of density 3.5 g/cm 3 , have been reported to overrespond in small fields 16 , 39 , 41 , 42 .…”

“…However, there is limited information in the literature on the effect of beam energy on the overresponse of solid‐state detectors in small radiation fields. Cranmer‐Sargison et al (11) investigated the effect of source parameterization changing incident beam energy over a small range (5.5–6.5 MeV) on diode small field correction factors using Monte Carlo simulations, finding marginal changes in correction factors across such small energy changes. Despite this, we hypothesize that the density effect in small radiation fields is dependent on beam energy and is the source of deviations in FFF:FF ratios for the Varian linac, as presented in Fig.…”

“…Several studies have reported on the response of ionization chambers, diodes, and the microdiamond 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 detector in small radiation fields for flattened beams, using either experiment or Monte Carlo simulations. Where necessary, small field correction factors $false(k_{Q_{clin}{Q}_{msr}}^{f_{clin},f_{msr}}false)$, as defined by Alfonso et al, (18) have been provided.…”