Implementing a newly proposed Monte Carlo based small field dosimetry formalism for a comprehensive set of diode detectors

Medical Physics - Tập 38 Số 12 - Trang 6592-6602 - 2011
Gavin Cranmer‐Sargison1,2, S. Weston3, J A Evans4, Narinder Sidhu5, David Thwaites6
1Department of Medical Physics, Saskatchewan Cancer Agency, Saskatoon, Saskatchewan S7N 4H4, Canada and Division of Medical Physics, Leeds Institute of Genetics, Health and Therapeutics, University of Leeds, Leeds LS2 9JT, United Kingdom
2Electronic mail: [email protected]
3Department of Medical Physics and Engineering, St James's University Hospital, Leeds LS9 7TF, United Kingdom
4Division of Medical Physics, Leeds Institute of Genetics, Health and Therapeutics, University of Leeds, Leeds LS2 9JT, United Kingdom
5Saskatoon Cancer Centre, Saskatchewan Cancer Agency, Saskatoon, Saskatchewan S7N 4H4, Canada and Department of Physics and Engineering, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada
6Institute of Medical Physics, School of Physics, University of Sydney, Sydney NSW 2006, Australia and Division of Medical Physics, Leeds Institute of Genetics, Health and Therapeutics, University of Leeds, Leeds LS2 9JT, United Kingdom

Tóm tắt

Purpose:The goal of this work was to implement a recently proposed small field dosimetry formalism [Alfonsoet al., Med. Phys. 35(12), 5179–5186 (2008)] for a comprehensive set of diode detectors and provide the required Monte Carlo generated factors to correct measurement.Methods:Jaw collimated square small field sizes of side 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, and 3.0 cm normalized to a reference field of 5.0 cm × 5.0 cm were used throughout this study. Initial linac modeling was performed with electron source parameters at 6.0, 6.1, and 6.2 MeV with the Gaussian FWHM decreased in steps of 0.010 cm from 0.150 to 0.100 cm. DOSRZnrc was used to develop models of the IBA stereotactic field diode (SFD) as well as the PTW T60008, T60012, T60016, and T60017 field diodes. Simulations were run and isocentric, detector specific, output ratios (ORdet) calculated at depths of 1.5, 5.0, and 10.0 cm. This was performed using the following source parameter subset: 6.1 and 6.2 MeV with a FWHM = 0.100, 0.110, and 0.120 cm. The source parameters were finalized by comparing experimental detector specific output ratios with simulation. Simulations were then run with the active volume and surrounding materials set to water and the replacement correction factors calculated according to the newly proposed formalism.Results:In all cases, the experimental field size widths (at the 50% level) were found to be smaller than the nominal, and therefore, the simulated field sizes were adjusted accordingly. At a FWHM = 0.150 cm simulation produced penumbral widths that were too broad. The fit improved as the FWHM was decreased, yet for all but the smallest field size worsened again at a FWHM = 0.100 cm. The simulated ORdet were found to be greater than, equivalent to and less than experiment for spot size FWHM = 0.100, 0.110, and 0.120 cm, respectively. This is due to the change in source occlusion as a function of FWHM and field size. The corrections required for the 0.5 cm field size were 0.95 (±1.0%) for the SFD, T60012 and T60017 diodes and 0.90 (±1.0%) for the T60008 and T60016 diodes—indicating measured output ratios to be 5% and 10% high, respectively. Our results also revealed the correction factors to be the same within statistical variation at all depths considered.Conclusions:A number of general conclusions are evident: (1) small field ORdet are very sensitive to the simulated source parameters, and therefore, rigorous Monte Carlo linac model commissioning, with respect to measurement, must be pursued prior to use, (2) backscattered dose to the monitor chamber should be included in simulated ORdet calculations, (3) the corrections required for diode detectors are design dependent and therefore detailed detector modeling is required, and (4) the reported detector specific correction factors may be applied to experimental small field ORdet consistent with those presented here.

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Medical Physics

Tập 38 Số 12

6592-6602

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