A Monte Carlo study on the PTW 60019 microDiamond detector

Abstract

Purpose Data on the output correction factor for small photon beam dosimetry of the microDiamond detector manufactured by the company PTW can be found in a variety of papers. Referring either to measurements or to Monte Carlo (MC) calculations, they show substantial disagreements particularly at very small fields. This work reports results of a further MC study aiming at a better understanding of how specific properties of the microDiamond detector are influencing its output correction factor and whether this can explain at least some of the disagreements. Methods In this study the method of a fluence‐based decomposition of the dose conversion factor was used which is considered as a useful tool to understand the response of a detector in nonreference conditions. This decomposition method yields the following three factors: (a) the stopping power ratio water to diamond, (b) a perturbation factor pint taking into account all fluence changes in the transition from a small water voxel at the point of dose determination to the bare diamond detector, and (c) a perturbation factor pext taking into account all additional fluence changes in the fully simulated diamond detector caused by the material and design details outside the sensitive volume. Results Monte Carlo calculated output correction factors were obtained for Co‐60, 6 MV and 10 MV photon beams showing that the maximum variation with field size remained in the order of 2% for quadratic field sizes larger than about 0.3 cm. For field sizes smaller than about 0.5 cm a clear under‐response is obtained at all three radiation qualities in agreement with all known MC calculations, however, in contrast to some measured result. The shape of the output correction factor can be well explained by an opposite mode of action between under‐response expressed by the perturbation factor pint and over‐response expressed by the perturbation factor pext where the first one is mainly influenced by volume averaging, and the second one by a back scatter effect of electrons from the diamond substrate into the sensitive volume. Conclusion The response of microDiamond detector can be well described under various measuring conditions by the dose conversion factor and the dependency of its fluence‐based subfactors on detector characteristics. Monte Carlo simulations offer an improvement in the understanding particularly of small‐field effects by relating the output correction factor to spectral fluence changes in the sensitive volume of the detector. The most significant influence factors are the finite size of the active volume and the presence of the high‐density diamond substrate causing a field size‐dependent backscattering. These perturbations are opposite in their effects. The diamond in the sensitive volume itself and in particular its density has almost no influence. Scattering of results at very small field sizes can be explained by different gradients of dose profiles around the beam axis at identical full width half maximum (FWHM) field size parameters and by possible deviati...

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate 192Ir brachytherapy sources

“…()) and is the dose to the mDD sensitive volume, both in the complete experimental setup. As done in our previous work, 10 and similar to the approach of several authors, 35–39 the factor f was decoupled into two sub‐factors, here named f E (energy dependence correction factor) and f V (volume‐averaging correction factor):…”

Development of Experimental Brachytherapy Dosimetry Using Monte Carlo Simulations for Detector Characterization

“…Marinelli et al (2016) showed that the actual dimensions were close to the nominal ones, but the discussion encouraged the manufacturer to update blueprints by including all metallic structures (the supporting pins and contacts). It did not affect MC-calculated small-field correction factors (Hartmann and Zink 2019), but had implications for our study. The k Q,Q0 values given in Paper I were calculated using complete blueprints.…”

Diamond Detectors for Radiotherapy X-Ray Small Beam Dosimetry

“…Moreover, the detector's electron density is different from the water ones, thus perturbing the secondary electron field (density effect) [3]. Quantification and correction of the combination of volume averaging and density effects (volume effect), can be addressed using MonteCarlo simulation to calculate correction factors to relate the detector's signal to the dose in water [4,5]. Therefore, volume averaging with source occlusion and lack of lateral electron equilibrium can cause a reduction of linac output and of measured output.…”