A Monte Carlo study on the PTW 60019 microDiamond detector
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2019-07-17 |
| Journal | Medical Physics |
| Authors | GĆ¼nther H. Hartmann, Klemens Zink |
| Institutions | Frankfurt Institute for Advanced Studies, Heidelberg University |
| Citations | 16 |
Abstract
Section titled āAbstractāPurpose 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 fluenceābased 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 p int 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 p ext 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 Coā60, 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 underāresponse 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 underāresponse expressed by the perturbation factor p int and overāresponse expressed by the perturbation factor p ext 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 fluenceābased subfactors on detector characteristics. Monte Carlo simulations offer an improvement in the understanding particularly of smallāfield 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 highādensity diamond substrate causing a field sizeādependent 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 deviations of the radius of the sensitive volume from the nominal radius. The backscattering effect also has an influence on the determination of profiles and for very small field sizes on the response at different rotation angles.