Dose-area product ratio in external small-beam radiotherapy: beam shape, size and energy dependencies in clinical photon beams.

In small-field radiotherapy (RT), a significant challenge is to define the amount of radiation dose absorbed in the patient where the quality of the beam has to be measured with high accuracy. The properties of a proposed new beam quality specifier, namely the dose-area-product ratio at 20 and 10 cm depths in water or DAPR20,10, were studied to yield more information on its feasibility over the conventional quality specifier tissue-phantom ratio or TPR20,10. The DAPR20,10may be measured with a large-area ionization chamber (LAC) instead of small volume chambers or semi-conductors where detector, beam and water phantom positioning and beam perturbations introduce uncertainties. The effects of beam shape, size and energy on the DAPR20,10were studied and it was shown that the DAPR20,10increases with increasing beam energy similarly to TPR20,10but in contrast exhibits a small beam size and shape dependence. The beam profile outside the beam limiting devices has been shown to have a large contribution to the DAPR20,10. There is potential in large area chambers to be used in DAPR measurement and its use in dosimetry of small-beam RT for beam quality measurements.

A Monte Carlo study on the PTW 60019 microDiamond detector.

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 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.

Multicenter evaluation of a synthetic single-crystal diamond detector for CyberKnife small field size output factors.

PURPOSE: The aim of the present work was to evaluate small field size output factors (OFs) using the latest diamond detector commercially available, PTW-60019 microDiamond, over different CyberKnife systems. OFs were measured also by silicon detectors routinely used by each center, considered as reference. METHODS: Five Italian CyberKnife centers performed OFs measurements for field sizes ranging from 5 to 60mm, defined by fixed circular collimators (5 centers) and by Iris(™) variable aperture collimator (4 centers). Setup conditions were: 80cm source to detector distance, and 1.5cm depth in water. To speed up measurements two diamond detectors were used and their equivalence was evaluated. MonteCarlo (MC) correction factors for silicon detectors were used for comparing the OF measurements. RESULTS: Considering OFs values averaged over all centers, diamond data resulted lower than uncorrected silicon diode ones. The agreement between diamond and MC corrected silicon values was within 0.6% for all fixed circular collimators. Relative differences between microDiamond and MC corrected silicon diodes data for Iris(™) collimator were lower than 1.0% for all apertures in the totality of centers. The two microDiamond detectors showed similar characteristics, in agreement with the technical specifications. CONCLUSIONS: Excellent agreement between microDiamond and MC corrected silicon diode detectors OFs was obtained for both collimation systems fixed cones and Iris(™), demonstrating the microDiamond could be a suitable detector for CyberKnife commissioning and routine checks. These results obtained in five centers suggest that for CyberKnife systems microDiamond can be used without corrections even at the smallest field size.