Evaluation of dose-volume metrics for microbeam radiation therapy dose distributions in head phantoms of various sizes using Monte Carlo simulations

摘要

This work evaluates four dose-volume metrics applied to microbeam radiation therapy (MRT) using simulated dosimetric data as input. We seek to improve upon the most frequently used MRT metric, the peak-to-valley dose ratio (PVDR), by analyzing MRT dose distributions from a more volumetric perspective. Monte Carlo simulations were used to calculate dose distributions in three cubic head phantoms: a 2cm mouse head, an 8cm cat head and a 16cm dog head. The dose distribution was calculated for a 4×4 mm 2microbeam array in each phantom, as well as a 16×16 mm 2array in the 8cm cat head, and a 32×32 mm 2array in the 16cm dog head. Microbeam widths of 25, 50 and 75 νm and center-to-center spacings of 100, 200 and 400 νm were considered. The metrics calculated for each simulation were the conventional PVDR, the peak-to-mean valley dose ratio (PMVDR), the mean dose and the percentage volume below a threshold dose. The PVDR ranged between 3 and 230 for the 2cm mouse phantom, and between 2 and 186 for the 16cm dog phantom depending on geometry. The corresponding ranges for the PMVDR were much smaller, being 249 (mouse) and 246 (dog), and showed a slightly weaker dependence on phantom size and array size. The ratio of the PMVDR to the PVDR varied from 0.21 to 0.79 for the different collimation configurations, indicating a difference between the geometric dependence on outcome that would be predicted by these two metrics. For unidirectional irradiation, the mean lesion dose was 102%, 79% and 42% of the mean skin dose for the 2cm mouse, 8cm cat and 16cm dog head phantoms, respectively. However, the mean lesion dose recovered to 83% of the mean skin dose in the 16cm dog phantom in intersecting cross-firing regions. The percentage volume below a 10% dose threshold was highly dependent on geometry, with ranges for the different collimation configurations of 287% and 3396% for the 2cm mouse and 16cm dog heads, respectively. The results of this study illustrate that different dose-volume metrics exhibit different functional dependences on MRT geometry parameters, and suggest that reliance on the PVDR as a predictor of therapeutic outcome may be insufficient.