Recommended dose voxel size and statistical uncertainty parameters for precision of Monte Carlo dose calculation in stereotactic radiotherapy

Abstract

Monte Carlo (MC)-based treatment planning requires a choice of dose voxel size (DVS) and statistical uncertainty (SU). These parameters effect both the precision of displayed dose distribution and time taken to complete a calculation. For efficient, accurate, and precise treatment planning in a clinical setting, optimal values should be selected. In this investigation, 30 volumetric modulated arc therapy (VMAT) stereotactic radiotherapy (SRT) treatment plans, 10 brain, 10 lung, and 10 spine were calculated in the Monaco 5.11.02 treatment planning system (TPS). Each plan was calculated with a DVS of 0.1 and 0.2 cm using SU values of 0.50%, 0.75%, 1.00%, 1.50%, and 2.00%, along with a ground truth calculation using a DVS of 0.1 cm and SU of 0.15%. The variance at each relative dose level was calculated for all SU settings to assess their relationship. The variation from the ground truth calculation for each DVS and SU combination was determined for a range of DVH metrics and plan quality indices along with the time taken to complete the calculations. Finally, the effect of defining the maximum dose using a volume of 0.035 cc was compared to 0.100 cc when considering DVS and SU settings. Changes in the DVS produced greater variations from the ground truth calculation than changes in SU across the values tested. Plan quality metrics and mean dose values showed less sensitivity to changes in SU than DVH metrics. From this study, it was concluded that while maintaining an average calculation time of <10 min, 75% of plans could be calculated with variations of <2.0% from their ground truth values when using an SU setting of 1.50% and a DVS of 0.1 cm in the case of brain or spine plans, and a 0.2 cm DVS in the case of lung plans.

Keywords: Monte Carlo; stereotactic radiotherapy; treatment planning.

Fig. 1

Calculated variance of the differences between the ground truth and a calculation with a given statistical uncertainty (SU) for all plans combined versus the relative dose of the interval of calculation. Each calculated variance is plotted at the mid‐point of the interval of consideration. The red dashed lines indicate the values of SU² for the SU settings investigated.

Fig. 2

The top row shows the variation in maximum dose, defined as 0.035 cc, from the ground truth when calculated using a 0.1 cm (left) or 0.2 cm (right) dose voxel size (DVS). The bottom row shows the variations for maximum dose, defined as 0.100 cc, calculated using a 0.1 cm (left) or 0.2 cm (right) DVS.

Fig. 3

The top row shows the relative variation in planning target volume (PTV) coverage from the ground truth plan for calculations completed using a 0.1 cm (left) and 0.2 cm (right) dose voxel size (DVS). The bottom row shows only plans calculated with a 0.1 cm DVS split by PTV ≤ 10 cc (left) and PTV > 10cc (right).

Fig. 4

The relative change in the maximum dose reported to the spinal cord and spinal cord planning risk volume contours. The maximum dose is defined as 0.035 cc for the 0.1 cm dose voxel size (DVS) plans (left) and 0.100 cc for the 0.2 cm DVS plans (right).

Fig. 5

The relative change in the mean gross target volume dose from the ground truth for calculations completed using a 0.1 cm dose voxel size (DVS) (left) and 0.2 cm DVS (right).

Fig. 6

The absolute change in the Paddick conformity index from ground truth for calculations completed using a 0.1 cm dose voxel size (DVS) (left) and 0.2 cm DVS (right).

Fig. 7

The absolute change in the Gradient Index from ground truth for calculations completed using a 0.1 cm dose voxel size (DVS) (left) and 0.2 cm DVS (right).