Use of dose-area product to assess plan quality in robotic radiosurgery

In robotic stereotactic radiosurgery (SRS), optimal selection of collimators from a set of fixed cones must be determined manually by trial and error. A unique and uniformly scaled metric to characterize plan quality could help identify Pareto-efficient treatment plans. The concept of dose-area product (DAP) was used to define a measure (DAP${}_{ratio}$) of the targeting efficiency of a set of beams by relating the integral DAP of the beams to the mean dose achieved in the target volume. In a retrospective study of five clinical cases of brain metastases with representative target volumes (range: 0.5-5.68 ml) and 121 treatment plans with all possible collimator choices, the DAP${}_{ratio}$ was determined along with other plan metrics (conformity index CI, gradient index R50%, treatment time, total number of monitor units TotalMU, radiotoxicity index f12, and energy efficiency index η50%), and the respective Spearman's rank correlation coefficients were calculated. The ability of DAP${}_{ratio}$ to determine Pareto efficiency for collimator selection at DAP${}_{ratio}$< 1 and DAP${}_{ratio}$< 0.9 was tested using scatter plots. The DAP${}_{ratio}$ for all plans was on average 0.95 ± 0.13 (range: 0.61-1.31). Only the variance of the DAP${}_{ratio}$ was strongly dependent on the number of collimators. For each target, there was a strong or very strong correlation of DAP${}_{ratio}$ with all other metrics of plan quality. Only for R50% and η50% was there a moderate correlation with DAP${}_{ratio}$ for the plans of all targets combined, as R50% and η50% strongly depended on target size. Optimal treatment plans with CI, R50%, f12, and η50% close to 1 were clearly associated with DAP${}_{ratio}$< 1, and plans with DAP${}_{ratio}$< 0.9 were even superior, but at the cost of longer treatment times and higher total monitor units. The newly defined DAP${}_{ratio}$ has been demonstrated to be a metric that characterizes the target efficiency of a set of beams in robotic SRS in one single and uniformly scaled number. A DAP${}_{ratio}$< 1 indicates Pareto efficiency. The trade-off between plan quality on the one hand and short treatment time or low total monitor units on the other hand is also represented by DAP${}_{ratio}$.