A novel index for assessing treatment plan quality in stereotactic radiosurgery

OBJECTIVE Stereotactic radiosurgery (SRS) is characterized by high levels of conformity and steep dose gradients from the periphery of the target to surrounding tissue. Clinical studies have backed up the importance of these factors through evidence of symptomatic complications. Available data suggest that there are threshold doses above which the risk of symptomatic radionecrosis increases with the volume irradiated. Therefore, radiosurgical treatment plans should be optimized by minimizing dose to the surrounding tissue while maximizing dose to the target volume. Several metrics have been proposed to quantify radiosurgical plan quality, but all present certain weaknesses. To overcome limitations of the currently used metrics, a novel metric is proposed, the efficiency index (eta(50%)), which is based on the principle of calculating integral doses: eta(50%) = integral doseTV/integral dose(PIV50%). METHODS The value of eta(50%) can be easily calculated by dividing the integral dose (mean dose x volume) to the target volume (TV) by the integral dose to the volume of 50% of the prescription isodose (PIV50%). Alternatively, differential dose-volume histograms (DVHs) of the TV and PIV50% can be used. The resulting eta(50%) value is effectively the proportion of energy within the PIV50% that falls into the target. This value has theoretical limits of 0 and 1, with 1 being perfect. The index combines conformity, gradient, and mean dose to the target into a single value. The value of eta(50%) was retrospectively calculated for 100 clinical SRS plans. RESULTS The value of eta(50%) for the 100 clinical SRS plans ranged from 37.7% to 58.0% with a mean value of 49.0%. This study also showed that the same principles used for the calculation of eta(50%) can be adapted to produce an index suitable for multiple-target plans (Gh(12Gy)). Furthermore, the authors present another adaptation of the index that may play a role in plan optimization by calculating and minimizing the proportion of energy delivered to surrounding organs at risk (OAR eta(50%)). CONCLUSIONS The proposed efficiency index is a novel approach in quantifying plan quality by combining conformity, gradient, and mean dose into a single value. It quantifies the ratio of the dose doing good versus the dose doing harm, and its adaptations can be used for multiple-target plan optimization and OAR sparing.