A novel index for assessing radiosurgery treatment plan quality.
One of the hallmarks of intracranial radiosurgery is a steep dose gradient from the periphery of the target into surrounding brain. Clinical studies have consistently backed up the importance of steep dose fall off through evidence from symptomatic complications (Flickinger, Korytko, Blonigen, Minniti). The available data suggests that there are threshold doses, above which, the risk of symptomatic radionecrosis increases with volume irradiated. It therefore makes sense to concentrate on limiting doses above these thresholds, ignoring lower doses that may be clinically irrelevant.
Several metrics have been proposed to quantify dose fall off. The Gradient Index (GI) (Paddick and Lippitz) remains the most commonly used metric, serving as a practical volumetric assessment of dose fall off. Upon the formulation of the GI the authors identified the limitation that this metric is not suitable for comparing plans of incongruent conformity. In order to overcome this limitation, Thomas et al proposed the AUC metric, as an alternative for comparative plan evaluation. The AUC metric is the integral area under the dose-volume histogram (DVH) between the 50% of the prescription dose (PD) and the prescription isodose (PI). This metric provides a useful dose-volume product (Energy in Joules) that quantifies dose fall off outside the target, which the authors have previously used as a predictor for normal tissue complications. This metric, however, still suffers from dependence to conformity and prescription dose whilst not accounting for dose deposition inside the target.
To overcome limitations of currently used metrics, we propose a novel metric, the Efficiency Index (EI), based on the same principle of integrating areas under differential DVHs:
EI=DminTV∫DmaxTV*dose/50%PD∫DmaxV*dose
where DminTV is the minimum dose in the target, Dmax is the maximum dose, PD is prescription dose, TV is target volume and V is the volume occupied by the 50%PD isodose line.
The EI can be easily calculated using differential DVHs of the TV and of volume V. The value is effectively the proportion of energy deposited inside the target within the 50% of PD isodose line. It has theoretical limits of 0 and 1, with 1 being perfect. It combines conformity, gradient and a high mean dose to the target into a single value.
The EI has been calculated for 40 clinical SRS plans (mean TV of 3.8 cc) with a GI range of 2.49 – 3.03 and a mean of 2.74. The calculated EI values ranged from 0.403-0.551 with a mean of 0.496.
