Significance of different conformity indices for evaluation of radiosurgery treatment plans for vestibular schwannomas

Abstract

Object. There are various kinds of conformity parameters currently in use, although several of them are limited and reflect only target volume coverage or normal tissue overdosage. Indices are reviewed with the goal of determining those that are most significant for the evaluation of radiosurgery treatment plans for patients with vestibular schwannoma, based on the authors' experience at the Novalis Shaped Beam Surgery Center. Methods. Fifty-five radiosurgery plans for patients with vestibular schwannomas (VSs) have been evaluated. In this paper the conformation number (CN) and dose-related CN (dCN) are evaluated, and a penalty for underdosed target volumes and overdosed normal tissue is incorporated. A strategy is discussed to apply these indices (CN and dCN) to define the optimal prescription isodose (PI). For a given radiosurgery treatment plan, permitting partial target underdosage may offer an improvement of the CN. Variations of different conformation indices have been calculated for varying prescription levels—for example, an isodose plan. The resulting graph for the CN is discussed in detail to illustrate its use in defining the optimal PI level. For the 55 cases of VSs reported on, the median CNmax result was 0.78. Conclusions. It is possible to achieve highly conformal dose distributions with Novalis radiosurgical system. The CN is the parameter of choice when evaluating radiosurgery treatment plans and scoring possible treatment plans. It takes into account both target underdosage and normal tissue overdosage and offers a valuable scoring parameter while avoiding false-perfect scores.