LAUSR

PURPOSE We describe a new approach to evaluate conformity of dose distributions in radiotherapy. METHODS The suggested conformity factor λ is defined by using existing conformity indices and expansion of the planning target volume (PTV). If the average distance ( d ¯ $\bar d$ ) between the PTV and reference isodose surface as well as an arbitrarily selected PTV expansion margin ( d exp ${d_{\exp }}$ ) are both much smaller than the size of the PTV, then λ approximately equals the ratio d ¯ d exp $\frac{{\bar d}}{{{d_{\exp }}}}$ . We use λ to analyze several cases of stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT). RESULTS In the case of SRS with a single target or multiple targets, treatment plans produced with the help of volumetric modulated arc therapy (VMAT) have smaller λ than plans produced by using dynamic conformal arcs (DCA). Likewise, it is demonstrated that in the case of SBRT, λ is reduced by employing VMAT instead of DCA. It is also shown that if the distance between the reference isodose surface and surface of the PTV is fixed, λ varies less with variations in PTV volume compared to frequently used conformity indices. CONCLUSIONS The described conformity factor λ can be applied clinically to compare and rank treatment plans for lesions of different sizes. It is suggested that conditions λ < 1 $\lambda < 1$ and λ $\lambda $ > 1 can be employed as "pass" and "fail" criteria, respectively, for dose conformity assessment with appropriate choice of d exp ${d_{\exp }}$ . This article is protected by copyright. All rights reserved.