Abstract 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 (dˉ$bar d$) between the PTV and reference isodose surface and an arbitrarily selected PTV expansion margin (dexp${d_{exp}}$) are both much smaller than the size of the PTV, then λ approximately equals the ratio dˉdexp$frac{{bar d}}{{{d_{exp}}}}$. 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 can be employed as “pass” and “fail” criteria, respectively, for dose conformity assessment with appropriate choice of dexp${d_{exp}}$.
展开▼
