Novel approach for the evaluation of dose conformity in radiotherapy.

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 ( ) between the PTV and reference isodose surface and an arbitrarily selected PTV expansion margin ( ) are both much smaller than the size of the PTV, then λ approximately equals the ratio . 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 and λ > 1 can be employed as "pass" and "fail" criteria, respectively, for dose conformity assessment with appropriate choice of .