Toward an improved assessment of dose conformity in radiotherapy.

Background: Evaluation of dose conformity is important to ensure minimum dose to normal tissue and sufficient dose coverage of the planning target volume (PTV). The existing conformity indices depend on the PTV volume and do not differentiate between two different scenarios: overdosing normal tissue and underdosing PTV.

Purpose: In this study, we introduce a novel index to assess conformity of dose distributions in radiotherapy.

Methods: The suggested conformity index is defined by the ratio of the volume representing actual "non-conformity" of the planned dose and the volume representing acceptable "non-conformity." The latter volume is produced by expanding the PTV. If both the average distance ( ) between the reference isodose surface and planning target volume and arbitrarily selected PTV expansion margin ( ) are much smaller than the size of the PTV, approximately equals the ratio . In this work, was utilized to analyze 90 cases of brain metastases treated with stereotactic radiation therapy (SRS) and 102 cases of lung cancer treated with stereotactic body radiation therapy (SBRT).

Results: For  = 0.1 cm, all considered SRS treatment plans were characterized by while 2 out of 102 SBRT plans had . The average values of for SRS and SBRT plans were 0.31 and 0.43, respectively. For  = 0.2 cm, all studied treatment plans had , and the average values of for SRS and SBRT plans were 0.15 and 0.25, respectively.

Conclusions: The suggested conformity index varies less with PTV volume than the RTOG and Riet-Paddick indices frequently used for evaluation of dose conformity. In addition, can be expressed as a sum of two terms which describe "over-coverage" and "under-coverage" of the treatment target. The results confirm that can be used for evaluation of dose conformity in SRS and SBRT.