Purpose: To classify the available plan evaluation indices and compare the dosimetric suitability of these indices.
Materials and methods: Available published plan evaluation indices were categorized. Conformity index (CI) into two groups, one group contains those CI formulas which do not consider critical structure and other group contains those CI formulas which consider planning target volume (PTV) coverage, normal tissue and critical structure sparing simultaneously. Various homogeneity index (HI) formulas extracted from literature. Structure data sets of 25 patients were taken under consideration comprising of various sites. For each patient, two plans were created using Volumetric Arc Therapy technique. First type of plan (Plan-A) were generated considering all tissue objectives for targets and Organ at Risks (OARs) whereas second type of plan (Plan-B) were generated considering only targets tissue objectives and excluding OARs tissue objectives during plan optimization and dose calculation. Planning evaluation parameters were compared between Plan-A and Plan-B.
Results: CI calculated by various formulas in two different scenarios presented <2% variation. Any commonly used CI formula failed to differentiate the two different planning situations. On comparison between HI of two different scenario, it is observed that there are four formulas of HI which showed negligible variation but two formulae: S-index and HI (D) showed marginal variation. It is also observed that when OARs are removed from optimization dose homogeneity improved which is specifically pointed by sigma index formula.
Conclusion: CI, which has assimilated the presence of OAR in their formulation, shows more reliability in plan evaluation. Sigma index was found to be more efficient formula while evaluating homogeneity of a treatment plan.
Keywords: Conformity index; homogeneity index; planning target volume.