A new index: Triple Point Conformity Scale (CS3) and its implication in qualitative evaluation of radiotherapy plan

Background: Across the history of radiotherapy, with gradual technological progress and various methods of irradiation, the purpose has always been to deliver homogeneously 100% of the prescribed dose to 100% of the target volume containing the identifiable tumour and/or tumour cells potentially present while limiting the dose to adjacent normal tissues. Material and methods: The formula for triple point conformity scale is: CS3 = (V-95 + V-100 + V-105)/3V(T). (a) Lower limit determination: CS3 = (V-T + 0.93 V-T + 0.0)/3V(T) = 0.643; (b) Upper limit determination: in order to find out an empirical relation in between V-105 and V-T, we studied over 593 cancer patients of various sites by taking planning target volume as target, and an empirical relation is derived out as: V-105/V-T = 0.0007. Hence, CS3 = (V-T + V-T + 0.0007 V-T)/3V(T) = 0.6667 similar to 0.667. Result: Upper and lower limits of CS3 have been calculated at 0.643 and 0.667, respectively. Maximum value of CS3 index is recorded 0.656 while minimum value is 0.478. Discussion: The CS3 scale constitutes an attractive tool because it could facilitate decisions during analysis of various treatment plans proposed for conformal radiotherapy. Its major advantages are its simplicity and integration of multiple parameters. Conclusion: The triple point conformity scale (CS3) provides better qualitative information about radiotherapy plans as compared to other conformity indices. This study advises the users to use the CS3 scale to evaluate a conformal radiotherapy plan which encompasses a wide range of relevant clinical volumes, and is able to extract qualitative dosimetric information.