New conformity indices based on the calculation of distances between the target volume and the volume of reference isodose

OBJECTIVE: To present conformity indices (CIs) based on the distance differences between the target volume (TV) and the volume of reference isodose (VRI). METHODS: The points on the three-dimensional surfaces of the TV and the VRI were generated. Then, the averaged distances between the points on the TV and the VRI were calculated (CIdistance). The performance of the presented CIs were evaluated by analysing six situations, which were a perfect match, an expansion and a reduction of the distance from the centroid to the VRI compared with the distance from the centroid to the TV by 10%, a lateral shift of the VRI by 3 cm, a rotation of the VRI by 45° and a spherical-shaped VRI having the same volume as the TV. The presented CIs were applied to the clinical prostate and head and neck (H&N) plans. RESULTS: For the perfect match, CIdistance was 0 with 0 as the standard deviation (SD). When expanding and reducing, CIdistance was 10 and -10 with SDs <1.3, respectively. With shifting and rotating of the VRI, the CIdistance was almost 0 with SDs >11. The average value of the CIdistance in the prostate and H&N plans was 0.13 ± 7.44 and 6.04 ± 23.27, respectively. CONCLUSION: The performance of the CIdistance was equal or better than those of the conventional CIs. ADVANCES IN KNOWLEDGE: The evaluation of target conformity by the distances between the surface of the TV and the VRI could be more accurate than evaluation with volume information.