A modified homogeneity index for multi- target radiotherapy plans by subtracting high-dose subvolumes

Background and Purpose Considering the limitations of current homogeneity index (HI) in multi-target plans, we have redefined the volume (V) PTV in the formula for calculating the HI value of target volumes. This redefinition could provide a reference for clinical applications. Methods and Material Since the three widely used HI formulas may not fully capture dose homogeneity in multi-target treatment plans, we have redefined the V PTV in the calculation formulas, which may allow for a better assessment of HI for the multi-target radiotherapy plans. Fifteen clinically treated breast cancer (BC) with conserving surgery patients and fifteen nasopharyngeal carcinoma (NPC) patients were chosen. We calculated the V PTV with the name of V PTVpro and the HI pro1 , HI pro2 , HI pro3 . A paired, two-tailed non-parametric Wilcoxon signed-rank test was utilized to compare V PTVori and V PTVpro , HI ori1 and HI pro1 , HI ori2 and HI pro2 , HI ori3 and HI pro3 . The correlations between HI ori1 and HI pro1 , HI ori2 and HI pro2 , HI ori3 and HI pro3 were also analyzed. We conducted a questionnaire survey among a panel of six senior radiation oncologists and physicists to validate the clinical applicability of this study. Results For Planning Target Volume (PTV) of BC and PTV1 of NPC, the V PTVpro were significantly smaller than the V PTVori and HI pro were significantly lower than HI ori . The correlation coefficient values of HI ori1 -HI pro1 , HI ori2 -HI pro2 , and HI ori3 -HI pro3 showed variations, though all demonstrated positive correlations with statistically significant p-values <0.01. The results for these two target volumes revealed a consistent pattern: the HI ori1 -HI pro1 pair exhibited the highest correlation coefficient values, while HI ori2 -HI pro2 consistently showed the lowest values. Meanwhile, clinical validation by the expert panel found that the V PTVpro and HI pro in this study do not affect clinical plan selection. All experts agreed that, compared to V PTVori and HI ori , V PTVpro and HI pro better align with their clinical practice for evaluating multi-target radiotherapy plans. Conclusions The V PTVpro calculation subtracts the overlapping high-dose target volume from the low-dose target volume. This adjustment is intended to enable a more accurate assessment of dose HI specifically within the low-dose target. Additionally, the V PTVpro is also applicable for HI calculation in single-target volume. We suggest using the V PTVpro to calculate the HI of the target for multi-target plans such as BC with conserving surgery and NPC. Meanwhile, we recommend employing formula 1 for the calculation of HI among the three formulas.