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

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.

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.

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.

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.