Abstract
Purpose: In dose‐painting by numbers, conventional plan quality indices such as homogeneity index (HI) and dose‐standard‐deviation (STD) have limitations in application. This study aims to introduce new indices for evaluating treatment plans of this radiotherapy technique. Methods: Deviation‐from‐prescription‐index (DFPI) was formulated as the volume‐weighted average of the deviation between prescription dose and planned dose. Two sub‐indices, index‐of‐hotness (IOH) and index‐of‐coldness (IOC) were also defined specifically for selected voxels of which planned dose was higher and lower than the prescribed dose, respectively. The new indices were first tested for 9 IMRT/VMAT plans having 3 different dose prescription levels. For comparison purpose, 4 conventional indices (three types of HI and STD) and difference between V98 and V110 [V110 + (100− V98)] were calculated in each prescription region, and then averaged. Using the Spearman rank correlation test, the relationship was investigated between the DFPI‐based ranks and the composite ranks based on all of 5 conventional indices. The new indices were also evaluated in 9 theoretically‐generated plans having a continuously varying function of dose prescription where the conventional indices are not applicable. Results: The plan quality ranks according to DFPI were found to be highly correlated with that from the composite of conventional indices (correlation coefficient = 0.95, where 1 means perfect correlation). It was also demonstrated that the new indices are applicable to dose painted plans in a continuously varying dose prescription. Conclusion: It was shown that the new plan quality indices formulated with voxel‐based prescription dose can be appropriately used for dose‐painting radiotherapy. The indices allow for a single‐value evaluation regardless of the number of prescribed dose levels, which cannot be achieved using the conventional indices.
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