ABSTRACT Dose conformity and steepness of dose fall-off at a target volume’s boundary are important quality criteria in treatment planning to predict complication rates in normal tissue. The clinical validity of correlations between a dose gradient measure and treatment complications are not proved for all but one of the common dose gradient measures. The author demonstrates that more-dimensional dose gradient measures exactly describe local dose gradient distributions towards organs at risk. As a result, incidences of radiogenic toxicities in normal tissue correlate on the dose gradient measures in a natural manner. Anisotropic dose gradient problems will be described by the superficially averaged dose gradient (SADG). The degrees of anisotropy can be quantified by a novel geometrical metric called the relative ellipsoidality (E). The correlations between the SADG and dose-volume metrics for normal tissue were investigated for the linac-based stereotactic radiotherapy of 25 malignant choroidal melanomas. The ratios of the mean values of the local SADG to the mean value of the global SADG were in a range of 1.2 to 1.7. The correlation between the volume of normal tissue receiving at least 10 Gy, V10 Gy, and the global SADG was strong: Pearson’s correlation coefficient r was 0.833 (p = 0.000). The mean dose values within the ipsilateral lacrimal glands correlated with r ≥ 0.870 (p = 0.000) on the local SADG. The relative ellipsoidalities of the isodoses of interest were in a range of 1/27.3 to 1/2.1. More-dimensional dose gradient measures are urgently required for lesions that are located in non-homogeneous normal tissue. Only exact descriptions of the dose gradient distributions enable reliable prognoses of treatment complications. The parameter E is a measure of the anisotropy of dose gradient distributions and allows the estimation of the quality of dose gradient indices with respect to the description of local dose gradients and prediction of complication rates.
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