Fig. 5.

(a) Illustration of the effect of PTV margin reduction on nearby critical structures in a meningioma patient. Shown on the axial T1 planning MRI are the contours for the CTV (orange), brainstem (cyan), and PTVs, with 2, 3, and 4 mm isotropic margins (increasing brightness in yellow), respectively. In this patient, relative to a 4 mm PTV, margins of 3 and 2 mm reduce the proportion of the brainstem overlapped by the PTV by 5.4 % and 10.8 %, respectively. (b) Quantification of volumetric improvements from PTV margin reduction across all patients. The top group summarizes the reduction in PTV volume relative to a 4 mm PTV; for example, a 2 mm margin reduces the PTV volume by a range of 11.9–42.7 % relative to a 4 mm margin. The other groups describe the reduction in PTV overlap of critical structures. The PTV of 32 patients, for example, overlaps the optic chiasm. Among this patient subgroup, reducing the PTV margin from 4 to 2 mm reduces the proportion of the optic chiasm overlapped by the PTV by a range of 0.0–35.9 %. In all groups, the boxplots delineate the median (vertical line), 25th and 75th percentiles (box edges), most extreme non-outlier values (whiskers), and outliers greater than 150 % of the interquartile range from the box edge (circles). Blue and orange boxplots quantify PTV margin reduction from 4 mm to 3 and 2 mm, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)