Gross target volume contouring in canine extra-axial brain tumors: Effects of magnetic resonance image slice thickness and time between subsequent image sets.

Accurate determination of the gross target volume (GTV) is critical in radiation treatment planning, as errors could result in underdosing of the tumor or overdosing of nearby organs at risk. This multicenter retrospective observational serial measurement study evaluated the effects of variations in MRI slice thickness and a time delay between the diagnostic (MRI-1) and RT planning (MRI-2) MRIs GTV contouring in dogs with presumed meningiomas. The hypothesis was that the GTV would increase in size with time on T1-weighted sequences with contrast. Inclusion required paired MRI acquisition within 3 months. The GTV was contoured on each MRI. Forty-six dogs were included. Slice thickness was significantly different (P < .001) between MRIs: MRI-1 had a median of 3.9 mm (range: 0.8-6 mm; only two dogs <2 mm), and MRI-2 had a median of 0.9 mm (range: 0.6-4.5 mm; only two dogs >2 mm). The median time between MRIs was 22 days (range: 8-74 days). The MRI-1 GTV was significantly different from MRI-2 GTV (P < .0001); thirty (65%) were larger, five were equal in size, and 12 were smaller than the MRI-2 GTV. This difference in GTV is likely due to the slice thickness differences between MRI acquisitions rather than changes in tumor size due to the short time interval between MRI-1 and MRI-2. This finding highlights the differences between diagnostic and RT treatment-planning MRIs. For brain tumor target contouring, an MRI at the same time as the RT planning CT with <1 mm slice thickness, 3D acquisitions, and anisotropic voxel is recommended.