Target definition for malignant gliomas: no difference in radiation treatment volumes between 1.5T and 3T magnetic resonance imaging.

Purpose: Currently, most high-grade glioma patients undergo a 1.5T brain magnetic resonance (MR) for radiation treatment planning. We hypothesized that 3T MR imaging (MRI) scanning is superior to 1.5T due to higher signal-to-noise ratio (SNR), and thus will result in more accurate quantification of tumor volumes. The purpose of this prospective study was to determine differences in radiation planning volumes for high-grade gliomas when scanned on 3T MR versus 1.5T MR.

Methods And Materials: In this prospective controlled trial, 23 patients with high-grade gliomas underwent brain MRI scanning in both 1.5T and 3T field strengths within a 24-hour period; no steroids or treatment changes were made in-between scans. After 3 investigators contoured the T2 fast low-angle inversion recovery (FLAIR) abnormality (gross tumor volumes or [GTV]) for all patients, clinical target volume (CTV) and planning treatment volumes (PTV) were defined. Calculations by an independent investigator included volumes, standard deviations, SNRs, and contrast-to-noise ratios (CNRs); statistical analysis was performed on raw data.

Results: Planning treatment volume ratios (3T:1.5T) for each investigator were 0.95 ± 0.12 (range, 0.64-1.10), 0.98 ± 0.10 (range, 0.64-1.16), and 0.99 ± 0.06 (range, 0.86-1.13). By paired 2-tailed t test, these volumes were not statistically different (P = .051), although there is a trend to 3T producing smaller volumes than 1.5T. Dice similarity coefficients were 0.90 ± 0.05, 0.90 ± 0.06, and 0.91 ± 0.05 for the investigators.

Conclusions: Planning target volumes for high-grade gliomas were similar at 3T and 1.5T MR using our standard imaging protocols. However, in some patients, the 3T MR may reveal substantially smaller tumor volume due to inferior conspicuity of the lesion. These findings imply that while overall the radiation target volumes are comparable, there are differences in CNR and SNR that lead to differences in individual patients. The 1.5T may be better for gaining conspicuity of the tumor.