Discrepant longitudinal volumetric and metabolic evolution of diffuse intrinsic Pontine gliomas during treatment: implications for current response assessment strategies

Based on clinical observations, we hypothesized that in infiltrative high-grade brainstem neoplasms, such as diffuse intrinsic pontine glioma (DIPG), longitudinal metabolic evaluation of the tumor by magnetic resonance spectroscopy (MRS) may be more accurate than volumetric data for monitoring the tumor's biological evolution during standard treatment. We evaluated longitudinal MRS data and corresponding tumor volumes of 31 children with DIPG. We statistically analyzed correlations between tumor volume and ratios of Cho/NAA, Cho/Cr, and NAA/Cr at key time points during the course of the disease through the end of the progression-free survival period. By the end of RT, tumor volume had significantly decreased from the baseline (P < .0001) and remained decreased through the last available follow-up magnetic resonance imaging study (P = .007632). However, the metabolic profile of the tumor tissue (Cho/Cr, NAA/Cr, and Cho/NAA ratios) did not change significantly over time. Our data show that longitudinal tumor volume and metabolic profile changes are dissociated in patients with DIPG during progression-free survival. Volume changes, therefore, may not accurately reflect treatment-related changes in tumor burden. This study adds to the existing body of evidence that the value of conventional MRI metrics, including volumetric data, needs to be reevaluated critically and, in infiltrative tumors in particular, may not be useful as study end-points in clinical trials. We submit that advanced quantitative MRI data, including robust, MRS-based metabolic ratios and diffusion and perfusion metrics, may be better surrogate markers of key end-points in clinical trials.