Comment to: Parametric Response Map as an Imaging Biomarker to Distinguish Progression from Pseudoprogression in High-Grade Glioma: Pitfalls in Perfusion MRI in Brain Tumors

Pseudoprogression is a recently recognized phenomenon which occurs mainly within the first 3 months after resection and radiochemotherapy of glioblastomas in up to 30% of cases. It refers to contrast enhancement with or without clinical deterioration which spontaneously resolves and which is not regarded as being related to tumor growth. Even with advanced imaging techniques it is not yet possible to distinguish reliably between true tumor progression and pseudoprogression in patients with high-grade glioma who received radiochemotherapy. Tsien et al. performed magnetic resonance imaging (MRI) dynamic susceptibility contrast (DSC) perfusion on 27 patients with high-grade glioma before radiochemotherapy and at week 1 and 3 during treatment. Clinical and conventional magnetic resonance (MR) parameters, including average percent change of cerebral blood volume (CBV) and cerebral blood flow (CBF), were evaluated as potential predictors of pseudoprogression and progression, respectively. In addition, a new postprocessing tool called parametric response map, which uses a voxel-by-voxel method of image analysis, was used to evaluate CBV and CBF. Given significant tumor heterogeneity after radiochemotherapy, Tsien et al. hypothesized that a voxel approach may be more sensitive than referring to the mean tumor average of CBV and CBF as it is done in standard whole-tumor statistical methods. Using the “parametric response map”, Tsien et al. found a reduction in CBV at 3 weeks after therapy to be a potential biomarker for true tumor progression, whereas all other parameters failed to differentiate between progression and pseudoprogression.