Quantitative T1 and T2 mapping in recurrent glioblastomas under bevacizumab: earlier detection of tumor progression compared to conventional MRI

Treatment with the humanized anti-vascular endothelial growth factor (VEGF) antibody bevacizumab in glioblastoma patients suppresses contrast enhancement via the reduction of vascular permeability, which does not necessarily indicate real reduction of tumor cell mass. Therefore, other imaging criteria are needed to recognize tumor growth under bevacizumab more reliably. It is still unknown, whether quantitative T1 mapping is useful to monitor the effects of anti-angiogenic therapy or to indicate a tumor progression earlier and more reliable compared to conventional magnetic resonance imaging (MRI) sequences. This raised the question whether quantitative T1 mapping is more suitable to monitor treatment effects of bevacizumab. Conventional and quantitative MRI was performed on six consecutive patients with recurrent glioblastoma before treatment with bevacizumab and every 8 weeks thereafter until further tumor progression. Quantitative T1 maps before and after intravenous application of contrast agent and quantitative T2 maps were performed to calculate serial differential maps and subtraction maps from one time point, subtracting contrast-enhanced T1 maps from non-contrast T1 maps. In five illustrative cases, tumor progression was documented earlier in differential T1 relaxation time (DiffT1) and T2 relaxation time (DiffT2) maps before changes in the conventional MRI studies were obvious. Four patients showed previous prolongation of T1 relaxation time in the DiffT1 maps, suggesting tumor progression, and subtraction maps revealed faint contrast enhancement matching with the areas of T1 prolongation. Our results emphasize that quantitative relaxation time mapping could be a promising method for tumor monitoring in glioblastoma patients under anti-angiogenic therapy. Quantitative T1 mapping seems to detect enhancing tumor earlier than conventional contrast-enhanced T1-weighted images.