Response assessment of anti-angiogenic therapy in recurrent high grade gliomas by molecular MR- and Pet- imaging

Bevacizumab, a humanized monoclonal antibody inhibiting the biological activity of VEGF, is successfully used as an anti-angiogenic agent in recurrent high grade glioma (rHGG) treatment. Anti-angiogenic therapy, however, causes difficulties in distinguishing between anti-vascular and true anti-tumor effects when using standard MRI criteria, a so-called "pseudo-response". The aim of this PhD thesis was (1) to evaluate functional and molecular neuroimaging response parameters during anti-angiogenic therapy in rHGG patients and (2) to compare two PET tracers to identify the most promising tracer for further neuroimaging of rHGG with respect to future use during anti-angiogenic treatment. In two independent studies, ADC (apparent diffusion coefficient) maps derived from diffusion-weighted MR images as well as FET-PET (18F-fluoro-ethyl-tyrosine) scans were analysed in rHGG patients at onset of anti-angiogenic treatment and at eight weeks follow-up. ADC histograms demonstrated significant changes in skewness in relation to progression-free survival (PFS6) at six months. Patients with increasing skewness following anti-angiogenic therapy had significantly shorter PFS than did patients with decreasing or stable skewness values. In the second study, MR scans according to RANO criteria (Response assessment in neurooncology) and FET-PET scans independently were able to predict PFS6. In four patients FET-PET was able to detect tumor progression earlier than MRI. In a third study we compared FET and FLT-PET (18F-fluoro-deoxy-thymidine) intra-individually and found that the sensitivity for detecting a HGG was higher for FET than for FLT. FET detected biologically active tumor even beyond the borders of contrast enhancement (CE) in T1 and FLT uptake was absent in tumors with no or moderate CE. In rHGG patients undergoing anti-angiogenic treatment, ADC maps and FET-PET, are predictive of treatment response. They contribute important information to response assessment based solely on MRI and RANO criteria. FET has a higher sensitivity than FLT in detecting HGG and shows tumor beyond the borders of contrast enhancement in MRI. (author)