GRM1 and neoplasm: However, the high accumulation and slow clearance from the brain reduce the selectivity of this tracer for identifying tumours from healthy brain tissues to non-targeted organs and could cause radiation-induced damage.206 In 2015, the authors designed three mGluR1-targeting PET tracers: [11C]4-6, which showed comparable tumour uptake rates (∼4.0–4.2%ID/g at 60 min).