[ 18F]Fluoromethylcholine PET/CT for CNS lymphoma assessment: a new tool

Background Central nervous system (CNS) lymphomas are a rare subset of lymphoma, which are associated with a poor outcome. The gold standard for CNS imaging is with gadolinium-enhanced magnetic resonance imaging (MRI); however, there are a number of limitations, including some patients with small persistent abnormalities from scarring due to focal haemorrhage or from a previous biopsy, which can be difficult to discern from residual tumour. [ 18 F]Fluoromethylcholine positron emission tomography–computed tomography (FCH-PET/CT) uses an analogue of choline, which due to the upregulation of choline kinase in tumour cells, allows increased uptake of FCH. As there is minimal background grey matter uptake of FCH, FCH-PET/CT can be used in CNS imaging and provide a useful tool for response assessment. Methods This is a cohort study, where we identified 40 patients with a diagnosis of primary or secondary CNS lymphoma between 1 st November 2011 and 10 th October 2019. Results 26 of the 40 patients (65%) had concordant results. Of the discordant results, 11 out of 14 had partial response (PR) on MRI but showed a metabolic complete response (mCR) on FCH-PET. The overall response rates (ORR) were similar between the two modalities (90% for MRI versus 95% with FCT-PET/CT). Conclusion We conclude that FCH-PET/CT is a reasonable alternative mode of imaging to gadolinium-enhanced MRI brain imaging, providing a new tool for assessment of CNS lymphoma.