Feasibility of dual tracer positron emission tomography using background subtraction in prostate cancer patients

Abstract

Introduction Positron emission tomography (PET) with two tracers is a novel medical imaging technique which has advantages such as shortening the duration of scans, however PET cannot explicitly distinguish between two tracers as both signals stem from 511 keV photon pairs. The aims of this thesis were to (1) determine the behaviour of [18]F - Fluoroethylcholine (FEC) within the prostate in prostate cancer patients for the time interval of 48 minutes to 71 minutes post injection for patients with prostate cancer and (2) to analyse the feasibility of dual tracer PET using FEC and [68]Ga labelled prostate-specific membrane antigen (PSMA). Methods Two retrospective studies based on PET data were analysed in prostate cancer patients. The first study analysed the stability of FEC by comparing the activity of FEC at two time points (48 and 71 minutes) for 6 patients. The second study involved a static PET acquisition of FEC followed by an injection and dynamic acquisition of PSMA. Signal separation was done by background subtraction, with the assumption (supported by the results from the first study) that FEC remained stable, to obtain values for the estimated PSMA activity. Kinetic modeling was performed to compare obtained parameters between the estimated PSMA activities and one patient who was only administered PSMA. Results The percent change in quantitative values for the whole prostate varied less than 10 % for 4 of the 6 patients in the first study. No significant wash out or wash in was observed. In the second study, the irreversible two-tissue compartment model was the best fit kinetic model for PSMA. The influx parameter Ki for all patients had the same order of magnitude for each patient with the value ranging from 0.01 to 0.08 1/min. Conclusions The findings in literature and results from the analysis of the choline stability study suggest that FEC remains trapped within cells, however, a full dynamic PET acquisition of FEC activity is advised. Estimated PSMA activity showed similar values for kinetic parameters with the PSMA only patient, however, issues representing true quantification were discussed as there exists a trade off between the benefits of acquired data with multiple tracer PET and quantification.