Reference tissue uptake of [18F]PSMA-1007 in positron emission tomography of recurrent prostate cancer.

Semi-quantitative uptake measurements are part of the PROMISE framework for PET/CT interpretation of tracers targeting the prostate-specific membrane antigen (PSMA), where tumor uptake is compared to uptake in reference tissues. [18F]PSMA-1007 has primarily hepatobiliary excretion and it is suggested to use spleen as reference tissue instead of liver. The aim of this study is to investigate the normal range and interpatient variability of [18F]PSMA-1007 uptake in reference tissue for prostate cancer patients, with particular focus on the splenic uptake. A total of 102 [18F]PSMA-1007 PET/CT scans of prostate cancer patients with biochemical recurrence (BCR) from three hospitals were included in the analysis. SUVmax and SUVmean measurements were performed in the blood pool, liver, spleen, and parotid glands. Results are reported as median with percentiles [5th, 95th]. For 15/102 (15%) and 20/102 (20%) of the patients, the SUVmax and SUVmean, respectively, were higher in the spleen than in the parotid glands. Median SUVmax of the reference tissues were 1.2 [0.6, 2.2] for blood, 15.8 [8.9, 26.2] for spleen, and 26.6 [14.5, 40.4] for the parotid glands. Median SUVmean were 11.9 [6.6, 20.7] for spleen and 19.3 [11.2, 32.1] for the parotid glands. In this multicenter cohort of BCR patients, [18F]PSMA-1007 reference tissue uptake is reported in accordance with the PROMISE protocol. A substantial number of patients present with higher uptake in the spleen than in parotid glands, for reasons that are currently unexplained. Care should be taken when interpreting these values until further studies are conducted. Question The normal range for [18F]PSMA-1007 uptake values in PROMISE reference tissues are not well documented, challenging standardized lesion categorization in prostate cancer PET interpretation. Findings In a multicenter cohort, splenic [18F]PSMA-1007 uptake exceeded parotid gland uptake in a notable subgroup (15%), leading to situations where PSMA expression scoring according to the PROMISE framework cannot be applied as intended. Clinical relevance Unexpectedly high splenic uptake relative to parotid gland uptake of [18F]PSMA-1007 in some patients may affect lesion categorization; careful interpretation is advised.