Evaluating the biodistribution for [ 68 Ga]Ga-PSMA-11 and [ 18 F]F-PSMA-1007 PET/CT with an inter- and intrapatient based analysis.

BACKGROUND: Liver uptake in [68 Ga]Ga-PSMA-11 PET is used as an internal reference in addition to clinical parameters to select patients for [177 Lu]Lu-PSMA-617 radioligand therapy (RLT). Due to increased demand, [68 Ga]Ga-PSMA-11 was replaced by [18 F]F-PSMA-1007, a more lipophilic tracer with different biodistribution and splenic uptake was suggested as a new internal reference. We compared the intra-patient tracer distribution between [68 Ga]Ga-PSMA-11 and [18 F]F-PSMA-1007.

METHODS: Fifty patients who underwent PET examinations in two centers with both [18 F]F-PSMA-1007 and [68 Ga]Ga-PSMA-11 within one year were included. Mean standardized uptake values (SUVmean ) were obtained for liver, spleen, salivary glands, blood pool, and bone. Primary tumor, local recurrence, lymph node, bone or visceral metastasis were also assessed for intra- and inter-individual comparison.

RESULTS: Liver SUVmean was significantly higher with [18 F]F-PSMA-1007 (11.7 ± 3.9) compared to [68 Ga]Ga-PSMA-11 (5.4 ± 1.7, p < .05) as well as splenic SUVmean (11.2 ± 3.5 vs.8.1 ± 3.5, p < .05). The blood pool was comparable between the two scans. Malignant lesions did not show higher SUVmean on [18 F]F-PSMA-1007. Intra-individual comparison of liver uptake between the two scans showed a linear association for liver uptake with SUVmean [68 Ga]Ga-PSMA-11 = 0.33 x SUVmean [18 F]F-PSMA-1007 + 1.52 (r = .78, p < .001).

CONCLUSION: Comparing biodistribution of [68 Ga]Ga and [18 F]F tracers, liver uptake on [68 Ga]Ga-PSMA-11 PET is the most robust internal reference value. Liver uptake of [18 F]F-PSMA-1007 was significantly higher, but so was the splenic uptake. The strong intra-individual association of hepatic accumulation between the two scans may allow using of a conversion factor for [18 F]F-PSMA-1007 as a basis for RLT selection.