OBJECTIVES: ¹⁸F-Fluoride PET/CT has been used for the detection of metastases from prostate cancer. In certain European countries, ⁶⁸Ga-PSMA PET/CT is being increasingly utilized for the diagnosis and restaging of prostate cancer. In this study, we compared the uptake of ⁶⁸Ga-PSMA and ¹⁸F-Fluoride in skeletal metastases from prostate cancer.

METHODS: Thirty patients with skeletal metastases from prostate cancer had both ⁶⁸Ga-PSMA and ¹⁸F-Fluoride PET/CT using Biograph mCT Flow 64 PET/CT, within a period of three months. All lesions were counted and SUVmax measured. Where disseminated metastases were observed, at least 5 lesions with distinguishably higher uptake (SUVmax) were included. A lesion was considered ‘match’, when it had comparable uptake of both ⁶⁸Ga-PSMA and ¹⁸F-Fluoride in identical lesions, and ‘mismatch’ if a lesion was PSMA-positive and Fluoride-negative. Lesions with negative-PSMA and positive-Fluoride were considered ‘reverse mismatch’.

RESULTS: A total of 196 lesions were analyzed. Match was found in 106, mismatch in 37, and reverse mismatch in 53 lesions. All osteoblastic lesions on CT were positive on ¹⁸F-Fluoride PET; 53/196 osteoblastic lesions were negative on ⁶⁸Ga-PSMA PET. Out of the 37 ¹⁸F-Fluoride negative lesions, 28 were localized in the bone marrow, and were also not detected by CT. Nine osteolytic lesions were positive on ⁶⁸Ga-PSMA PET, but not on ¹⁸F-Fluoride PET.

CONCLUSION: This study demonstrates that in comparison to ¹⁸F-Fluoride PET, ⁶⁸Ga-PSMA PET has a better ability to identify active bone metastases from prostate cancer. The lesions which showed uptake of ¹⁸F-Fluoride, but no ⁶⁸Ga-PSMA expression, were mostly related to other disease processes with increased bone metabolism, such as degenerative or inflammatory changes, suspected Paget’s disease, non-union fracture, orthopedic surgery, bone infarction, and possibly healed previous metastases. The probability of ¹⁸F-Fluoride uptake of non-PSMA expressing metastases cannot be ruled out. For such cases, ¹⁷⁷Lu labeled bisphosphonate radionuclide therapy could be beneficial.