Comparative assessment of skeletal uptake on Ga-68 PSMA PET/CT versus F-18 Fluoride PET/CT in metastatic prostate cancer

Objectives F-18 Fluoride PET/CT has been used for assessing metastases from various cancers, including prostate cancer. Recently, Ga-68 PSMA PET/CT has become the imaging modality of choice in the diagnosis and restaging of metastatic prostate cancer in certain European countries. In this study the uptake of Ga-68 PSMA and F-18 Fluoride in skeletal metastases of prostate cancer has been compared.

Methods Thirty patients (mean age: 68.8y) with skeletal metastases from prostate cancer undergoing Lu-177 PSMA-radioligand therapy (PRLT) had both Ga-68 PSMA PET/CT (mean radioactivity: 158.9 MBq) and F-18 Fluoride PET/CT (mean radioactivity: 259.6 MBq) for restaging 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 and SUVmax were included. A lesion was considered a ‘match’, when it had comparably similar uptake of both Ga-68 PSMA and F-18 Fluoride in identical lesions, and a ‘mismatch’ if a lesion was PSMA-positive and Fluoride-negative. Lesions with negative-PSMA and positive-Fluoride were considered as ‘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 the osteoblastic lesions on CT were positive on F-18 fluoride PET/CT; 53/196 osteoblastic lesions were Ga-68 PSMA negative. Out of the 37 F-18 fluoride negative lesions, 28 were localized in the bone marrow, and unable to be identified by CT. Nine osteolytic lesions were positive on Ga-68 PSMA, but not on F-18 fluoride PET/CT.

Conclusions This study demonstrates that in comparison to F-18 Fluoride PET/CT, Ga-68 PSMA PET/CT has a better ability to identify active bone metastases from prostate cancer. The lesions which showed uptake of F-18 Fluoride, but no uptake of Ga-68 PSMA, were related to other disease processes such as degenerative or inflammatory changes, possible Paget’s disease, reactive changes post previous post fracture, and possibly healed previous metastases. However, the possibility of F-18 Fluoride uptake of non-PSMA expressing metastases cannot be ruled out, except by follow-up studies.