Focal Unspecific Bone Uptake on [¹⁸F]PSMA-1007 PET: Evaluation Analog PROMISE Criteria and Validation via PET/CT Follow-Up

Background: Focal unspecific bone uptake (UBU) is common in [¹⁸F]PSMA-1007 PET/CT, yet its clinical significance remains unclear, causing uncertainty in treatment decisions. Material and Methods: We retrospectively analyzed 99 prostate cancer patients (age 69 ± 7) who underwent [¹⁸F]PSMA-1007 PET/CT scans (3 MBq/kg; uptake time 70 ± 14 min) for staging and follow-up (after 13.0 ± 7.2 months). Semiquantitative assessment using the miPSMA score, analogous to the PROMISE criteria, evaluated the prevalence of UBU and bone metastases. Results: In the initial PET/CT scan, 56 patients had 230 lesions classified as UBU. A total of 19 patients were found to have bone metastases and UBU, while 24 patients had no focal bone uptake. UBU distribution was as follows: ribs (50%), spine (30%), pelvis (15%), and other sites (5%). There were no significant differences in age, Gleason score, injected tracer dose, uptake time, SUV_peak of UBU, or SUV_mean in the spleen and parotid gland between patients with and without UBU. Follow-up showed stable miPSMA-score and CT appearance in 44/56 patients with UBU (79%), minor changes in 5/56 patients (8%), and new bone metastases in 7/56 patients (12%). Patient-specific analysis indicated at least one bone metastasis initially classified as UBU in 3/56 patients (5%) and new bone metastases in 4/56 patients (7%). In total, 4 of the 24 patients (17%) without initial focal uptake developed osseous metastases at follow-up. Conclusions: No significant differences were found between patients with or without UBU. Only a small portion of UBU (2%) evolved into metastases, a lower rate than the development of new osseous metastases, which appears to be independent of UBU.