Background: The purpose of this study was to investigate the imaging properties of pulmonary metastases and benign opacities in Ga-68-PSMA positron emission tomography (PET) in patients with prostate cancer (PC). Methods: Ga-68-PSMA-PET/CT scans of 739 PC patients available in our database were evaluated retrospectively for lung metastases and non-solid focal pulmonary opacities. Maximum standardized uptake values (SUVmax) were assessed by two- and three-dimensional regions of interest (2D/3D ROI). Additionally CT features of the lesions, such as location, morphology and size were identified. Results: Ninety-one pulmonary metastases and fourteen opacities were identified in 34 PC patients. In total, 66 PSMA-positive (72.5%) and 25 PSMA-negative (27.5%) metastases were identified. The mean SUVmax of pulmonary opacities was 2.2 +/- 0.7 in 2D ROI and 2.4 +/- 0.8 in 3D ROI. The mean SUVmax of PSMA-positive pulmonary metastases was 4.5 +/- 2.7 in 2D ROI and in 4.7 +/- 2.9 in 3D ROI; this was significantly higher than the SUVmax of pulmonary opacities in both 2D and 3D ROI (p<0.001). The mean SUVmax of PSMA-negative metastases was 1.0 +/- 0.5 in 2D ROI and 1.0 +/- 0.4 in 3D ROI, and significantly lower than that of the pulmonary opacities (p<0.001). A significant (p<0.05) weak linear correlation between size and 3D SUVmax in lung metastases (rho(Spearman)=0.207) was found. Conclusion: Based on the SUVmax in Ga-68-PSMA-PET alone, it was not possible to differentiate between pulmonary metastases and pulmonary opacities. The majority of lung metastases highly overexpressed PSMA, while a relevant number of metastases were PSMA-negative. Pulmonary opacities demonstrated a moderate tracer uptake, significantly lower than PSMA-positive lung metastases, yet significantly higher than PSMA-negative metastases.
