99mTc-PSMA SPECT/CT Versus 68Ga-PSMA PET/CT in the Evaluation of Metastatic Prostate Cancer.

BACKGROUND Tc-prostate-specific membrane antigen (PSMA) SPECT/CT is less expensive and readily available modality compared with Ga-PSMA PET/CT for imaging prostate cancer (PC). The aim of this study is to compare the value of these 2 modalities in patients confirmed or suspicious to have metastatic prostate cancer. PATIENTS AND METHODS Twenty-two patients with the mean age of 66.6 ± 10.1 years were studied using Tc-PSMA SPECT/CT and Ga-PSMA PET/CT, with less than 7 days interval between the 2 imaging procedures. Whole-body PET/CT was done 60 minutes after IV injection of 185 MBq (5 mCi) of Ga-PSMA. Tc-PSMA SPECT/CT was performed 3 hours after IV injection of 555 to 740 MBq (15-20 mCi) of Tc-PSMA. The images of each modality were interpreted independently, and the results were compared according to patient-based as well as region-based analyses. RESULTS In patient-based evaluation, both Tc-PSMA SPECT/CT and Ga-PSMA PET/CT scans were positive in 95.45% (21/22). In region-based evaluation, Ga-PSMA PET/CT detected 53 regions (median of 2 regions per patient; range, 0-5), whereas 43 (median of 2 regions per patient; range, 0-5) were detected by Tc-PSMA SPECT/CT. Most of these differences could be explained by lower detection rate of Tc-PSMA SPECT/CT in prostate bed (n = 6). PET/CT detected more involved regions than SPECT/CT (P = 0.007), whereas similar frequency of extraprostatic lesions were diagnosed in both modalities (P = 0.102). Significant correlation was also demonstrated between serum prostate-specific antigen level and imaging parameters of disease extension detected by 2 modalities. CONCLUSIONS Tc-PSMA SPECT/CT could be a potential substitute for Ga-PSMA PET/CT in high-risk patients, except when evaluation of prostate bed is of major concern.