Background 99mTc–prostate-specific membrane antigen (PSMA) SPECT/CT is less expensive and readily available modality compared with 68Ga-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 99mTc-PSMA SPECT/CT and 68Ga-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 68Ga-PSMA. 99mTc-PSMA SPECT/CT was performed 3 hours after IV injection of 555 to 740 MBq (15–20 mCi) of 99mTc-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 99mTc-PSMA SPECT/CT and 68Ga-PSMA PET/CT scans were positive in 95.45% (21/22). In region-based evaluation, 68Ga-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 99mTc-PSMA SPECT/CT. Most of these differences could be explained by lower detection rate of 99mTc-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 99mTc-PSMA SPECT/CT could be a potential substitute for 68Ga-PSMA PET/CT in high-risk patients, except when evaluation of prostate bed is of major concern.

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