Tc-99m-PSMA SPECT/CT Versus Ga-68-PSMA PET/CT in the Evaluation of Metastatic Prostate Cancer
January 22, 2022 — According to a study published in the journal of Clinical Nuclear Medicine, researchers using radiopharmaceutical products of Pars Isotope Co. have shown that Tc-99m-Prostate Specific Membrane Antigen (PSMA) SPECT/CT could be a potential substitute for Ga-68-PSMA PET/CT in high-risk patients, except when evaluation of prostate bed is of major concern.
A prospective study of patients confirmed or suspicious to have metastatic prostate cancer is conducted to investigate the value of Tc-99m-PSMA SPECT/CT as a cost-effective and readily accessible method in comparison with Ga-68-PSMA PET/CT.
Twenty-two patients with a mean age of 66.6 ± 10.1 years were studied using Tc-99m-PSMA SPECT/CT and Ga-68-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-68-PSMA. Tc-99m-PSMA SPECT/CT was performed 3 hours after IV injection of 555 to 740 MBq (15–20 mCi) of Tc-99m-PSMA. The images of each modality were interpreted independently, and the results were compared according to patient-based as well as region-based analyses.
In patient-based evaluation, both Tc-99m-PSMA SPECT/CT and Ga-68-PSMA PET/CT scans were positive in 95.45% (21/22). In region-based evaluation, Ga-68-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-99m-PSMA SPECT/CT. Most of these differences could be explained by the lower detection rate of Tc-99m-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). A significant correlation was also demonstrated between serum prostate-specific antigen level and imaging parameters of disease extension detected by 2 modalities.
By the way of conclusion, Tc-99m-PSMA SPECT/CT could be considered as an alternative method to Ga-68-PSMA PET/CT in high-risk patients, except when the evaluations are focused on prostate bed rather than distant areas. However, further studies, adding more delayed pelvic SPECT/CT images or more delayed WB scan, might obviate this pitfall. Also, the candidates of Lu-177-PSMA-RLT might benefit from comparable diagnostic performance and lower cost of SPECT/CT versus PET/CT. Also, in patients with suspected bone scan, it can be considered as a substitute or complementary method.