An Analysis of the Diagnostic Performance of Tc-99m PSMA SSPECT/CT in Biochemically Recurrent Prostate Cancer Compared with Ga-68 PSMA PET/CT: A Single-center, Prospective Study.

Objective: Biochemical recurrence (BCR) after initial management of Prostate Carcinoma (PC) is frequent. Subsequent interventions rely on disease burden and metastasis distribution. Ga prostate-specific membrane antigen positron emission tomography/computed tomography (PSMA PET/CT) is an excellent imaging modality in BCR. However, Ga is radionuclide generator produced and has restricted availability. mTc-labeled PSMA could be a potential cost-effective alternative. We compared the performance of Tc-PSMA single-photon emission CT (SPECT)/CT and Ga-PSMA PET/CT in BCR with a serum prostate surface antigen (PSA) level of <20 ng/mL.

Materials And Methods: The prospective study included 25 patients with BCR and at least one lesion on a Ga-PSMA PET/CT. All patients underwent 99 mTc-PSMA SPECT/CT, and disease distribution and metastatic burden were compared with Ga-PSMA PET/CT. The maximum standard uptake value (SUVmax) and the tumor-to-background ratio (TBR) were computed and analyzed.

Results: The mean age and serum PSA (SPSA) were 69.72 ± 6.69 years and 5.65 ± 6.07 ng/mL. Eleven patients (44%) had SPSA ≤2 ng/mL. Recurrent sites were noted in the prostate (19, 76%), prostatic bed (3, 12%), and pelvis lymph nodes (LNs) (13, 52%). Distant metastasis to bones (13, 52%), lungs (5, 20%), and retroperitoneal LNs (2, 8%) were noted. Both modalities were concordant for the recurrent disease at the prostate, prostatic bed, bone, and lung lesions. Tc-PSMA could localize pelvis LNs in most patients (10/13, 76.9%). The site-specific sensitivity and specificity between the two modalities were not significantly different ( > 0.05). TBR shows excellent correlation with SUVmax (0.783, < 0.001). Four (16%) patients were understaged with Tc-PSMA due to the nonvisualization of the subcentimeter size LNs. No patient with systemic metastases was understaged.

Conclusions: Tc-PSMA SPECT/CT has good concordance with Ga-PSMA PET/CT in BCR, even at low PSA levels. However, it may miss a few subcentimeter LNs due to lower resolution. Tc-PSMA SPECT/CT could be a simple, cost-effective, and readily available imaging alternative to PET/CT.