Data and images courtesy of Instituto Nacional de Cancerología, Tlalpan, Mexico City, Mexico
Background
The overexpression of prostate-specific membrane antigen (PSMA) imaged through PET/CT and SPECT/CT agents is correlated with the malignancy of prostate cancer. Emerging data has shown the superiority of PSMA imaging with PET/CT over conventional imaging for primary prostate cancer diagnoses; however, access to PSMA imaging has been a challenge in low- and middle-income countries where high demand and expensive costs impede the growth of PSMA PET/CT imaging. In this case, a male in his 70s with PSMA-avid prostate cancer underwent 18F-PSMA-1007 PET/CT and 99mTc-iPSMA SPECT/CT imaging within a 24-hour interval to compare both agents in an effort to increase clinical utility.
History
A routine 18F-PSMA-1007 PET/CT was performed to stage a high-risk prostate cancer patient. PET/CT findings confirmed PSMA expression in the tumors. The patient was administered with 10.76 mCi (398.12 MBq) intravenous (IV) injection of 18F-PSMA-1007, and approximately 1 hour later, a whole-body acquisition was conducted on a Biograph™ PET/CT system.
Figure 1: Coronal CT, 18F-PSMA-1007 PET/CT, and coronal and sagittal PET MIP images confirm PSMA expression in metastatic disease.
Findings
As observed in Figure 1, 18F-PSMA-1007 PET/CT images demonstrate pelvic and retroperitoneal lymph node metastases.
The patient then underwent 99mTc-iPSMA SPECT/CT within a 24-hour interval to compare to the 18F-PSMA-1007 PET/CT PSMA expression. The patient was administered with 20 mCi (740 MBq) IV injection of 99mTc-iPSMA, and approximately 5 hours later, a multi-bed SPECT/CT was conducted on a Symbia™ SPECT/CT system. 99mTc-iPSMA SPECT/CT images in Figure 2 demonstrate similar PSMA-avid lesions in comparison to 18F-PSMA-1007 PET/CT.
The clinical value and accuracy of 99mTc-iPSMA SPECT/CT is demonstrated in Figures 3 and 4 when compared to 18F-PSMA-1007 PET/CT. In countries where PSMA imaging is not widely available, 99mTc-iPSMA SPECT/CT can play an integral role in expanding access to accurate prostate cancer diagnoses.
Figure 2: Coronal CT, 99mTc-iPSMA SPECT/CT, and coronal and sagittal SPECT MIP images confirm PSMA expression in lesions, which was previously confirmed in 18F-PSMA-1007 PET/CT.
Discussion
This case demonstrates the importance of imaging PSMA expression not only with PET/CT but also with SPECT/CT. According to the National Comprehensive Cancer Network (NCCN) Guidelines, it is recommended to perform PSMA imaging on patients with unfavorable intermediate-, high-, and very high-risk prostate cancer for primary staging. NCCN guidelines also indicate that PSMA PET/CT can be an alternative to conventional imaging. As 1 in 8 men are diagnosed with prostate cancer worldwide, access to PSMA imaging will increasingly become a challenge in underdeveloped countries as demand increases. Besides prostate cancer staging, PSMA PET/CT imaging is also utilized to qualify patients with castrate-resistant prostate cancer for 177Lu-PSMA-617 treatment. 99mTc-iPSMA SPECT/CT may become a useful agent in qualifying patients in countries where PSMA PET/CT imaging is limited.1,2
Conclusion
99mTc-iPSMA SPECT/CT is useful in the initial diagnosis of patients with high-risk prostate cancer. 99mTc-iPSMA is an available alternative for staging prostate cancer in countries with emerging economies that lack the sufficient infrastructure such as cyclotrons or PET imaging. Although 99mTc-iPSMA SPECT/CT demonstrated a slightly lower lesion detectability compared to 18F-PSMA PET/CT, there was no impact on the patient’s initial staging nor treatment planning.
Examination protocol
Scanners: Biograph mCT 20 Excel PET/CT and Symbia T6 SPECT/CT