Application of PSMA PET Scans in Prostate Cancer
The Optimal Tool for Detecting Distant Metastases of Prostate Cancer
The Prostate-Specific Membrane Antigen (PSMA) has been extensively used in the imaging diagnosis and treatment of prostate cancer due to its high specificity and binding affinity to prostate cancer cells. PET scans using various radiolabeled PSMA preparations are currently recognized as the best tools for detecting metastasis in lymph nodes and distant sites of prostate cancer.
High Detection Rate of Ga-68-PSMA-11 PET
According to a meta-analysis conducted by Hope and colleagues, for initial diagnostic staging, the sensitivity and specificity of Ga-68-PSMA-11 PET are 74% and 96%, respectively. When PSA levels are >2 ng/mL, the detection rate rises to 94%. Even with PSA levels <2 ng/mL, 67% of patients can still detect recurring or metastatic lesions using Ga-68-PSMA-11 PET. A prospective, multicenter study by Hofman et al. of 302 high-risk prostate cancer patients found that PSMA PET-CT has a significantly higher accuracy (92% vs. 65%; p<0.0001) than traditional imaging studies such as CT, MRI, or bone scans, whether it's for detecting pelvic lymph nodes or distant metastases. A large study in 2022 by Monica Abghari-Gerst involving 2,005 patients post initial curative treatment confirmed that, at PSA levels >2 ng/mL, the Ga-68-PSMA-11 PET detection rate exceeds 90% and it's over 70% when PSA is between 0.5-2 ng/mL. Currently, Ga-68-PSMA-11 PET is the most widely-used and research-backed PET scan for prostate cancer diagnosis and treatment.
Two Limitations of F-18-PSMA-1007 PET Usage
In addition to the radiolabeled PSMA-11 with Ga-68, there are now PSMA preparations labeled with the positron isotope F-18. The advantage of the F-18 isotope is its longer half-life, allowing the drug to remain effective longer. It is also suitable for production and long-distance transportation, and a larger dose can be used to enhance imaging quality. However, its drug distribution properties differ slightly from those of Ga-68-labeled PSMA-11. The two more studied drugs are F-18-DCFPyL (brand name: Pylarify) and F-18-PSMA-1007. F-18-DCFPyL has drug distribution properties similar to Ga-68-PSMA-11 but is currently unavailable in Taiwan. On the other hand, F-18-1007, an F-18-labeled PSMA preparation officially listed in the European Pharmacopoeia, is available in Taiwan, with several PET centers capable of producing it. Due to its extremely low metabolism in urine, F-18-PSMA-1007 PET offers excellent image quality and detection rates, with over 90% detection rates at PSA >1 ng/mL and over 70% when PSA is between 0.5-1 ng/mL. It's a great option for suspected recurrence. The two main limitations are its high metabolism in the liver, making it harder to detect liver metastases, and a relatively high false-positive rate, primarily in bones and nerve ganglia.
F-18-PSMA-1007 PET is More Suitable for Detecting Suspected Recurrence
A comparative study by Rauscher and others found that 102 patients suspected of prostate cancer recurrence underwent both F-18-PSMA-1007 and Ga-68-PSMA-11 PET scans. Of the 369 lesions detected by F-18-PSMA-1007 PET, only 124 were confirmed metastatic lesions of prostate cancer, with the number of false-positive lesions nearly double that of true lesions. In contrast, the same patients who underwent Ga-68-PSMA-11 scans detected 178 lesions, of which 126 were confirmed metastatic lesions of prostate cancer. Due to these two limitations, I personally believe that F-18-PSMA-1007 PET can be used to detect suspected recurrence, but it's not recommended for use in high-risk staging during the initial diagnosis of prostate cancer. This is because the commonly occurring false-positive lesions in bones might deprive patients of the opportunity for aggressive curative treatment.
Author: Dr. Huang Yuyi (Department of Nuclear Medicine)