Imagine trying to find a single match from a matchbook in a large room. Not an easy task, is it? But if the lights were dimmed and the match struck, its location would be immediately apparent.
This is the basic idea behind PSMA imaging, a newly approved method for detecting prostate cancer that is spreading or metastatic. The process is based on a minimally radioactive tracer called Gallium-68 PSMA-11. The tracer is delivered in tiny amounts by injection and travels around the body and onto a protein called PSMA, which is found in high amounts on prostate cancer cell surfaces. The labeled cells then light up on whole-body imaging with a positron emission tomography (PET) scan.
According to the new FDA approval, doctors can perform a PSMA PET scan to look for metastases in men with rising PSA levels after prostate cancer treatment, or if they suspect cancer is metastatic in a newly diagnosed patient. The scans have unprecedented resolution: they can detect tumors that are only a few millimeters in size and allow doctors to find and treat metastases before they become more dangerous.
The key study that led to PSMA approval was published in 2019 by staff at the University of California at Los Angeles and the University of California at San Francisco. The researchers enrolled 635 men with rising PSA levels after surgery or radiation for prostate cancer. All men received a full body PSMA PET scan and suspicious findings were recorded for the prostate bed (the local anatomy near the prostate), lymph nodes, skeletal structures, and other organs. Teams of independent experts reviewed the PSMA-PET data and their interpretations were, in turn, validated by pathologists who looked at the actual tissue samples under a microscope. When tissue samples were not available for pathologist review (known as histopathology), PSMA-PET findings were confirmed or excluded with additional imaging tools or with PSA measures after cancer treatment.
The results showed that the PSMA-PET scan correctly marked metastases confirmed by histopathology in 84% of the cases. The accuracy was better for scans that were further confirmed with other imaging tools and PSA readings. In these cases, PSMA-PET identified metastatic tumors in 92% of the cases. The higher a man’s PSA, the more likely the scans will find metastatic cancer.
The new approval only applies to Gallium-68 PSMA-11 manufactured at UCLA and UCSF, as well as PSMA PET scans performed at these two institutions. However, other PET imaging agents that bind to PSMA proteins are being expedited by the FDA and should, according to Dr. Jeremie Calais, a UCLA physician who led the research, to be approved in 2021.
“If this new PSMA scan becomes more widespread, it will once again expand the diagnostic capabilities of doctors who care for men with prostate cancer,” said Dr. Marc Garnick, Professor of Medicine at Gorman Brothers at Harvard Medical School and Beth Israel Deaconess Medical Center, Editor of Harvard Health Publishing Annual report on prostate diseasesand Editor-in-Chief of HarvardProstateKnowledge.org. “Importantly, the scans provide a more accurate assessment of whether there are cancer deposits outside the area of the prostate that are normally not detected by currently available diagnostic studies. This, in turn, will help inform more specific treatments and allow a more accurate assessment of the effectiveness of our treatments. “