Imagine trying to find a single match from a book of matches in a large room. Not an easy task, right? But if the lights were dimmed and the match was lit, then 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 metastasizing. The method relies on a minimally radioactive tracer called gallium-68 PSMA-11. Delivered in tiny amounts by injection, the tracer travels throughout the body and gloms onto a protein called PSMA that is found at high levels on prostate cancer cell surfaces. The labeled cells will then light up on whole-body imaging with a positron-emission tomography (PET) scan.
Per the FDA’s new approval, doctors can give a PSMA-PET scan to hunt for metastases in men with rising PSA levels after prostate cancer treatment, or if they suspect cancer is metastasizing in a newly diagnosed patient. The scans have unparalleled resolution: able to detect tumors only a few millimeters in size anywhere in the body, they allow doctors to find and treat metastases before they become more dangerous.
The pivotal study leading to PSMA’s approval was published in 2019 by collaborators at the University of California, Los Angeles and the University of California, San Francisco. The investigators enrolled 635 men with rising PSA levels after surgery or radiation for prostate cancer. All the men got a whole-body PSMA-PET scan, and suspicious findings were recorded for the prostate bed (the local anatomy in the vicinity of 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 the pathologist’s review (which is called histopathology), PSMA-PET findings were confirmed or ruled out using additional imaging tools, or with PSA measures taken after cancer treatment.
Results showed that PSMA-PET scan correctly flagged metastases confirmed by histopathology 84% of the time. 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 92% of the time. Importantly, the higher a man’s PSA, the more likely the scans were to find metastatic cancer.
The new approval applies only to gallium-68 PSMA-11 manufactured at UCLA and UCSF, and to PSMA-PET scans given at those two institutions. However, other PET imaging agents that bind to PSMA proteins are under accelerated review at the FDA, and should be approved in 2021, according to Dr. Jeremie Calais, a UCLA physician who helped lead the research.
“When this new PSMA scan becomes more widely available, it will again add to the diagnostic capabilities of physicians caring for men with prostate cancer,” said Dr. Marc Garnick, the Gorman Brothers Professor of Medicine at Harvard Medical School and Beth Israel Deaconess Medical Center, editor of the Harvard Health Publishing Annual Report on Prostate Diseases, and editor in chief of HarvardProstateKnowledge.org. “Importantly, the scans enable a more precise evaluation of whether cancer deposits are present outside the area of the prostate gland that are not normally detected by currently available diagnostic studies. This in turn will help inform more specific treatments and enable a more accurate assessment of the effectiveness of our treatments.”