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Imagine trying to find a single match from a matchbook in a large room. Not an easy task, right? But if the lights were dimmed and the match was lit, its location would be immediately apparent.
This is the basic idea of PSMA imaging, a newly approved method for detecting prostate cancer that is spreading or metastasizing. The method is based on a weakly radioactive tracer called gallium-68 PSMA-11. Delivered in small amounts by injection, the tracer travels throughout the body and slips on a protein called PSMA that is found in high levels on the surfaces of prostate cancer cells. The labeled cells will then light up on whole body imaging with a positron emission tomography (PET) scan.
According to new FDA approval, doctors can perform a PSMA-PET scan to look for metastases in men with rising PSA levels after treatment for prostate cancer, or if they suspect cancer is metastasizing. in a newly diagnosed patient. The scans have unmatched resolution: able to detect tumors as small as a few millimeters anywhere in the body, they allow doctors to find and treat metastases before they become more dangerous.
The pivotal study leading to the approval of the PSMA was published in 2019 by collaborators from the University of California at Los Angeles and the University of California at San Francisco. Investigators recruited 635 men with rising PSA levels after surgery or radiation therapy for prostate cancer. All the men underwent a PSMA-PET whole body scan and suspicious results 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 examined the actual tissue samples under a microscope. When tissue samples were not available for the pathologist’s examination (referred to as histopathology), PSMA-PET results were confirmed or excluded using additional imaging tools, or with PSA measurements taken after cancer treatment.
The results showed that the PSMA-PET scan correctly reported histopathologically confirmed metastases 84% of the time. The accuracy was better for scans that were 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 detect metastatic cancer.
The new approval applies only to gallium-68 PSMA-11 manufactured at UCLA and UCSF, and PSMA-PET scans performed at these two facilities. However, other PET imaging agents that bind to PSMA proteins are under expedited review by the FDA and are expected to be approved in 2021, according to Dr. Jeremie Calais, a UCLA physician who helped lead research.
“When this new PSMA test becomes more widely available, it will again add to the diagnostic capabilities of physicians treating men with prostate cancer,” said Dr Marc Garnick, Gorman Brothers Professor of Medicine at Harvard Medical School and Beth Israel Deaconess Medical Center, editor-in-chief of Harvard Health Publishing Annual report on prostate diseases, and editor of HarvardProstateKnowledge.org. “Importantly, the scans allow a more accurate assessment of the presence of cancerous deposits outside the prostate area 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.
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