New research suggests that repeating a Prostate-Specific Membrane Antigen (PSMA) PET scan later can reveal cancer that was missed the first time. PET stands for Positron Emission Tomography, an advanced medical imaging test that reveals how the tissues and organs are functioning.
Among patients with rising Prostate-Specific Antigen (PSA) levels but no visible disease on their initial scan, a second test detected cancer in 56 percent of cases. The results changed treatment plans for nearly half of the patients.
According to the findings published in the July issue of The Journal of Nuclear Medicine, prostate cancer can return after surgery or radiation therapy without immediately producing symptoms or a tumor that doctors can see.
Instead, the first warning may be a rising PSA level in the blood, a situation known as biochemical recurrence. The increase suggests that prostate cells remain somewhere in the body, but it does not reveal their location.
This can make treatment decisions difficult. Doctors may need to determine whether the cancer is confined to the area where it began, has reached nearby lymph nodes, or has spread to distant organs. Each pattern can require a different strategy.
PSMA PET imaging is designed to help solve that problem. A radioactive tracer attaches to prostate-specific membrane antigen, a protein often found at high levels on prostate cancer cells. The scan then highlights areas where the tracer accumulates, potentially exposing deposits that conventional imaging cannot easily detect.
On why some cancers escape detection, the study observed that even this sensitive technology has limits, as about 30 percent of patients with suspected recurrence have no detectable cancer on their first PSMA PET scan, despite rising PSA levels.
It found that the disease may be microscopic, express too little PSMA, or not yet produce a signal strong enough to distinguish it from surrounding tissue. “There is little information on the utility of repeating a PSMA PET after an initial negative scan.
“In our study, my colleagues and I sought to determine the benefit of a second PSMA PET scan, as well as to assess predictors for positive PSMA PET scans,” said professor of Radiology at the University of Toronto and head of the Division of Molecular Imaging at the Joint Department of Medical Imaging at Princess Margaret Cancer Centre in Toronto, Ur Metser.
Metser and his colleagues examined 210 participants in the Registry for Recurrent Prostate Cancer in Ontario. Each patient had undergone more than one PSMA PET scan after the first produced a negative result.
The researchers tracked PSA levels, how quickly PSA concentrations doubled, whether the repeat scan found cancer, and whether the new information altered the patient’s care. They also classified the disease as local recurrence, locoregional, oligometastatic (fewer than five positive disease sites), or extensive metastatic (more than five positive disease sites).
The second scan uncovered recurrent cancer in 56 percent of the patients. Those discoveries prompted physicians to change management plans in nearly 50 percent of cases, with the greatest effect seen among patients with oligometastatic disease.
Finding a limited number of tumors can be particularly important because it may allow doctors to consider treatments directed at specific cancer sites rather than relying only on therapies that affect the entire body.
