UCSF is leading the United States evaluation of a new imaging technique that is helping patients and physicians better target prostate cancer treatment. Called PSMA-PET, it is especially useful in men who have rising levels of prostate specific antigen (PSA) after initial treatment. This condition, called biochemical recurrence, is the earliest sign that the cancer may have returned. PSMA-PET is much more accurate than conventional imaging in locating small metastases, and thus may allow men to choose targeted treatments, rather than systemic therapies, for recurrent cancer. PSMA-PET can also be used to help guide decisions about initial treatment of prostate cancer.
Diagnostic radiologist Thomas Hope, MD, oversees the PSMA-PET research study at UCSF and serves as the principal investigator for what will become a multicenter study of this prostate cancer imaging tool.
How it works
Positron emission tomography (PET) has been used in conjunction with computed tomography (CT) for many years to image other types of cancers, according to Hope. A tracing agent, often a sugar molecule, is labeled with minute amounts of a short-lived radioactive element. The resulting radioisotope is then injected into the patient. An imaging study is conducted after the tracer molecules have been taken up by tumor cells. The positrons emitted by the molecules as the radioactivity decays are mapped over CT images to locate cancer metastases.
PET scanning has historically not been helpful in prostate cancer, however, because prostate cancer cells do not absorb the commonly used tracing agents. Instead radiologists have had to rely on less sensitive imaging options: magnetic resonance imaging (MRI) to detect cancer in the prostate gland, bone scans to reveal bone metastases, and CT to show spread of the cancer to other soft tissues.
In an effort to make PET useful in prostate cancer, many research groups have attempted to develop imaging agents that would target prostate-specific membrane antigen (PSMA), a protein that is overexpressed on prostate cancer cells. However, it was not until German researchers developed PSMA-11 and made it widely available that PSMA-PET became a powerful tool in imaging patients with the disease. To perform each study, PSMA-11 is labeled with the radioactive element gallium-68. The radioisotopes, which have a very short half-life, are made specifically for each patient on the day of the scan.
"PSMA-PET allows us to determine the extent of prostate cancer more than any other imaging test currently available,” said Peter R. Carroll, MD, MPH, professor and chair, Department of Urology. “That lets us focus our efforts on tailored treatment, whether it be to the prostate alone, regional or distant lymph nodes, or other sites.”
UCSF was the first site in the United States to use this form of imaging for prostate cancer, and in the past year has performed PSMA-PET in 350 men, many of whom travelled to UCSF from around the country. As the technique becomes more widely available at other centers, UCSF will coordinate enrollment of patients into an FDA trial that is evaluating PSMA-11 under an investigational new drug protocol.
A New Tool for Defining Treatment Options
In patients with biochemical recurrence after prostatectomy, PSMA-PET detects localized disease in about 50 percent of those who have a PSA measuring less than 2 ng/mL. This may allow men to treat the recurrence with targeted therapies, such as surgery or external beam radiation, rather than systemic therapies such as androgen deprivation, sparing them unwanted side effects.
PSMA-PET can also help guide initial treatment decisions. When the study is performed before prostatectomy, it detects up to 70 percent of lymph node metastases, as compared to 40 percent with conventional imaging, said Hope. This can help men determine whether they want to add systemic therapy to their initial treatment plan.
“Molecular imaging with PSMA-PET has the potential to revolutionize our approaches for detecting and treating prostate cancer,” said UCSF radiation oncologist Felix Feng, MD. “With PSMA-PET, we can now see prostate cancer, in the prostatic area, in lymph nodes, and in bones, that we could not previously detect, and this has significant ramifications for how we approach this disease clinically. I think that molecular imaging approaches like PSMA-PET will eventually become a standard of care for patients with aggressive prostate cancers.”