Using positron-emission tomography (PET) with a radioactive tracer identifies levels of progesterone receptor (PR) in patients with estrogen receptor (ER)-positive breast cancer, researchers reported in Nature Communications. The diagnostic tool would help clinicians determine which patients are most likely to respond to hormone therapies for breast cancer.

Researchers tested the imaging in 43 women with ER-positive, HER2-negative locally advanced, recurrent, or metastatic breast cancer. They injected the women with FNPP, a radioactive tracer that binds to PR and lights up on PET scans, then administered 24-hour estrogen challenges to determine the tumors’ response to the hormone.

The PET scan showed notable increases in PR among 28 of the patients, all of whom later responded to treatment with hormone therapies such as tamoxifen or letrozole. The other 15 women had either no or modest increases in PR and did not end up responding to hormone treatment.

 “FFNP-PET before and after administration of estradiol over 24 hours can be accomplished in as little as two days before estrogen therapy is started and discriminates likely responders from nonresponders with high accuracy,” the researchers wrote. “While the results of this study are extremely promising, our findings need to be confirmed in a multicenter trial before this method can be used to guide clinical practice.”

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