Applications of FES-PET Imaging in Breast Cancer Treatment Decision-Making

Gary Ulaner, MD, PhD: Now, that we’ve introduced what FES is—a radiotracer composed of estrogen that’s bound to something, that emits radiation and allows us to noninvasively evaluate estrogen receptor [ER]—let’s talk about a few potential applications of this technology. The first to note, and potentially the most important, is that it helps you determine estrogen receptor status in metastatic lesions and helps you determine whether estrogen receptor–targeted therapies are going to be valuable in an individual patient.

Normally, imaging is used to detect the lesion and then determine if the lesion is getting bigger or smaller on treatment. But FES is not being used to determine whether lesions are getting bigger or smaller. Rather, it’s specifically being used to help medical oncologists, like Dr Jeremy Force, determine whether they should or should not use estrogen receptor–targeted therapies on this patient. Even if the patient is ER-positive on immunohistochemistry, if they don’t have enough FES uptake, then we can reliably predict that this patient will not respond to estrogen receptor–targeted therapy and that these patients should receive another form of therapy, such as chemotherapy.

Let’s look at a few examples of how this is applied in clinical practice. These are patients with metastatic ER-positive breast cancer receiving modern therapy, including an AI [aromatase inhibitor] and a CDK4/6 inhibitor. The FDG [fluorodeoxyglucose]–PET scan is the furthest on the left. Let’s look at the patient on the top. The FDG-PET shows us that there are FDG-avid sites of malignancy in this patient. The FES-PET, which is done before treatment shows that these lesions are FES avid. If we look back to that diagram that I showed on the prior slide, the patients are above the cutoff of FES avidity.

This predicts a good chance that the patient will respond to estrogen receptor–targeted therapy. Indeed, this patient received the estrogen receptor targeted therapy. And the post-treatment FDG, demonstrates a marked reduction in the amount of detectable disease. That’s a win both for the FDG-PET, which can measure the extent of disease, as well as for the FES-PET, which was used to help select estrogen receptor–targeted therapy for this patient.

Now, let’s look at the patient on the bottom. This patient also has FDG-avid malignancy. But the FES-PET is essentially predominantly negative. This patient is down below our cutoff, our threshold for FES PET-avidity. It predicts this patient will not respond to endocrine therapy. This patient in this clinical trial, and all patients, did go on to endocrine therapy. As expected, patients with low FES-avidity did not respond to estrogen receptor–targeted therapy. This patient has the same or even slightly more disease after a course of therapy than previously. This is a modern adaptation of the initial study that I showed, and it was performed at Washington University, confirming that even in modern ER-targeted therapy, we can use the FES-PET to help us determine whether patients will respond to estrogen receptor–targeted therapy. It’s a very useful scan for these patients.

Transcript Edited for Clarity