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GU Cancers 2023 | Unanswered questions for the use of PSMA PET in the staging of prostate cancer

David M. Nanus, MD, Weill Cornell Medicine and New York Presbyterian Hospital, New York City, NY, talks on the current uses of PSMA PET scans in prostate cancer, such as in the staging of newly diagnosed, high-risk patients or those with non-metastatic castrate-resistant prostate cancer (CRPC). Dr Nanus discusses the utility of PSMA PET compared to conventional imaging techniques, and explores the potential of using PSMA PET to detect oligometastatic disease or assess response to treatment. This interview took place at the ASCO GU Cancers Symposium 2023 in San Francisco, CA.

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Transcript (edited for clarity)

So, while it’s clear that PSMA PET scan has great utility in newly-diagnosed high-risk patients, I think most people would agree on that, and it’s widely used today across the world, if it’s available. There are some other areas where it’s utility is really not proven. So, if you think about prostate cancer, one area that comes up, not infrequently, is the patient who’s already castrate-resistant and has no evidence of metastatic cancer...

So, while it’s clear that PSMA PET scan has great utility in newly-diagnosed high-risk patients, I think most people would agree on that, and it’s widely used today across the world, if it’s available. There are some other areas where it’s utility is really not proven. So, if you think about prostate cancer, one area that comes up, not infrequently, is the patient who’s already castrate-resistant and has no evidence of metastatic cancer. So, are M0 CRPC. And that’s generally on conventional imaging, where you say they have no evidence of metastatic cancer despite a rising PSA.

In that setting, PSMA PET scan is generally used, which is similar to the patients with castrate-sensitive disease who, with a biochemical relapse post-definitive therapy, most would recommend, in fact, all guidelines recommend PSMA PET CT. So those early areas, I think it’s pretty universally used.

Where it’s a little bit confusing and really where there’s not a whole lot of evidence, is patients that are metastatic castrate-resistant prostate cancer. So, you have a patient who’s on hormonal therapy or ADT maybe with androgen-receptor selective inhibitor, and their PSA’s going up. Is there any utility in that setting to do PSMA PET? Frequently they’ll have conventional imaging and it doesn’t show any evidence of progression, but if you do PSMA PET CT, maybe it will show progression. And then it’s confusing, “Well, should I change therapy because the PSMA PET CT shows progression, but all the studies were done without PSMA PET CT?”

I think what a lot of investigators and clinicians are looking for as evidence for what’s called oligo progression, meaning that one site of disease is getting worse. And so if you have a patient on therapy, we know with a lot of therapies and many diseases where if you have an escape lesion or one lesion’s getting worse on, let’s say, immunotherapy or systemic therapy like hormonal therapy, and you treat that one lesion, maybe you can put the patient back into a remission. That has not been proven, but there are some prospective trials treating oligo-metastatic prostate cancer to see if that actually translates to improvement in survival.

The other area, which is even more confusing, is response to therapy. So people think of a PET scan, right? Let’s say in lymphoma you have a FDG PET or in lung cancer, and patients get treated and they respond and you can use the PET CT to gauge response, and it’s widely accepted. So, many clinicians will use PSMA PET scan in that setting, and to see, “Is the patient responding even though they have widely metastatic disease?” Instead of getting conventional imaging, they use PSMA PET CT and all those areas are really not… The utility or the benefit or what improvement you’ll have in that setting, has really not been well-defined.

Up until now, the FDA does not look at PSMA PET CT as a surrogate for conventional imaging to see progression or response. So, I think there’ll be more studies in that area. There are a lot of reports. I mean many, many reports, mostly single institution. So as that data gets aggregated, I think we’ll be able to define in what scenario is a PSMA PET CT useful in patients with castrate-resistant, widely metastatic prostate cancer.

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