F-18-FES (Fluoroestradiol)¹⁸F-fluoroestradiol

Quick Answer

F-18-fluoroestradiol (FES) binds the estrogen receptor (ER), phenotyping ER expression across all lesions in a single whole-body PET. It is FDA-approved as an adjunct for detecting ER-positive lesions in recurrent/metastatic breast cancer, resolving heterogeneous or biopsy-inaccessible disease and informing endocrine-therapy decisions. Two facts define its use: recent ER-blocking therapy causes false negatives, and intense hepatobiliary excretion makes it unreliable for hepatic lesions.

Mechanism

FES (16α-[¹⁸F]fluoroestradiol) binds the estrogen receptor with high affinity; regional uptake correlates with ER expression measured by immunohistochemistry. Because it competes for the receptor, agents that occupy or down-regulate ER (tamoxifen, fulvestrant) block FES binding.

Biodistribution

Hepatobiliary metabolism and excretion produce intense liver and bowel activity (limiting hepatic-lesion assessment). Physiologic uterine uptake occurs in premenopausal patients; blood-pool and renal clearance are also seen.

Clinical Indications

• Confirming ER expression in metastatic/recurrent breast cancer — especially heterogeneous or biopsy-inaccessible disease.
• Endocrine-therapy planning — predicting benefit and clarifying discordant biopsy results.
• Whole-body ER phenotyping when a single biopsy cannot represent all lesions.

Protocol Notes

• Typical adult activity ~6 mCi (222 MBq); uptake ~60–80 min.
• Hold ER-blocking therapy appropriately (tamoxifen/fulvestrant cause false negatives) — timing is protocol-dependent.
• Standard PET/CT acquisition; report reference regions and SUV of index lesions.

Interpretation Highlights

• Lesion uptake above blood pool/background indicates ER expression; correlate with sites of known disease.
• Discordance (FDG-positive but FES-negative) can flag ER-loss/heterogeneity relevant to endocrine-therapy decisions.
• Not for hepatic lesions (physiologic liver/bowel activity).

Reporting Checklist

• Confirm ER-blocking drugs were held appropriately (timing) — the key false-negative source.
• Report ER-avidity by lesion, capturing heterogeneity.
• Note the hepatobiliary limitation for liver lesions.
• Correlate with FDG/anatomic imaging and receptor history.

Common Pitfalls

• Imaging during active ER-blocking therapy → false negatives.
• Liver/bowel physiologic activity obscuring adjacent/hepatic lesions.
• Over-interpreting low uptake in inherently ER-low/negative histologies.

Board Pearls

F-18-FES binds the estrogen receptor, phenotyping ER expression across every lesion in one whole-body PET — capturing heterogeneity or discordance a single biopsy misses and informing endocrine-therapy decisions.

Recent ER-targeting therapy (tamoxifen/fulvestrant) occupies the receptor and causes false negatives — hold per protocol; intense hepatobiliary excretion makes FES unreliable for hepatic lesions.

FES is most useful when disease is biopsy-inaccessible or heterogeneous, or when discordant biopsy/response raises the question of ER loss. Uptake correlates with ER IHC, so a genuinely ER-negative lesion is expected to be FES-low — the value is in mapping the distribution of ER across the whole body, not diagnosing the primary.

Related Pages

• Disease: Breast cancer (FES role by setting); tracer: FDG (paired phenotyping).
• Pitfalls: Pearls, pitfalls & normal variants.

Figure / Diagram Suggestions

• An ER-binding mechanism cartoon with the ER-blocker false-negative.
• A whole-body ER heterogeneity teaching plate (FES vs FDG discordance).

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