Negative 18 F-FES PET/CT Despite Estrogen Receptor Positivity by Immunohistochemistry.

Lawal, Ismaheel O; Li, Xiaoxian; Sakach, Elizabeth A; Farley, Clara R; Meisel, Jane L; Baum, Yoram; Chaudhary, Krishna K; Wang, Qun et al. · Clin Nucl Med · 2026

case_series · Level IV

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Abstract

Immunohistochemistry (IHC) is routinely used to determine the estrogen receptor (ER) expression status of breast cancer. 18 F-fluoro-estradiol ( 18 F-FES) PET/CT is indicated for select breast cancer patients with ER+ breast cancer, and patients with tracer-avid disease typically enjoy favorable responses to endocrine therapy (ET). We present 2 patients with metastatic breast cancer that were ER+ on IHC but lacked tracer avidity on 18 F-FES PET/CT. We hypothesize that despite ER detection on IHC, these receptors were nonfunctional, lacking the ability to bind estrogen. Since functional ER is a prerequisite for response to ET, these patients are not likely to respond to ET.

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