Abstract
Invasive lobular carcinoma (ILC) presents significant diagnostic challenges due to its subtle presentation on mammography. Its low metabolic activity further limits the detection of metastatic lesions using fluorine-18 ((18)F)-fluorodeoxyglucose-positron emission tomography (FDG-PET). In contrast, (18)F-fluoroestradiol-PET (FES-PET), which targets functional estrogen receptor (ER) expression, enhances visualization of ER-positive metastases. We present a case of metastatic right breast ILC with only subtle suspicious findings on initial mammography and ultrasound. As part of the metastatic workup, FES-PET/MRI was utilized and identified additional metastatic lesions that were not metabolically active on the initial FDG-PET/CT. This case underscores the challenges of ILC detection in breast imaging and highlights the emerging role of FES-PET in its management.