A Diagnostic Dilemma in a Breast Cancer Patient on Tamoxifen: Perihepatic Endometriosis Masquerading as Metastatic Disease.

Breast cancer metastasis to the liver carries a poor prognosis, and the identification of hypermetabolic hepatic or perihepatic lesions on 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) in a patient with a known oncological history typically raises immediate concern for disease progression. We report the case of a 49-year-old woman with a prior history of breast cancer, managed on tamoxifen, in whom PET/CT and conventional CT imaging demonstrated findings highly suggestive of widespread osseous and peritoneal metastases. Surgical excision of the dominant perihepatic lesion revealed endometriosis on histopathological examination, with no evidence of malignancy. The coexisting skeletal lesions, by contrast, represented true osseous metastases and were treated with systemic therapy. This case illustrates the potential for endometriosis, particularly in the context of tamoxifen-associated endometrial stimulation, to generate false-positive findings on functional imaging, and underscores the essential role of tissue biopsy in guiding oncological decision-making.