BONE MARROW BIOPSY REVEALS OCCULT INVASIVE LOBULAR CARCINOMA WITHOUT HYPERMETABOLIC BREAST LESION ON FDG PET CT

Bone marrow metastasis (BMM) from breast cancer is exceedingly rare and more often associated with invasive lobular carcinoma (ILC) than with invasive ductal carcinoma. ILC is characterized by a diffuse infiltrative growth pattern that often escapes detection onmammography, leading to delayed diagnosis and advanced disease. It also demonstrates low FDG avidity, with hypermetabolic activity typically limited to metastatic sites. We describe a 64-year-old woman who presented with chest pain, cytopenias, renal impairment, and hypercalcemia. Imaging revealed diffuse bony lucencies on FDG PET/CT without hypermetabolic activity in the breasts, raising concern for hematologic malignancy. Bone marrow biopsy, however, confirmed metastatic ILC (ER positive, PR negative, HER2 negative, GATA3 positive). Subsequent breast imaging identified a large right breast mass with axillary involvement. She was initiated on endocrine therapy with letrozole and ribociclib, along with monthly zoledronic acid, resulting in partial improvement of cytopenias, normalization of calcium levels, and effective symptom control. This case highlights the stealth nature of ILC, the limitations of conventional imaging, and the indispensable role of bone marrow biopsy with immunohistochemistry in the evaluation of unexplained cytopenias. Although BMM portends a poor prognosis, advances in endocrine and targeted therapy offer improved outcomes and underscore the importance of reporting such cases to guide clinical practice.