A 74-year-old woman presented with fatigue and normocytic anemia (hemoglobin 64 g/L and mean corpuscular volume 82 fL). Erythrocyte sedimentation rate was 131 mm/h; creatinine, 150 g/L; and ferritin, 1608 g/L. Protein electrophoresis showed no monoclonal protein, but κ free light chains were elevated (65.5 mg/L [normal range is 3.3–19.4]), with a κ/λ ratio of 2.33. Skeletal studies showed mottled lucencies, compression fractures, and diffusely heterogeneous magnetic resonance imaging signal. Nonsecretory multiple myeloma (MM) was suspected. Bone marrow (BM) examination revealed extensive infiltration by poorly differentiated cells with eccentric nuclei (panel A) and cytoplasmic luminal inclusions (panel B). Cells were positive for cytokeratin 7 (panel C) and estrogen receptor (panel D) and negative for CD138, colon, and lung markers. Pathology suggested metastatic breast cancer, lobular subtype. Direct investigation of the patient identified a mammary mass. Initial fine-needle aspiration was nondiagnostic, as is common with lobular carcinoma. The patient succumbed to traumatic subdural hematoma before core biopsy could be performed.

BM infiltration by breast cancer can mimic MM and should be in the differential diagnosis for female patients. Lobular carcinoma is often subtle clinicoradiologically, warranting a high index of suspicion and core biopsy in atypical cases. As CD138 can be positive in both MM and carcinoma, supplementary markers (IRF4, keratins, etc.) should be employed as appropriate.