Lytic Skeletal Lesions with CRAB Features: Metastatic Breast Carcinoma Mimicking Multiple Myeloma on FDG PET/CT

Diffuse marrow infiltration associated with anaemia, hypercalcemia, renal dysfunction, and elevated beta-2 microglobulin typically raises a strong suspicion for multiple myeloma. However, certain solid malignancies may closely mimic this presentation. We report a 57-year-old female presenting with chronic lower back pain and a right breast lump. Magnetic resonance imaging (MRI) spine demonstrated diffuse abnormal marrow signal with multiple vertebral compression fractures, initially suggestive of multiple myeloma. Mammography revealed irregular spiculated high-density lesions in the right breast [Breast Imaging Reporting and Data System (BI-RADS) category 5] without right axillary lymphadenopathy. Laboratory evaluation showed anaemia, hypercalcemia, renal dysfunction, and elevated lactate dehydrogenase; however, serum protein electrophoresis did not reveal an M-spike, and serum free light chain ratio was near normal. 18 F-FDG PET/CT demonstrated FDG-avid right breast lesions, mediastinal and bilateral hilar lymphadenopathy, and extensive intensely FDG-avid predominantly lytic skeletal lesions with vertebral collapse, closely mimicking multiple myeloma. No FDG-avid right axillary nodes were identified. Bone marrow biopsy confirmed metastatic osteosclerotic epithelial deposits consistent with breast carcinoma. This case highlights a diagnostic pitfall where metastatic breast carcinoma can simulate multiple myeloma, emphasising the importance of multimodality correlation and histopathological confirmation.