Challenging Diagnosis of Lytic Bone Lesions Between Multiple Myeloma and Bone Metastasis of Primary Breast Cancer.

Lytic bone lesions include various differential diagnoses, such as bone metastasis of cancer, multiple myeloma, primary bone cancers, and infections. Here, we report a rare case of primary breast cancer complicated by lytic bone lesions mimicking bone metastasis, which was subsequently diagnosed as multiple myeloma. Despite the development of several imaging modalities, such as magnetic resonance imaging and positron emission tomography/computed tomography, diagnosing lytic bone lesions with either multiple myeloma or tumor metastasis is highly challenging. Urinalysis is a noninvasive diagnostic method that includes useful diagnostic information; thus, physicians should evaluate urine protein levels when lytic bone lesions are observed.