Case Presentation: A 41-year-old male with a recently diagnosed left iliac soft tissue mass presents to our institution complaining of severe pain in his lower back and left hip limiting his ability to walk. The patient denied any recent fevers or chills, night sweats, allergies, or a family history of malignancies. The pelvic MRI on admission showed a large 6x8cm soft tissue mass in the left ilium along with diffuse bone marrow (BM) metastasis throughout the sacral spine with extension into the bilateral ilium and femur. CBC was notable for leukocytosis to 26.2 K/uL with monocytosis (1.06 K/uL) and eosinophils (9.77 K/uL). Peripheral blood smear revealed mature granulocytes, monocytosis and eosinophilia without left shift. Chemistry profile was remarkable for mild hypercalcemia (10.4 mg/dL) but no protein gap or other abnormality. 24-hour urine protein was elevated to 1775mg. Serum protein electrophoresis disclosed no M-spike but urine electrophoresis was notable for Bence-Jones proteinuria and serum lambda light chains were elevated (lamda 1357 mg/L, kappa 4.57 mg/L, and lambda/kappa ratio of 334.4). FIP1L1-PDGFRA fusion protein and next-generation sequencing for myeloproliferative neoplasms (MPNs) were negative. The BM biopsy showed plasma cell neoplasm with 80-90% cellularity. The pelvic biopsy was consistent with plasmacytoma. A diagnosis of light chain multiple myeloma (MM) was confirmed and the patient was initiated on treatment before discharge.
Discussion: Eosinophilia is commonly encountered in the inpatient setting and requires a focused approach to determine etiology. Eosinophilia is usually transient and secondary to infection, drugs, or allergic reactions. Due to the level of severity in this patient, it was important to screen for any signs of end-organ damage and order diagnostic testing as appropriate. In our patient, we believed that eosinophilia was a paraneoplastic phenomenon (due to eosinophilic cytokine production by tumor cells) secondary to MM. It was through this structured approach that the diagnosis of MM was made. Our patient’s MM is classified as light chain MM due to the lack of serum M-spike but positive serum light chains.
Conclusions: Evaluation of peripheral eosinophilia requires screening for signs of end organ damage, ruling out secondary causes related to infection or medications, and then completing serum testing and BM biopsy to evaluate for hematological abnormalities. Light chain MM is a hematological neoplasm that can present without the traditional serum findings of anemia, protein gap and serum M-spike and may be associated with peripheral eosinophilia.