Synchronous Extensive-Stage Small Cell Lung Cancer and Multiple Myeloma Detected by Marked Hypergammaglobulinemia

A 75-year-old woman presented with nausea, exertional dyspnea, and right-sided chest pain. Chest imaging showed a right hilar mass with right pleural effusion. Initial assessment favored primary lung cancer with carcinomatous pleuritis. However, laboratory tests at admission revealed markedly elevated levels of total protein with hypoalbuminemia, resulting from monoclonal IgG-kappa (κ) gammopathy (IgG: 7,700 mg/dL). Thoracentesis demonstrated an exudative effusion; pleural fluid cytology and transbronchial tumor biopsy confirmed small cell lung cancer (SCLC). Anemia, markedly increased serum free κ light chain levels, and atypical plasmacytosis in the bone marrow confirmed the presence of multiple myeloma. She was then diagnosed with extensive-stage SCLC and concomitant multiple myeloma. Dose-reduced carboplatin plus etoposide was initiated for SCLC with clinical improvement and no severe acute toxicity; treatment for myeloma was planned after stabilization of SCLC. This case highlights that extreme hypergammaglobulinemia in a patient with suspected lung cancer should trigger prompt evaluation for multiple myeloma. Dual malignancy can coexist and may be overlooked if clinicians focus on a single diagnosis.