Development of Symptomatic Bone Marrow Metastasis After Complete Response to Immunochemotherapy in Squamous Cell Lung Carcinoma

Introduction Symptomatic bone marrow metastasis in squamous cell lung cancer after achieving a complete response to immunochemotherapy has not been reported previously. Case Presentation A 63‐year‐old Japanese man was referred to our hospital for further examination of an abnormal chest radiograph. Whole‐body computed tomography revealed left hilar swelling and osteolytic lesions. Bronchoscopic examination led to a diagnosis of squamous cell lung carcinoma with multiple bone metastases. He was treated with carboplatin/Nab‐paclitaxel and pembrolizumab, and achieved complete response as determined by positron emission tomography with fluorodeoxyglucose‐computed tomography. Despite pembrolizumab maintenance therapy for 1 year, thrombocytopenia, anemia, and leukoerythroblastosis occurred. Bone marrow biopsy revealed squamous cell carcinoma cells within fibrotic tissue, confirming the diagnosis of symptomatic bone marrow metastases. The patient died 1 month after this diagnosis. Conclusion Recurrence presenting bone marrow metastasis should be considered in patients with squamous cell lung cancer, even achieving complete response to immunochemotherapy, particularly those with bone metastases.