Marked Leukocytosis and Subclavian Vein Occlusion from Pancoast Tumor Mimicking Cellulitis: A Case Report.

Introduction: Lung cancer can present with atypical clinical manifestations that may mimic benign conditions. We report an extremely rare case of lung cancer initially misdiagnosed as cellulitis due to unilateral upper limb swelling and elevated inflammatory markers. Case Presentation: A 65-year-old woman with a history of heavy smoking presented with progressive swelling and pain in the left upper limb, accompanied by leukocytosis and elevated inflammatory markers. She was initially diagnosed with cellulitis and treated with antibiotics without improvement. Contrast-enhanced computed tomography revealed a left apical lung tumor invading the chest wall and occluding the subclavian vein. Histopathological examination confirmed primary non-small-cell lung carcinoma, favoring adenocarcinoma. The patient also exhibited tumor-associated extreme leukocytosis and experienced rapid disease progression despite supportive care, dying 37 days after admission. Discussion: This case illustrates the diagnostic challenges posed by lung cancer presenting with atypical, infection-like manifestations. Pancoast tumors involving the subclavian vein can cause upper limb edema, while tumor-associated leukocytosis can mimic inflammatory responses. Because extreme leukocytosis in solid tumors is associated with poor prognosis, early recognition is crucial. Conclusion: Even when clinical findings suggest infection, malignancy should be considered in the differential diagnosis, particularly in patients with risk factors such as smoking or advanced age. Prompt chest imaging is essential to avoid diagnostic delay in such cases.