Atypical lung carcinoid with prominent mucinous stroma: unusual pathologic finding in pulmonary carcinoid

Abstract Background: Atypical carcinoids (ACs) are uncommon neuroendocrine tumors of the lung, and the presence of a prominent mucinous stroma is an exceptionally rare feature that may complicate diagnosis. Highlighting such variants is important for avoiding misclassification with mucin-producing pulmonary tumors. Case Presentation: A 34-year-old nonsmoking female presented with mild chest pain, minimal cough, and night sweats. Imaging revealed a well-circumscribed right middle-lobe lesion measuring 4.1 cm, and bronchoscopy showed a white-tan, fleshy mass filling the bronchial lumen. Pathology Findings: Histologic examination demonstrated a well-defined neoplasm with organoid, trabecular, follicular, pseudopapillary, and solid patterns and striking mucinous stromal change. Immunohistochemical staining was positive for CK AE1/AE3, synaptophysin (diffuse), chromogranin (diffuse), and TTF1, and negative for CK7, CK20, and CDX2. Focal necrosis and a mitotic rate of up to 5/2 mm2 supported the diagnosis of AC. Conclusion: This case represents a rare morphologic variant of AC distinguished by a prominent mucinous stroma, a feature that may obscure the diagnosis by simulating other mucin-producing lung tumors. Recognizing this uncommon pattern is crucial, as it directly impacts diagnostic accuracy and subsequent clinical decision-making.