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Thymic carcinoid with prominent mucinous stroma. Report of a distinctive morphologic variant of thymic neuroendocrine neoplasm.

Four cases are described of a distinctive morphologic variant of thymic carcinoid that was characterized by abundant stromal mucin admixed with the neuroendocrine elements resulting in a histologic picture reminiscent of metastatic mucin-secreting carcinoma. The patients were three men and a woman, aged 22 to 43 years. The tumors presented with symptoms of chest discomfort, cough, and dyspnea and were described as large anterior mediastinal masses on chest radiographs and computerized scans. Histologically, all cases showed nests and strands of tumor cells embedded in an abundant lightly eosinophilic, mucinous stroma with small cellular clusters as well as scattered single tumor cells seen floating in the mucin. The mucinous matrix was negative for periodic acid Schiff's and mucicarmine stains; alcian blue stains at pH 2.5 showed strong positivity of the mucinous material; this reaction was abolished by treatment with hyaluronidase, indicating the presence of nonepithelial stromal mucosubstances. Immunohistochemical stains showed strong positivity of the tumor cells with CAM 5.2, chromogranin, synaptophysin, and neuron-specific enolase, and negative staining with carcinoembyronic antigen and epithelial membrane antigen. Electron microscopy done in one case showed abundant dense-core cytoplasmic neurosecretory granules; there was no evidence of glandular secretory activity by the tumor cells. The tumors in two patients behaved in a highly aggressive fashion, with invasion of the chest wall, recurrence, and metastases to the lungs, pleura, and axillary, retroperitoneal, and mesenteric lymph nodes. Thymic carcinoid should be considered in the differential diagnosis of mediastinal neoplasms displaying prominent mucinous features. Application of immunostains and electron microscopy will be of value for establishing the correct diagnosis in this setting.