Thymic Atypical Carcinoid Mimicking Recurrent Type A Thymoma on Frozen Section: A Diagnostic Pitfall Resolved by Intraoperative Imprint Cytology

Thymic neuroendocrine neoplasms (tNENs) are rare anterior mediastinal tumors with aggressive behavior and can be misdiagnosed as type A thymoma on small biopsies or intraoperative frozen sections, although accurate distinction is critical for prognosis and management. Type A thymoma, while generally considered a low-grade malignant tumor with a favorable prognosis, comprises a small subset that exhibits aggressive features and develops distant metastases after surgical resection; these tumors are classified as atypical type A thymomas. A 72-year-old woman had a history of resected atypical type A thymoma two years earlier. Surveillance computed tomography revealed a 15-mm mediastinal nodule located anterior to the superior vena cava with intense fluorodeoxyglucose uptake on positron emission tomography-computed tomography. Frozen sections showed a proliferation of small- to medium-sized polygonal and short spindle cells arranged in solid nests and trabeculae without a lymphocyte-rich background, and were interpreted as recurrent atypical type A thymoma. In contrast, imprint cytology demonstrated monomorphic small- to medium-sized tumor cells with round to oval nuclei, finely granular "salt-and-pepper" chromatin, inconspicuous nucleoli, loose cohesion, and scattered rosette-like structures, strongly suggesting a tNEN. Permanent sections revealed nests, trabeculae, and rosettes of small- to medium-sized polygonal cells with granular chromatin and approximately four mitoses per 10 high-power fields, without large confluent necrosis. Immunohistochemistry showed diffuse positivity for CD56, chromogranin A, synaptophysin, and insulinoma-associated protein 1; a Ki-67 index of about 20%; negativity for CD5, CD117 (c-KIT), p63 (TP63), CK5/6, and CD20; and the absence of TdT/CD99-positive immature T cells, supporting a diagnosis of thymic atypical carcinoid. This case highlights the complementary value of imprint cytology and an appropriate immunohistochemical panel, in addition to frozen sections, in avoiding misclassification of tNENs as type A thymoma.