Metastatic medullary thyroid carcinoma or calcitonin-secreting carcinoid tumor of lung? A diagnostic dilemma in a patient with lung mass and thyroid nodule

Calcitonin-secreting neuroendocrine tumors are rare and have been reported in literature as case reports or case series in various organs including lung, pancreas, larynx, bladder, and ovary. These tumors have similar cytologic features and immunohistochemical profile to medullary thyroid carcinoma and thus it is difficult to distinguish them, especially when calcitonin-secreting neuroendocrine tumors are intermediate or higher grade and there is a mass lesion in the thyroid gland. Here, we report a rare case of calcitonin secreting atypical carcinoid tumor of the lung in a patient with thyroid nodule. However, after extensive ancillary studies on the thyroid gland, no tumor was detected and subsequent resection specimen revealed a pulmonary atypical carcinoid tumor with metastasis to level 11 lymph node. Being aware of this entity has significant clinical, diagnostic, and therapeutic implications and can prevent unnecessary thyroidectomies with subsequent possible morbidities.