Anaplastic Thyroid Carcinoma With Predominant Intrathoracic Progression: A Case Report.

While most thyroid cancers have a favorable prognosis, anaplastic thyroid carcinoma (ATC) is highly aggressive and results in a poor prognosis. ATC frequently arises from preexisting benign goiters or highly differentiated thyroid cancers, typically presenting with rapidly progressing local symptoms and often accompanied by distant metastasis. We report a case of ATC that rapidly progressed to intrapulmonary metastases and malignant pleural effusion. Despite a previous histological diagnosis of benign goiter and the absence of local symptoms at the final presentation, the patient died just 20 days after the onset of symptoms. In patients with thyroid tumors accompanied by rapid progression of intrathoracic lesions, ATC must be considered as a differential diagnosis, even if they were previously diagnosed with a benign goiter.