Abstract
Malignant mesenchymal thyroid tumors are one of the rarest types of thyroid cancer. Clinically, these tumors present as a rapidly growing thyroid mass. Due to their rarity and nonspecific findings, they are not the first conditions that come to mind during differential diagnosis. We report herein the case of an 87-year-old woman presenting with a rapidly growing thyroid mass in whom the differential diagnosis of anaplastic cancer was challenging. Following work up, the patient was diagnosed with primary fibrosarcoma of the thyroid, a rare type of malignant mesenchymal thyroid tumor. Because she declined surgery and her clinical condition was unsuitable for chemotherapy, she was treated with palliative radiotherapy. Primary thyroid fibrosarcoma is a rare cause of thyroid cancer and should be considered in the differential diagnosis of rapidly growing thyroid masses.