Thyroid Fibrosarcoma as a Rare Differential Diagnosis of Anaplastic Thyroid Cancer: A Case Report.

Introduction: Thyroid gland fibrosarcomas are very rare tumors, with only very few cases have been reported in the literature. Their similarity to anaplastic thyroid cancer poses a diagnostic challenge, often leading to misdiagnosis. Case Report: We report the case of an 87-year-old female with a history of left thyroid nodule who underwent a left lobectomy and subsequently, received levothyroxine therapy. She presented with a rapidly growing mass on the right thyroid gland. Her thyroid function was normal. Ultrasound revealed an ill-defined hypoechoic mass measuring 4 cm on the right thyroid. Fine-needle aspiration biopsy (FNAB) was performed, and cytology indicated Bethesda VI for anaplastic thyroid carcinoma. After the total thyroidectomy, the surgical pathological examination revealed a high-grade fibrosarcoma with extension into the strap muscle. Lymphovascular and perineural invasion was noted. Immunohistochemical staining showed positivity for smooth muscle actin (SMA), and negative for paired-box gene 8 (PAX8), transcription factor 1 (TTF-1), thyroglobulin, and epithelium markers (AE1/AE3). Following surgery, adjuvant therapy with radiation and chemotherapy using ifosfamide was administered. However, the disease progressed with lung metastasis. The treatment was changed to administration of pazopanib, resulting in dramatic improvement of lung metastasis. However, the disease continued to progress, and patient passed away within 2 years after treatment initiation. Conclusions: Although fibrosarcoma of the thyroid gland is exceedingly rare, it should be considered in the differential diagnosis of anaplastic thyroid carcinoma. Immunohistochemistry (IHC) plays a crucial role in supporting the diagnosis. A multidisciplinary approach is essential for its management. In addition to surgery, emerging adjuvant therapies with kinase inhibitors have shown promise in improving patient survival.