Due to progressive dyspnea, a male patient aged 59 underwent medical examination in 2003 in a local hospital. Neck ultrasound and fine-needle aspiration biopsy (FNAB) of a suspect lesion in the thyroid gland revealed the presence of a malignant neoplasm, i.e. mesenchymal tumor. Immunocytochemistry for epithelial membrane antigen, chromogranin A and leukocyte common antigen (CD45) was negative, while vimentin and S-100 were positive. The patient was referred to a university hospital center, where further oncologic work-up was done. Neck ultrasound revealed a tumor in the left lobe of the thyroid, with extension to the aortic arch. After repeated FNAB, cytologic diagnosis of primary thyroid fibrosarcoma was established. Due to the locally advanced and consequently inoperable disease, primary radiotherapy to the neck region (64 Gy in 32 fractions) was applied, followed by 6 cycles of chemotherapy with doxorubicin. After completion of therapy, computed tomography scan demonstrated significant regression of primary disease, but it was still not amenable to surgical treatment. Thus, the decision of the oncology board was active surveillance of the patient. During 9-year follow up, no signs of progression or activity of the disease were found.