A case of thyroid fibromatosis, a rare lesion of this gland.

Thyroid fibromatosis is a very rare lesion; to our knowledge, there are only four cases reported in the medical literature. Herein, we report the clinical case of a woman with thyroid fibromatosis with a long follow-up (11 years). A 63-year-old female patient, with an increasing multinodular goitre without compressive symptoms, was admitted to total thyroidectomy. The histology revealed a spindle-cell proliferation with fibroblastic characteristics with no atypia and thin capillary vessels. Immunohistochemistry was positive for beta-catenin, focally to desmin and alfa-actin and negative for cytokeratins and CD34. Thyroid cells did not display any features of papillary thyroid cancer. These characteristics were compatible with thyroid fibromatosis. For the past 11 years, the patient has been periodically followed up with neck CTs and she has not shown any signs of recurrence. Thyroid fibromatosis has been associated with invasion of surrounding structures in previous reported cases. However, this aggressive behaviour was not observed in our patient. The most challenging differential diagnosis is with papillary thyroid cancer with fibromatosis-like stroma, in which the malignant component is usually peripheral. Therefore, in these cases, it is mandatory to perform an extensive examination of the resected sample. LEARNING POINTS:Fibromatosis is a mesenchymal lesion that consists of an infiltrative proliferation of fibroblasts without atypia.Thyroid fibromatosis is a rare entity in this gland. In previously reported cases, it has been associated with an invasive behaviour but this was not the case in our patient.When spindle-cell proliferation with fibroblastic/myofibroblastic characteristics is detected on thyroid histology, it is mandatory to exclude a papillary thyroid carcinoma with fibromatosis-like stroma.