Anaplastic Transformation in Femoral Metastasis of Papillary Thyroid Carcinoma with Lung Involvement: a Rare Case of Late Recurrence after 47 Years

We report a case of a 75-year-old woman who experienced late recurrence in the lung as papillary thyroid carcinoma (PTC) 47 years after her first surgery, followed by rapid progression to distant metastasis and anaplastic transformation in the femur within one year.Despite 4 months of surgery, chemotherapy and radiotherapy, she eventually died.Immunohistochemical (IHC) analysis of the distant metastatic lesion revealed loss of TTF-1 and thyroglobulin, positive staining for PAX-8 and BRAF, and high expression of Ki-67, consistent with anaplastic transformation of PTC.Although anaplastic transformation in distant metastases is extremely rare, this case demonstrates its occurrence even after a long period, highlighting the diagnostic value of IHC staining in such extraordinary presentations.