SOX11 expression in a case of papillary thyroid carcinoma with fibromatosis/fasciitis-like stroma containing BRAF c.1799_1801delTGA and CTNNB1 c.133T>C mutations

We describe a case of papillary thyroid carcinoma with fibromatosis/fasciitis-like stroma (PTC-FLS) that contained the rare BRAF c.1799_1801delTGA (p.V600_K601delinsE) mutation, which has not previously been reported in this tumour, as well as the CTNNB1 c.133T>C (p.S45P) mutation. We also report the novel observation that spindle cells of the mesenchymal component exhibit diffuse nuclear but not cytoplasmic expression of SOX11, whereas the malignant epithelial cells did not. This suggests that immunoreactivity for SOX11 can be an alternative diagnostic tool for evaluating cases of PTC-FLS where the nuclear expression of β-catenin is ambiguous.