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.

Department of Microbiology and Immunology National University of Singapore Singapore Singapore

Department of Pathology Faculty of Medicine in Plzen Charles University Plzen Czech Republic

Department of Pathology National University Health System 5 Lower Kent Ridge Road Singapore 119074 Singapore

Department of Pathology National University Hospital Singapore Singapore

Molecular and Genetic Laboratory Biopticka Laboratory Ltd Plzen Czech Republic

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