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.
Unlike BRAF(V600E), the most common mutation, K601E is strongly associated with follicular-patterned cancer, particularly with the encapsulated follicular variant of PTC, and may also be found in follicular thyroid carcinomas.
Unlike the most common BRAF mutation seen in PTC carcinoma (BRAF(V600E)), this patient's mutation was a BRAF(K601E) mutation that previously has been associated with some cases of the follicular variant of PTC.
Moreover, there is a close association between the type of mutation and the PTC histotype: BRAF(V600E) is associated with conventional PTC and with histological variants of PTC displaying a prominent papillary growth pattern, whereas BRAF(K601E) is associated with the follicular variant of PTC.
The V599E mutation is highly prevalent in PTC with a papillary or mixed papillary follicular growth pattern, and the K600E mutation is apparently restricted to the follicular variant of PTC.