Papillary thyroid carcinoma with fibromatosis/fasciitis-like/desmoid-type stroma: When to suspect and how to diagnose.

Currently, it is known that more than 60% of adults have one or more thyroid nodules, of which less than 5% are malignant. Among them, papillary thyroid carcinoma (PTC) is the most common thyroid malignancy, accounting for 90% of all thyroid carcinomas. The World Health Organization (WHO) 5th edition endocrine tumours classification includes several PTC subtypes. Of these, PTC with fibromatosis/fasciitis-like/desmoid-type stroma stands out not only for being one of the least frequent subtypes but also for its characteristic histology (biphasic structure) and genetic pattern, which presents mutations in both BRAF and CTNNB1. Despite these distinctive features, it remains a challenging neoplasm to suspect and easy to misdiagnose, primarily due to its radiological imaging and cytological findings. In this study, we report a classic case, review the literature, discuss when suspicion should arise, and provide a practical diagnostic guide.