Multifocal Fibrosing Thyroiditis: an Under-recognized Mimicker of Papillary Thyroid Carcinoma

Multifocal fibrosing thyroiditis (MFT) is an enigmatic entity, characterized by multiple fibrotic scar-like lesions with a paucicellular fibrotic center surrounded by a cellular peripheral area with reactive-appearing follicular cell atypia and variable chronic inflammation. Although poorly recognized and likely underreported in surgical pathology, the entity is considered rare with only 65 cases to date–including the current one reported to expand on the preoperative findings of this under-recognized entity. The average age of the patients is 46.8 years (range 15–71 years), 94% are female, with female to male ratio of 15:1. Individual MFT lesions typically have a superficial location. The average number of fibrotic lesions is 15.4 (range 2–51 per MFT case). Their average size is 3.1 mm (range 0.4–15.1). MFT is a disorder of diseased thyroids, typically found postoperatively in glands removed for other reasons, such as chronic lymphocytic/Hashimoto thyroiditis (32.3%), follicular nodular disease (nodular hyperplasia) (30.1%), hyperthyroidism/diffuse hyperplasia (Graves disease) (9.2%). Intriguing is the association with papillary thyroid carcinoma–present in 38.5% of MFT cases, and particularly with sub-centimetric and multifocal papillary thyroid carcinoma, with which MFT can be confused. Cases where MFT is the only thyroid pathology (7.7%) can be preoperatively mistaken for papillary thyroid carcinoma, due to worrisome ultrasound (US) and cytologic features, both of which are here documented for the first time as a component of this article. Wider recognition of MFT and of its cytologic and ultrasound features at preoperative evaluation may reduce unnecessary thyroidectomies.