Papillary Thyroid Carcinoma with Desmoid-Like Fibromatosis: Double Trouble?

A plethora of histological PTC subtypes exists, many of them with distinct correlations to underlying genetics and prognosis [1]. PTC with desmoid-type fibromatosis (PTC-DTF) is an exceedingly rare subtype consisting of two distinct components, a mainly BRAF p.V600E-mutated PTC intermingled with a CTNNB1 driven soft tissue neoplasm [2, 3]. To date, only around 50 cases have been reported in the literature [4]. The tumor has previously been described as “PTC with nodular fasciitis-like stroma,” but this nomenclature is no longer endorsed in the new WHO classification given the distinct characteristics of this disease [3]. The exact proportions of the PTC and DTF components are not established for this entity, and the extent of the desmoid-type stroma may vary in individual cases [5]. Interestingly, most PTC-DTF cases are categorized as Bethesda V-VI lesions and are rarely described as schwannoma or fibroma [5]. The histological diagnosis of PTC-DTF requires the use of appropriate immunohistochemical biomarkers, as illustrated in this case. The VE1 antibody helps highlight the PTC component, and the nuclear accumulation of beta-catenin in the DTF component also helps to distinguish this entity from PTCs with a reactive, desmoplastic stroma [3].

As PTC-DTF is infrequently encountered, little is known regarding the association to clinical parameters and patient outcomes. While distant metastases have not been defined yet, PTC-DTFs often display locally infiltrative disease (extrathyroidal extension into strap muscle or adjacent structures) as well as central and lateral lymph node metastases [5]. Interestingly, while most nodal metastases only contain the PTC component, subsets of cases display nodal metastases including both the PTC and DTF components [5]. The PTC component seems to be RAI avid, and the mesenchymal component is believed to lack RAI avidity. Other adjuvant oncologic therapies have also been used, but the rarity of PTC-DTF makes any correlations to patient outcome unreliable [4]. However, pathologists should be aware of this rare subtype of PTC given the unique characteristics of these neoplasms to enable their distinction from other entities.

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