A solid fleshy pink-tan encapsulated tumor replaces the thyroid. There appears to be a rim (capsule) at the periphery of the mass.
Follicles exhibit nuclear features of papillary carcinoma. Nuclear clearing and overlap is evident. There is scalloping of colloid.
Here is another case but the nuclear features are not obvious. There are some nuclear grooves.
Follicles are variably sized and contain deeply eosinophilic colloid.
Some of the follicles are quite distended with colloid. There is some nuclear clearing, but no nuclear pseudoinclusions were identified in this 6 cm mass.
Note the irregular dissection by dense sclerotic bands.
Again, this encapsulated entirely follicular tumor nuclear features of papillary carcinoma, although inclusions were not identified specifically. There is significant nuclear overlap and irregularity, with chromatin clearing.
Extensive searching finally yields an intranuclear inclusion (arrow). The number of intranuclear inclusions in FVPTC is usually less than that seen in conventional PTC (DeLellis).
The follicular variant of papillary thyroid carcinoma (FVPTC) has proved to be a diagnostic challenge to pathologists. There has been much debate regarding the prognostic implications of this variant, but much studies have shown an identical, if not slightly improved prognosis, compared to classic PTC (Salajegheh).
Grossly, these tumors are often encapsulated and invasion into the capsule can occur. Histologically, variably sized follicles comprise much of the tumor. Papillary structures are rarely and only focally present. Deeply eosinophilic colloid with scalloped edges may also be present in the lumina. The nuclear features of FVPTC are similar to those seen in classic PTC i.e. large clear nuclei with grooves, nuclear pseudoinclusions and nuclear overlap/crowding. In fact, the key feature to distinguish FVPTC from follicular adenoma/carcinoma are the nuclear features (DeLellis, Thompson). However, a subset of encapsulated tumors may have subtle nuclear features, making diagnosis more problematic than usual.
In terms of molecular aberrations, the BRAF K601E mutation appears unique to PVPTC (Trovisco). However, it is not as frequent as the BRAF V600E mutation seen in classic PTC, and interestingly, the V600E mutation has about 2.5x the kinase activity compared to the K601E mutation (Salajegheh).
Some studies document a lower frequency of cervical node metastases and extra-thyroidal extension than conventional papillary carcinoma. However, long term outcomes appear highly similar.
→FVPTC must be distinguished from follicular adenoma/carcinoma.
DeLllis RA, Lloyd RV, Heitz PU, Eng C. Tumors of Endocrine Organs: WHO Classification of Tumours. Lyon; IARC Press; 2004: 60.
Salajegheh A, Petcu EB, Smith RA, Lam AK. Follicular variant of papillary thyroid carcinoma: a diagnostic challenge for clinicians and pathologists. Postgrad Med J. 2008 Feb;84(988):78-82.
Thomspon LDR. Endocrine Pathology: Foundations in Diagnostic Pathology. Philadelphia, PA: Elsevier; 2006: 88-89.
Trovisco V, Vieira de Castro I, Soares P et al. BRAF mutations are associated with some histological types of papillary thyroid carcinoma. J Pathol. 2004 Feb;202(2):247-51.