Endocrine Pathology Online
Pathology of Follicular Thyroid Carcinoma
Full thickness capsular transgression and or vascular invasion in or beyond the capsule,
are the absolute defining features for follicular carcinoma of the thyroid gland.
Since this is an architectural feature which may not be evident until many blocks are
examined, frozen section is not recommended in diagnosis of follicular thyroid lesions.
Subdivision into minimally invasive and widely invasive subtypes (obvious
macroscopic invasion) is prognostically useful as is grading.
Beware of the rare non-encapsulated variant which may be widely invasive without
any capsule in which to see the invasion.
Where histological features of papillary carcinoma are also present, the diagnosis
becomes that of follicular variant papillary carcinoma, which behaves as papillary
True vascular invasion must be distinguished from impaction of tumour cells into
vessels at cut-up.
Elastin stains and immunohistochemistry for endothelial markers are possibly
overrated because true vascular is generally apparent without their use.
Aberrant cytological features, necrosis and vascular invasion within the tumour
confined by the capsule point to a diagnosis of "atypical adenoma".
These have a benign course.
Fine needle aspiration cytology may implant follicular epithelium into the capsule
giving a pseudomalignant appearance.
Over diagnosis of follicular neoplasms is a risk if the features of hyperplastic nodules
in multinodular goiter are overlooked.
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