HURTHLE
CELL CARCINOMA OF THE THYROID – WHY IS IT REPORTED SEPARATELY?
Oncocytic
follicular cells of the thyroid (the so-called Hürthle cells) are
large polygonal cells with marked eosinophilic, granular cytoplasm reflective
of overly abundant mitochondria and can be observed in both benign and
malignant conditions of the thyroid gland. Focal oncocytic trans-differentiation
results in neoplasia consisting predominantly of Hürthle cells
that can represent benign Hürthle cell adenomas or malignant Hürthle cell carcinoma (HCC). Hürthle cells are also seen in the
oncocytic variant of the papillary thyroid carcinoma. HCC represents only 1 – 3
% of all thyroid cancers and is currently classified as a variant of follicular
thyroid cancer (according to the World Health Organization).
The distinction
between adenoma and carcinoma can generally only be definitively made on
histological examination of a resection specimen, based on the presence or
absence of capsular and/or vascular invasion (which are the hallmarks of HCC, and
is defined by the presence of oncocytes, at least 75 % of the entire tumor). Therefore,
preoperative diagnosis of HCC, either radiologically or based on results of FNA
cytology (FNAC) is not possible. FNA cytology can typically be classified as
atypia of undetermined significance / follicular lesion of undetermined
significance (AUS / FLUS) (Bethesda III). Both benign and malignant oncocytic
lesions can demonstrate marked cytologic atypia, complicating the
interpretations of results of FNAC. A cytologic diagnosis of suspicious for HC
neoplasm (Bethesda VI) carries a 15 – 30 % risk of malignancy.
HCC has
some distinctive clinical characteristics such as:
I.A more
aggressive biological behavior compared to the other differentiated thyroid
cancers
II.Higher
rate of distant metastases
III.Decreased
iodine avidity compared with other differentiated thyroid cancers
IV.Relatively
higher risk for lymph node metastases (5 – 13 %) compared to follicular thyroid
cancer and increased risk for nodal recurrence
V.Reduced 10-yr
survival compared to follicular cancer, especially in the presence of lymph
node or distant metastases
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