Atypia in Follicular Neoplasm: Making a Case for Follicular Variant of Papillary Thyroid Carcinoma
Berrin Ustun, David Chhieng, Constantine Theoharis, Manju Prasad, Adebowale J. Adeniran
Department of Pathology, Yale School of Medicine, New Haven, CT, USA
ABSTRACT
Background: The Bethesda thyroid classification describes follicular neoplasm (FN) as a cellular lesion showing microfollicular architecture with scant or absent colloid. Fine-needle aspiration (FNA) diagnosis of FN is a screening test that does not differentiate between a benign and malignant tumor. The majority of thyroid nodules (up to 80%) diagnosed as FN are benign upon histologic examination. This study is designed to determine the predictive value of cytologic diagnosis in a subset of FN and offer a practical guide for thyroid physicians by identifying significant risk factors for malignancy based on cytologic atypia.
Design: Based on a retrospective review of cytologic diagnosis between January 2008 and December 2011, all thyroid FNA cases with the diagnosis of FN were reviewed. A subset with cytologic atypia – some features suggestive but not diagnostic for Papillary thyroid carcinoma follicular variant (FVPTC) – was identified. The PPV of the cytologic interpretation of FN with atypia for neoplasia (including adenoma and carcinoma) and that for malignancy were calculated.
Results: A total of 38 cases of thyroid FNA (29 female and 9 male) with the cytologic diagnosis of FN with atypia (and with surgical follow-up) were identified (representing 12% of the total number of cases diagnosed as FN with surgical follow-up over this time period). All patients had undergone either lobectomy with completion thyroidectomy or total thyroidectomy. The 38 FNA samples resulted in the following distribution of final histological diagnosis: Neoplastic – 30/38 (out of which 26 were malignant), Benign – 8/38. The positive predictive value for neoplasia and malignancy were 78% and 68% respectively. The malignant cases were predominantly FVPTC (19/26). Others included classic PTC (5/26) and follicular carcinoma (2/26).
Conclusions: The reported incidence of malignancy in FN is 10%-30%. FN with subtle atypical features has a much higher rate of malignancy (68%). The main diagnostic challenge is to differentiate FVPTC from other follicular lesions. Subclassifying FN based on presence of atypia has implications for management. This subset of patients will benefit from a more aggressive follow-up including immediate referral for lobectomy.
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