Cytologic Features of Thyroid Lesions Diagnosed as Indeterminate for Neoplasia Which Predict Follicular Neoplasm on Fine Needle Aspiration Biopsy of Thyroid (FNAB): A 5-Year Retrospective Study in a Tertiary Care Hospital in Ontario, Canada

IT Ahmed, KT Mai, S Islam. The Ottawa Hospital, The University of Ottawa, Ottawa, ON, Canada.

Background: Follicular lesions (FL) of the thyroid encompass non-neoplastic and neoplastic lesions. Diagnosis of follicular lesion is a challenging area in the interpretation of thyroid FNAB. Objectives: The purpose of the study is two-folds: 1). To identify cytologic features that will more accurately predict neoplasia (FN) and 2). To recognize diagnostic pitfalls in the differential diagnosis of follicular lesions (FL).

Design: A computer database search was performed for FL, indeterminate for neoplasia on FNAB of thyroid between January 2003 and August 2008 from the archives of the Cytopathology section at our institution. A total of 504 cases were retrieved with a diagnosis of FL; 205 (40.6%) had a histological follow-up and were retained for the study.

Results: Histological follow-up of 205 cases showed follicular Adenoma (FA) in 53 (25.8%), hurthle cell adenoma (HA) in 19 (9.2%), multi-nodular goitre (MGN) in 95 (46.3%), papillary carcinoma, follicular variant (PTCFV) in 25 (12.2%), thyroiditis (THY) in 8 (3.9%) and follicular carcinoma (FC) in 5 (2.4%) patients. Cytologic features that accurately predict FA (48 specimens, 91%), FC (4 specimens, 80%) and PTCFV (23 specimens, 92%) are tight microfollicle formations (defined as acinar structures formed by crowed enlarged nuclei with chromatin clearing and inconspicuous nucleoli) present in >60% of the smears and scant to nil colloid. Another important cytological fearture than we found in our study is the presence of nuclear grooves in more than 5% of follicular cells, which predicts presence of neoplastic lesion. However, presence of nuclear grooves in more than 30% of follicular cells are predictive of papillary carcinoma. Nuclear grooves in more than 5% of follicular cells were found in FA (16 specimens, 30%), FC (2 specimens, 40%) and PTCV (16 speicmens,64%) on FNAB. The most common diagnostic pitfall is follicular cells wrapped up in clotted blood and endothelial cells (MGN, 87 specimens).

Conclusions: The cytologic features that will increase diagnostic specificity of FN (FA, FC and PTCFV) are tight microfollicle formations present in more than 60% of the smears, nuclear grooves in >5% of cells and scant colloid. The most common diagnostic pitfall is clotted blood.