Diagnostic value of a cytomorphological subclassification of follicular patterned thyroid lesions: a study of 927 consecutive cases with histological correlation.

Background: Fine-needle aspiration cytology (FNAC) has proved to be an effective diagnostic tool in patients with thyroid nodules. Several reporting schemes have been suggested to define the risk of malignancy and consequent clinical management. To date, however, among lesions showing a follicular pattern, FNAC is still unable to differentiate between benign and malignant ones. The aim of our study was to evaluate whether a subclassification of follicular cytologic specimens, based on cytoarchitectural patterns, could differentiate categories with a different risk of malignancy, thus improving the clinical management of patients harboring follicular nodules.

Methods: We report a cohort of 927 consecutive cases who underwent thyroid surgery in our hospital between 2000 and 2008. Each patient underwent FNAC before surgery. All the cytologic specimens were divided into five categories (Thy 1: inadequate material, Thy 2: benign, Thy 3: indeterminate, Thy 4: suspicious for malignancy, Thy 5: malignant). Thy3 specimens were further divided into three subcategories (Thy 3a, or "follicular lesions of indeterminate significance": scant colloid, microfollicular pattern, or small clusters of thyrocytes with round nuclei usually without, but sometimes with, minimal cellular pleomorphism; Thy 3b, or "follicular neoplasm": absence of colloid, small clusters, or microfollicles of medium-large sized cell populations arranged in cohesive groups with nuclear overlapping, crowding, and pleomorphisms; and Thy 3c or "Hurthle-cell neoplasm": scant colloid, sheets or clusters of oxyphilic cells).

Results: Thy 1 specimens (51 cases on the whole) proved to be malignant in 5.88% (3 cases), Thy 2 specimens (319) in 3.45% (11 cases), Thy 4 specimens (91) in 84.62% (77 cases), and Thy 5 specimens (172) in 98.84% (170 cases). Thy 3 specimens (294 cases) proved to be malignant in 17.35% as a whole, but when divided into the three subcategories, the percentage of malignant cases was significantly different between the Thy 3a group (4.95%) and the Thy 3b and Thy 3c groups (25.0% and 22.77% respectively).

Conclusions: This study supports the National Cancer Institute consensus showing a different risk of malignancy for "follicular lesions of undetermined significance” compared with "follicular neoplasms" and "Hurthle cells neoplasms," which are more suspect for malignancy. This subclassification could improve clinical management of thyroid nodules, helping to better select patients for surgery or follow up.