Follicular Lesion of Undetermined Significance (FLUS)

Atypia of Undetermined Significance

Epidemiology and Etiology:

Common sites:

Gross features:

Cytologic features:

• AUS:
• Cells (follicular, lymphoid, or other) with architectural and/or nuclear atypia that is not sufficient to be suspicious or malignant, but more marked than can be confidently called benign
• Any one of the following scenarios:
• Prominent microfollicular population that does not otherwise fulfill criteria for follicular neoplasm or suspicious
• E.g. in a sparsely cellular aspirate with scant colloid
• Predominance of Hurthle cells in a sparsely cellular aspirate with scant colloid
• Hindered interpretation of nuclear features due to sample preparation artifact (e.g. air-drying artifact, pale and slightly smudgy chromatin, and/or mildly irregular nuclear contours, clotting artifact with apparent cellular crowding)
• Moderately or markedly cellular sample composed virtually exclusive population of Hurthle cells, yet the clinical setting suggests a benign Hurthle cell nodule (Lymphocytic/Hashimoto thyroiditis, multinodular goiter)
• Focal features suggestive of papillary carcinoma (nuclear grooves, enlarged nuclei with pale chromatin, irregular nuclear contour and shape) in an otherwise predominantly benign appearing sample (esp. in the setting of lymphocytic thyroiditis)
• Cyst-lining cells which may appear atypical (nuclear grooves, prominent nucleoli, elongated nuclei and cytoplasm, intranuclear cytoplasmic inclusions) in an otherwise benign-appearing sample
• Virtually always accompanied by hemosiderin-laden macrophages
• Minor population of follicular cells with nuclear enlargement often with prominent nucleoli (e.g. history of radioactive iodine, carbimazole, or other pharmaceutical agents, repairative changes such as cystic degeneration and or hemorrhage)
• Atypical lymphoid infiltrate in which flow cytometry is desireable, insufficient for “suspicious for malignancy”
• Not otherwise categorized
• FLUS:
• Follicular cells meeting the above criteria
• Compromised specimen (sparse cellularity, obscured by blood or excessive clotting) often

Immunophenotype:

Marker:

Sensitivity:

Specificity:

Molecular features:

Other features:

• ~5-15% risk of malignancy (20-25% risk of malignancy for those that end up being resected)
• In general (depending on the clinical circumstances), clinical approach is to repeat FNA after a reasonable interval
• In general, AUS frequency should be ~ 7% of all thyroid FNA interpretations

References:

• Ali SZ & Cibas ES (Eds.) The Bethesda System for Reporting Thyroid Cytopathology. Springer, New York, 2010.