Atypia of Undetermined Significance/Follicular Lesion of Undetermined Significance

Abstract

The classification of “indeterminate” lesions (those not clearly benign or malignant) in thyroid cytopathology has long been a source of confusion for both pathologists and clinicians. There has been much variation in how cytopathologists perceive, interpret, and report such aspirates, especially when the uncertainty relates to follicular lesions. Clinicians have tended to lump interpretations like “follicular lesion,” “atypical,” “follicular neoplasm,” “indeterminate for neoplasia,” and “suspicious for malignancy” into a single “indeterminate for malignancy” category for conceptual and even management purposes. Follow up studies, however, have shown significantly different clinical outcomes for distinct subcategories within the generic indeterminate category. For this reason, it is advisable to define and distinguish categories with distinct risk associations for malignancy, like “Suspicious for follicular neoplasm” versus “Suspicious for malignancy (e.g., papillary carcinoma).” In a minority of cases, the cytologic and/or architectural atypia encountered is of uncertain significance: it is of an insufficient degree to qualify for any of the suspicious categories. Such cases have a lower risk of malignancy and deserve to be separated from the suspicious categories.