Subcategorisation of AUS/FLUS thyroid lesions as per the 2017 Bethesda System for Reporting Thyroid Cytopathology: a retrospective study from a tertiary care centre analysing risk of malignancy (ROM) of the different subcategories

Abstract

Aims: The 2017 Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) recommends subclassification of atypia of undetermined significance (AUS)/follicular lesion of undetermined significance (FLUS) into six subcategories. The present study evaluates the risk of malignancy (ROM) and risk of neoplasm (RON) among these.

Methods: All thyroid aspirates reported as AUS/FLUS over a 4.5-year period, with available histology, were reviewed and subclassified as per TBSRTC. ROM and RON were calculated and compared.

Results: Of 2554 thyroid aspirates, 281 (11.0%) were AUS/FLUS. Eighty-one with available histology were evaluated. ROM was 51.8%. Cytologic and architectural atypia (AUS-C&A) was the most prevalent (62.9%), followed by Hürthle cell type (19.6%), AUS-A (11.1%), AUS-not otherwise specified (NOS) (7.4%), cytologic atypia (AUS-C) (4.9%) and atypical lymphoid cells (1.2%). Papillary thyroid carcinoma (PTC) and adenomatous goitre (AG) were the most common histological diagnoses (27% each). On histology, AUS-C had 2/4 PTC and 2/4 AG on histology. AUS-A had 4/9 follicular neoplasm (FN) and 2/9 non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) while AUS C&A had 18/51 PTC, 13/51 AG, 11/51 NIFTP and 5/51 FN. ROM and RON were similar across subcategories, ROM was the highest for AUS-C&A (58.8%), AUS-C (50%) and AUS-NOS (50%). NIFTP reclassification as non-malignant reduced ROM to 35.8% (absolute reduction of 16% and a relative decrease of 31%) with the greatest relative decrease seen in AUS-A (50%), followed by AUS-C&A (37%), and none in others.

Conclusions: AUS/FLUS subcategorisation helped to indicate risk for the more likely neoplasm, whether PTC or FN. ROM was the highest for cases with cytological atypia but did not differ significantly across different subcategories. NIFTP changed the ROM of AUS-A and AUS-C&A, since both NIFTP and FN have microfollicles.

Keywords: cancer; cytopathology; endocrine pathology; thyroid cancer.