The Australian experience with the Bethesda classification system for thyroid fine needle aspiration biopsies

Fine needle aspiration biopsy (FNAB) is the initial investigation of choice for thyroid nodules. The Bethesda system, which classifies thyroid FNABs into different categories each linked to a risk of malignancy has been widely adopted. However, the risk of malignancy implied by each Bethesda category is likely to vary due to population characteristics and inconsistency in the application of diagnostic criteria. We present our experience of the Bethesda system in 2076 thyroid nodules from 1410 patients. Categories were as follows: 266 (12.8%) were category 1 (B1) non-diagnostic, 1551 (74.7%) category 2 (B2) benign, 97 (4.7%) category 3 (B3) atypia of uncertain significance, 98 (4.7%) category 4 (B4) suspicious for follicular neoplasm, 16 (0.8%) category 5 (B5) suspicious for malignancy and 48 (2.3%) category 6 (B6) malignant. Surgery was performed on 425 nodules from 315 patients. Malignancy rates in the target nodules were B1 4.2%, B2 0.26%, B3 9.3%, B4 15.3%, B5 79% and B6 100%. Twelve patients with B3 nodules underwent repeat FNAB, with eight reclassified as B2, one as B3, one as B1 and two as B4. An incidental microcarcinoma separate to the target nodule was identified in 11.1%. As applied in our institution, and despite very sparing use of B3 and B5 categories, our audit has demonstrated risks of malignancy broadly in keeping with that predicted. Of note, the risk of malignancy in the clinically indeterminate categories of B1, B3 and B4 were all at the lower ranges of those predicted in the Bethesda atlas and mostly lower than those reported by other studies.