Bethesda classification of thyroid FNAC

What it is

Fine-needle aspiration cytology of the thyroid is read by the Bethesda system. Thyroid FNAC will provide you with a Bethesda score and a cytopathologic category, and those are the only two things you need to know about it. The categories are six:

• I: non-diagnostic or inadequate
• II: benign, colloid or thyroiditis
• III: atypia or follicular lesion of undetermined significance, FLUS
• IV: follicular neoplasm or suspicious for follicular neoplasm, FN
• V: suspicious for malignancy
• VI: malignant

Category against malignancy risk against management

• Category I, non-diagnostic: malignancy risk 1 to 4%; repeat the FNA.
• Category II, benign, colloid or thyroiditis: risk under 3%; observe, with US at 12 to 18 mo. US follow-up is the whole of it.
• Category III, atypia or FLUS: risk 10 to 30%; repeat the FNA, or US follow-up, or molecular testing, or lobectomy.
• Category IV, follicular neoplasm: risk 25 to 40%; diagnostic lobectomy.
• Category V, suspicious for malignancy: risk 50 to 75%; surgery, lobectomy or thyroidectomy.
• Category VI, malignant: risk 97 to 99%; definitive surgery, near-total thyroidectomy.

One line on the table is worth holding: the suggested treatment is the treatment prior to availability of molecular testing. That is why molecular testing appears as an added choice for the atypia category in the newer table and not in the older one.