As the thyroid gland is fairly accessible, fine needle aspiration cytology is easily performed.
In the UK, thyroid cytology may be categorised into 5 classes. Although it broadly resembling the system used in breast cytology/biopsy, each category generally refers to a set of specific conditions, with an increasing risk of malignancy the higher the category.
Modifications to the original classification were made in 2009 by the Royal College of Pathologists in consultantion with the British Society for Clinical Cytology. These were made mainly to address the several issues, e.g. cystic lesions were not easily categorised in the original categories, and suspicious cells which were not obviously follicular lesions were not easily classified.
In the US, the Bethesda system is preferred.
- Non-diagnostic due to inadequacy (i.e. less than 6 groups of 10 follicular cells) or technical artefact, precluding meaningful interpretation. Implies need to repeat +/- ultrasound guidance.
- Non-neoplastic (usually colloid goitre or thyroiditis). Two Thy2 FNA results 3-6 months apart is generally sufficient to exclude malignancy. If patients has significant compressive symptoms or high risk factors, surgery may still be appropriate.
- Follicular lesion. FNA cannot reliably distinguish between a follicular adenoma and a follicular adenocarcinoma. Implies need for lobectomy for histological confirmation.
- Suspicious of malignancy (medullary/papillary/anaplastic or lymphoma)
- Diagnostic of malignancy (medullary/papillary/anaplastic or lymphoma)
- Non-diagnostic for cytological diagnosis
- Non-diagnostic for cytological diagnosis - cystic lesion
- Non-neoplastic, cystic lesion (the difference between this category and Thy1c is clinical discretion, i.e. if the operator is confident clinically or radiologically that the lesion is in keeping with a cyst)
- Neoplasm possible – atypia/non-diagnostic
- Neoplasm possible, suggesting follicular neoplasm
- Suspicious of malignancy
- ↑ RCP Guidelines for the management of thyroid cancer. 2nd edition (or pdf version).
- ↑ Royal College of Pathologists' Guidance on the Reporting of Thyroid Cytology 2009.
- ↑ Baloch ZW, Cibas ES, Clark DP, Layfield LJ, Ljung BM, Pitman MB, Abati A. The National Cancer Institute Thyroid fine needle aspiration state of the science conference: a summation. CytoJournal. 2008; 5:6.(Epub) (Link to article – subscription may be required.)