Thyroid cytology

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As the thyroid gland is fairly accessible, fine needle aspiration cytology is easily performed.

Contents

Classification Systems

In the UK, thyroid cytology may be categorised into 5 classes.[1] 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.[2]

In the US, the Bethesda system is preferred.[3]

Original Categories

Thy1 
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.
Thy2 
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.
Thy3 
Follicular lesion. FNA cannot reliably distinguish between a follicular adenoma and a follicular adenocarcinoma. Implies need for lobectomy for histological confirmation.
Thy4 
Suspicious of malignancy (medullary/papillary/anaplastic or lymphoma)
Thy5 
Diagnostic of malignancy (medullary/papillary/anaplastic or lymphoma)

Modified

Thy1
Non-diagnostic for cytological diagnosis
Thy1c
Non-diagnostic for cytological diagnosis - cystic lesion
Thy2
Non-neoplastic
Thy2c
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)
Thy3a
Neoplasm possible – atypia/non-diagnostic
Thy3f
Neoplasm possible, suggesting follicular neoplasm
Thy4
Suspicious of malignancy
Thy5
Malignant

References

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