Positive (malignant) aspirate

Thyroid cancer (although the most common endocrine malignancy) is rare, accounting for less than 1% of solid tumours. Annual UK incidence of thyroid malignancy is approximately 2.3 per 100,000 women and 0.9 per 100,000 men. Primary thyroid malignancies can be divided into differentiated (papillary and follicular carcinoma), medullary and anaplastic carcinomas, being in the proportions of 94%, 5% and 1% respectively in iodine-sufficient areas. Although the prognosis is greatly variable between different types, it is estimated that overall only 9% of patients diagnosed with thyroid cancer die of their disease.

Non-Hodgkin lymphomas - although rare - may also involve the thyroid as a secondary tumour. As primary lymphomas usually occur within a background of Hashimoto's thyroiditis (and are therefore difficult to differentiate cytologically and clinically) they are sometimes a cause of false-negatives, and often classified as suspicious rather than positive.

General criteria for malignancy

• High cellularity
• 3D clusters
• Absent or scanty, dense colloid
• Lack of bare nuclei
• Single cells with dense or granular cytoplasm
• Nuclear membrane convoluted (grooving-pseudoinclusions)
• Coarsely granular chromatin