Ultrasound features of malignancy

Microcalcification:
Most specific (90%) sign of malignancy, seen as hyperechoic foci without shadowing
Seen in approx. 40% of thyroid ca
Most common in papillary ca
Also seen rarely in other ca and adenoma and Hashimoto's
Other types of calcifications include, large calcification (necrosis of ca, goitre), Microcalcification with reverberation artefact (benign nodule), peripheral calcification (MNG)

Absence of halo:
Neither sensitive or specific
Complete halo is highly suggestive of a benign lesion
Halo is absent in more than 50% of benign lesion
15% of papillary ca have complete halo

Taller than wider(AP more than transverse):
As similar in breast ca
Specific sign (90%)

Vascularity:
More than 50% of solid hypervascular lesions are benign
Malignant lesions show marked central flow; peripheral flow is more common with benign lesions
Complete avascular nodule is very unlikely to be malignant

Hypoechoic solid nodule:
Sensitive but not specific
Marked low echogenicity is suggestive of malignancy

Size:
More than 4 cm are more likely to be malignant
Not a sensitive or specific sign
Direct invasion:
Specific sign of malignancy
Common with anaplastic ca, lymphoma, sarcoma

Lymph node involvement:
Occur in about 20%
Common with papillary ca, medullary ca

References:
Hoang JK et al. US Features of Thyroid Malignancy: Pearls and Pitfalls. RadioGraphics 2007;27:847-860