Ultrasound findings of papillary thyroid microcarcinoma: a review of 113 consecutive cases with histopathologic correlation.

Papillary thyroid microcarcinoma (PTMC) is a malignant thyroid tumor of less than 1 cm in size. Ultrasound (US) is the preferred imaging modality for PTMC. This study retrospectively evaluated the US results from 113 consecutive cases of PTMC with 127 nodules and correlated the results with the histopathologic findings. The results showed none of the PTMC nodules had a cystic element. Half of the PTMC nodules had a well-defined boundary and the remaining half had an ill-defined boundary. A percent (6.3%) of the nodules had a halo sign. Microcalcification was present in approximately half of the PTMC nodules. The cancerous tissue with a papillary structure was hypoechogenic and the amount of fibrous stroma determined whether the nodule echogenicity was heterogeneous or homogeneous. Heterogeneous echogenicity was associated with a fibrous stroma proportion of >20%. Only PTMC nodules with predominantly follicular structures containing a large proportion of colloid demonstrated isoechogenicity. Well-defined boundaries on US were associated with well-defined histologic margins, with or without the presence of an intact fibrous pseudo-capsule. A halo sign with a well-defined boundary detected by US was associated with the presence of an intact fibrous pseudo-capsule. Half of the PTMC nodules with ill-defined boundaries exhibited infiltration into the surrounding thyroid tissue, while the remaining half of the PTMC nodules with ill-defined boundaries on US presented irregular nodule margins by histology, rather than nodule infiltration. Psammoma bodies detected at a frequency ≥5 per ×200 microscopic field of view were detectable on US as microcalcifications.