Cathepsin B and Cathepsin D Expression in Follicular Adenomas and Carcinomas of the Thyroid Gland.

Biologic features that distinguish follicular adenomas (FAs), minimally invasive follicular carcinomas (MICs), and extensively invasive follicular carcinomas (EICs) of the thyroid gland are not well understood. Endogenous proteases, including cathepsin B (CB) and cathepsin D (CD), have been linked to tumor progression in other malignancies and may be important in the different biologic behavior of these follicular thyroidal lesions. Archival paraffin-embedded tissue sections from 16 FAs, 12 MICs, and 4 EICs were studied for immunohistochemical expression of CR and CD. Percent of tumor staining, intensity of staining, and intracytoplasmic staining pattern were assessed. Increased intensity of staining with CD was observed in follicular carcinomas compared to adenomas and was greatest in the EIC (p < 0.04). An increase in diffuse cytoplasmic staining pattern with CD (p < 0.008) and CB (p < 0.02) was observed in follicular carcinomas compared to FAs. The increased intensity of staining with CD in the follicular thyroid carcinomas and the increased diffuse cytoplasmic staining pattern with CD and CR in these carcinomas suggest that these proteinases may play a role in their propensity to invade and metastasize and, therefore, their more aggressive behavior. Furthermore, this diffuse cytoplasmic staining suggests an altered intracellular processing of these proteinases in the carcinomas.