Follicular thyroid carcinoma with an unusual glomeruloid pattern of growth.

We describe an uncommon thyroid tumor in a 56-year-old woman. The widely infiltrating, angioinvasive neoplasm, 5 cm in diameter, exhibited a peculiar architectural growth pattern characterized by follicles with round to oval epithelial tufts growing within, often supported by a fibrovascular core mimicking the renal glomerulus. Colloid-empty follicles, tubular or elongated, were lined by pseudostratified tall, columnar cells with clear cytoplasm. Nuclei were round to oval, with evenly distributed, slightly coarse chromatin. Tumor cells were positive for thyroid transcription factor-1, thyroperoxidase, thyroglobulin, cytokeratin 18, Hector Battifora mesothelial cell, and vimentin. Scattered cells positive for S100, Wilms tumor 1 (WT1), and cytokeratins AE1/AE3 were found, with no reaction detected for cytokeratins 34betaE12, 5/6, 7, 19, or 20. There were PAX8-PPARgamma rearrangement and N-RAS mutation. No mutations were found for APC or BRAF genes, nor were RET/PTC rearrangements detected. Because of the distinctive histologic features, we propose naming this tumor follicular thyroid carcinoma with an unusual glomeruloid pattern of growth.