Answer: Cystic parathyroid adenoma

Histology: Nests of neuroendocrine cells are separated by broads bands of dense collagenized stroma with extensive hemosiderin deposition. There is cystic change within the center of the mass. The fibrosing reaction involves the capsule of the thyroid gland, but there is no overt invasion into the thyroid parenchyma or into other structures of the neck. Vascular invasion is not identified. The tumor cells have uniform round nuclei with inconspicuous nucleoli, and pale to clear cytoplasm. Mitotic figures are not identified.

Discussion: Dense fibrous banding is a characteristic feature of parathyroid carcinoma, but is not a very specific finding. Dense fibrous bands are also encountered as a degenerative or retrogressive change in parathyroid adenomas, particularly those that have undergone cystic degeneration (i.e. cystic parathyroid adenoma). One feature that is helpful in recognizing fibrous banding as a degenerative feature is the associated hemosiderin deposition. Because of the non-specificity of fibrous banding, the diagnosis of malignancy (i.e. parathyroid carcinoma) must be supported by other more specific morphologic findings. These include local tumor invasion into adjacent structures, vascular invasion, and metastatic spread. While mitotic figures are more frequently encountered in parathyroid carcinomas compared to adenomas, the presence of atypical mitotic forms are specific for malignancy. Medullary thyroid carcinoma may enter the differential diagnosis, but the fibrotic bands do not stain with Congo red, the tumor cells are not immunoreactive for calcitonin, and the presence of hyperparathyroidism is usual known at the time of surgery.