Answer: Parathyroid adenoma

Histology: Nodules of cellular parathyroid tissue are separated by bands of densely collagenized fibroconnective tissue with focal deposition of hemosiderin. Atrophic thyroid follicles are interspersed between the parathyroid nodules. The parathyroid cells are relatively uniform without significant atypia. Mitotic figures are not identified. Invasion of angiolymphatic spaces is not appreciated.

Discussion: The reliable distinction between a parathyroid adenoma and a parathyroid carcinoma is notoriously difficult. The recognition of a parathyroid carcinoma is aided by the presence of certain microscopic features including:
1) vascular invasion
2) mitotic activity,
3) dissecting bands of dense collagenized fibrous tissue, and
4) local tumor invasion.

By themselves, however, these features may not be entirely diagnostic of malignancy such that interpretation of the microscopic findings must be done with a considerable degree of caution.

Vascular Invasion: Vascular invasion is regarded as the most reliable evidence of malignancy; but unequivocal vascular invasion is noted in only about 15% of parathyroid carcinomas.

Mitotic Activity: Mitotic figures should not be used as a sole feature of malignancy as mitotic activity can be appreciated in a subset of adenomas.

Dense Fibrous Banding: Dense fibrous banding is the least specific finding and should not be used as sole criteria of malignancy. Fibrous banding is frequently encountered in adenomas as a degenerative change. Fibrous banding is also encountered in the bed of a previous excision site in a recurrent parathyroid adenoma.

Local Tumor Invasion: Evaluation of invasion is also notoriously difficult, particularly when there is suspected invasion of the thyroid gland. Parathyroid glands are frequently located within the thyroid capsule and sometimes within the thyroid parenchyma. Consequently, the intimate association of parathyroid and thyroid tissue is not necessarily indicative of thyroid invasion by a parathyroid carcinoma. Interpretation of invasion is further complicated by the fact that nodules of parathyroid tissue can become implanted in surrounding tissue as a result of prior surgery.

Obtain a good medical history before making a diagnosis of parathyroid malignancy. In the present case, it was discovered that the patient developed recurrent hypercalcemia one year after the incomplete removal of a parathyroid adenoma from the same site as the present lesion. Based on this history, the microscopic findings were interpreted as “recurrent nodules of parathyroid adenoma in a prior surgical field.”