Atypical morphological features in secondary and tertiary hyperparathyroidism mimic malignancy: a detailed...

Patients with secondary or tertiary hyperparathyroidism due to chronic renal failure who fail medical treatment require sub-total or total parathyroidectomy. The histological characteristics of parathyroid glands resected in this setting are not well studied; however, there is awareness that the findings may mimic malignancy. We conducted a detailed clinicopathological study of parathyroid resection specimens from an unselected Royal North Shore Hospital (RNSH) cohort (n=110) and an external consultation cohort (n=44). The most common atypical features observed in the RNSH cohort were fibrous bands (43%) and fibrous capsule (29%), while sheet-like/trabecular architecture (12%), invasive/pseudoinvasive growth pattern (4%) and mitotic activity (9%) were less prevalent. Clinical follow-up data were available for these cases, and none showed recurrence or metastasis. In the external consultation cohort, fibrous bands (84%) and fibrous capsule (66%) were also the most frequently observed features; however, invasive/pseudoinvasive growth pattern (34%) and sheet-like/trabecular architecture (34%) ​were more commonly seen in this group. One case in this cohort demonstrated unequivocal vascular invasion, fulfilling the diagnostic criteria for parathyroid carcinoma. However, at 5-year ​follow-up, there was no evidence of recurrence or metastasis. In summary, atypical histological features are common in parathyroid glands resected for secondary or tertiary hyperparathyroidism and should not be misinterpreted as atypical parathyroid tumour or carcinoma. Parathyroid carcinoma is exceedingly rare in this context, and even when strict diagnostic histological criteria are met, the long-term prognosis may be favourable.