Diagnostic feature of parathyroid carcinoma is

Parathyroid carcinomas may be circumscribed lesions that are difficult to distinguish from adenomas. Cells are usually uniform and resemble normal parathyroid cells and of enclosed in a capsule. Diagnosis of carcinoma based on cytologic detail is unreliable , and invasion of surrounding tissues and metastasis are the only reliable criteria. Ref: Robbins and cotrans 9e Pg 1102