Atypical parathyroid adenoma

The presence of a fibrous capsule and traversing thick, hyalinised fibrous bands seen in this case are not usually seen within a regular parathyroid adenoma and are a noted feature of atypical parathyroid adenoma, along with a solid and trabecular growth pattern and scattered mitotic figures. 

Parathyroid Carcinomacarcinoma vs. Atypical Parathyroid Adenoma atypical parathyroid adenoma

Parathyroid CarcinomacarcinomaThe definitive diagnosis of parathyroid carcinoma is the presence of metastatic disease or localised invasion into surrounding structures.  HistologicalHistological features of parathyroid carcinoma include:-  Capsule

• capsule invasion and involvement of surrounding structures-  Lymphovascular / Perineural
• lymphovascular/perineural invasion-  Elevated
• elevated mitotic rate with atypical mitotic figures

Atypical Parathyroid Adenomaparathyroid adenomaAtypical adenomas lack the definitive evidence of parathyroid carcinoma but display some abnormal features:-  Thick

• thick hyalinised fibrous bands-  Incomplete
• incomplete capsular invasion-  Prominent
• prominent trabecular growth pattern-  Necrosis
• necrosis (real, tumour-associated necrosis)-  Increased
• increased mitotic rate (>1/10hpf) -  uncommonuncommon

Note:  AdenomasAdenomas likely need to show more than just one abnormal feature to be designated as 'atypical', with some suggesting 2+ to qualify for a diagnosis.

ImmunohistochemistryPTH is a great stain to determine the parathyroid nature of the cells.   There aren'tThere are not any definitive or diagnostic immunohistochemical stains you can use to distinguish between benign and malignant (or potentially malignant) parathyroid neoplasms, however, the following have been used in research settings:

Adenoma (vs. 'Normal 'normal'):  neurofilament, GFAP, vimentin, less intense PTH and chromograninParathyroid Carcinoma:  Ki67, galectin-3, cyclin D1, bcl2, MDM2