Answer: Pituitary adenoma

Histology: The marrow of the bone is infiltrated by a mononuclear cell process. On high power examination, the cells have fine chromatin and eosinophilic cytoplasm. A vague nesting pattern is apparent. By history, this patient had previously undergone resection for a pituitary adenoma.

Discussion: Metastatic carcinoma to bone would typically demonstrate more cytologic atypia as well as desmoplastic reaction. Meningioma represents another benign lesion which can involve bone; however, the cells of the meningioma would be more syncytial in their growth pattern, would demonstrate whorling, and would demonstrate intranuclear cytoplasmic inclusions. The reactive bone in the current lesion demonstrates changes that resemble those of Paget’s disease; however, in the setting of the adenoma involving bone, these are best interpreted as reactive.

As the current lesion shows, not all epithelioid cell lesions involving bone represent malignancies. Meningiomas, pituitary adenomas, and sinonasal inverted papillomas may involve bone yet bone invasion is not a criterion for malignancy in these specific lesions.