Figure 3b-8: Prolactinoma with atypia and resistance to dopamine agonist treatment. Prolactinomas consist of sheets of relatively small, monomorphic cells (top left) with a cap-like (‘Golgi-pattern’) staining for prolactin (top right). Most pituitary adenomas of any lineage, including lactotroph adenomas, do not show any mitotic figure in routine stains. Some lactotroph adenomas in men can be resistant to dopamine agonists and histologically atypical, as in this example: Mitotic figures are common, the Ki67/MIB-1 proliferation index is high (10-20% [usually <3%], bottom left) and nuclear p53 is overexpressed (bottom right). This indicates a high likelihood of tumour recurrence if incompletely excised (as proved to be the case in this instance).