Oncocytic Transformation in Pituitary Adenomas: Immunohistochemical Analyses of 65 Cases

CONTEXT: Oncocytic change in pituitary adenomas has been evaluated by electron microscopy and more recently by immunohistochemistry. The clinical significance of this change is not well known, although some reports suggest a relationship with more aggressive behavior. OBJECTIVE: To assess the frequency of oncocytic change in pituitary adenomas and to correlate this finding with clinicopathologic factors. DESIGN: We studied oncocytic change in a series of 65 pituitary adenomas by immunohistochemistry. According to the percentage of oncocytic cells stained by antimitochondrial antibody, adenomas were classified in 3 groups: 50% or more, 10% to 49%, and 1% to 9% of oncocytic cells. RESULTS: Eight cases (12.3%) showing at least 50% of oncocytic cells were classified as oncocytic adenomas: 6 were gonadotroph adenomas and 2 were null-cell adenomas. Among the remaining cases, 9 (14%; all gonadotroph adenomas) showed 10% to 49% oncocytic cells, and in 14 cases (21.5%; 5 gonadotroph adenomas, 6 somatotroph adenomas, 2 corticotroph adenomas, and 1 thyrotroph adenoma) between 1% and 9% were shown. Patients with adenomas that showed oncocytic change presented more frequently at a higher average age (P =.05), but no relationship with extrasellar extension or proliferative activity measured by Ki-67 was observed. In somatotroph adenomas, cases with oncocytic change showed higher percentages of Ki-67 (P =.05) but no correlation with extrasellar extension or cytokeratin staining (dot pattern versus perinuclear) was found. CONCLUSION: Adenomas with oncocytic change present more frequently in older patients, but they are not clinically more aggressive. In addition, somatotroph adenomas with oncocytic cells show similar cytokeratin pattern and higher proliferative activity, which is not correlated with local aggressiveness.