Predictors of aggressive clinical phenotype among immunohistochemically confirmed atypical adenomas

Despite formal pathological criteria, not all atypical pituitary adenomas display clinically aggressive behavior. We set out to determine which factors predict a clinically aggressive phenotype among a cohort of atypical pituitary adenomas. Medical records were retrospectively reviewed from April 2008 to July 2015. Of 569 pituitary adenomas, 47 (8.3%) patients were surgically treated for atypical adenomas as defined by the WHO criteria. Clinically aggressive adenomas were defined as occurring in those patients who necessitated additional therapeutic intervention after the index (first) surgery, including additional surgery, medical therapy, or radiosurgery. Forty-seven patients with histopathological and immunohistochemical confirmation of atypical adenomas were identified and of these, 23 were noted to have a clinically aggressive course. Among the remaining 24 patients, the disease remained quiescent after the index surgery. On univariate analysis, clinically aggressive lesions were more likely to have a larger axial diameter on MRI (2.9±1.9cm vs. 1.9±0.7cm, p=0.02), greater incidence of cavernous sinus invasion (65.2% vs. 20.8%, p<0.01), and greater incidence of clival extension (60.9% vs. 0, p<0.01) on preoperative imaging. The two groups were equivalent with regard to immunohistochemical staining for ACTH, HGH, LH, FSH, PRL, and TSH. Clinically aggressive lesions, however, trended towards a greater average MIB-1 proliferative index (7.5%±4.9 vs. 6.0%±3.6, p=0.03). On multivariate analysis, the MIB-1 proliferative index trended towards statistical significance (p=0.06) as an independent predictor of clinical aggressiveness. Atypical pituitary adenomas are defined by a rigid set of immunohistochemical markers, but not all necessarily demonstrate an aggressive clinical phenotype.