Best Post of December '08: What's an 'atypical pituitary adenoma'?

Next up in our "Best of the Month" series is December, 2008's pick. This was an easy choice since, because it was the holiday season, there was only one post that month! Here it is:

We recently had a case at our institution of an atypical pituitary adenoma, confirmed by Bernd Scheithauer at the Mayo Clinic. What is implied by the designation of a pituitary adenoma as being “atypical”? Burger, Scheithauer, and Vogel -- in their textbook Surgical Pathology of the Nervous System and Its Coverings by (4^th Edition, 2002), page 469 – have the following to say on the matter: “In an effort to identify tumors likely to behave in an aggressive manner, a histologic category intermediate between ordinary or ‘typical’ adenomas and pituitary carcinoma has be established. The designation ‘atypical adenoma’ denotes tumors showing increased proliferative activity, that is, more than an occasional mitosis, an MIB-1 labeling index exceeding 3%, and p53 immunoreactivity. Because combinations of these findings are clearly associated with invasion and/or recurrence, the designation ‘atypical’ earmarks potentially more aggressive lesions. Its utility in identifying tumors capable of metastasis remains to be established in prospective studies.” In an associated photo caption, it is mentioned that nucleolar prominence is also a common feature of atypical pituitary adenoma.