Assessment of Mitotic Activity in Pituitary Adenomas and Carcinomas

Assessment of mitotic activity represents one of the oldest and most routinely used histopathologic methods of evaluating the biological aggressiveness of human tumors. In the case of pituitary tumors, however, the relevance of this approach as a means of gaging tumor behavior remains ill-defined. In this article, the relationship between the mitotic index and biological aggressiveness of pituitary tumors was evaluated in a series of 54 pituitary adenomas and 6 primary pituitary carcinomas. All tumors were fully classified by immunohistochemistry and electron microscopy; adenomas were further stratified on the basis of their invasion status, the latter being defined as gross, operatively, or radiologically apparent infiltration of dura or bone. Mitotic figures were present in 11 tumors, 10 being either invasive adenomas or pituitary carcinomas. A significant association between the presence of mitotic figures and tumor behavior was noted, as evidenced by progressive increments in the proportion of cases expressing mitotic figures in the categories of noninvasive adenoma, invasive adenoma, and pituitary carcinoma (3.9, 21.4, and 66.7%, respectively; Fisher’s exact test, two-tailed,p<0.001). The mitotic index, however, appeared to be a less informative parameter, being extremely low in all cases (mean=0.016%±0.005 [±SEM]). Although the mean mitotic index in pituitary carcinomas (0.09%±0.035) was significantly higher than the mean mitotic index of either noninvasive adenomas (0.002%±0.002) or invasive adenomas (0.013%±0.005), no practical threshold value capable of distinguishing these three groups was evident. Comparison of the mitotic index with Ki-67 derived growth fractions in these tumors revealed a significant but weak linear correlation (r=0.41,p<0.01). These data suggest that when, mitotic figures are present, they do provide some indication of the behavior and invasive potential of pituitary tumors. For routine diagnostic purposes, however, the discriminating power of this parameter is somewhat limited, being superseded by alternative and more informative methods of growth fraction determination such as that provided by the Ki-67 immunolabeling.