Proliferative Activity and Invasiveness among Pituitary Adenomas and Carcinomas: An Analysis Using the MIB-1 Antibody

The monoclonal antibody Ki-67 reacts with a human nuclear cell proliferation-associated antigen that is expressed in all active parts of the cell cycle. Recently we have raised monoclonal antibodies, MIB 1-3, against recombinant parts of the Ki-67 antigen. These antibodies are true Ki-67 equivalents, as demonstrated by immunostaining of fresh specimens, biochemistry, and molecular biological techniques. Formalin-fixed, paraffin-embedded sections routinely processed for immunohistochemistry failed to stain for Ki-67 and MIB 2. Antibodies MIB 1 and MIB 3 labelled mitotic figures, while non-mitotic proliferating cells were negative under these conditions. However, when dewaxed microwave oven-processed paraffin sections of formalin-fixed tissues were used, MIB 1 and MIB 3 gave strong nuclear staining of those cells presumed to proliferate under a variety of normal and neoplastic conditions. Moreover, routine decalcification or depigmentation techniques did not alter the immunoreactivity of MIB 1 and MIB 3 with microwave-processed paraffin sections. This method is highly reproducible, easy to perform at low cost, and no additional technical skill is needed because after microwave treatment just routine immunohistochemical methods are used. Since we have successfully applied this new method to sections obtained from paraffin blocks stored for a long time (in one case more than 60 years), the assessment of cell kinetics through the detection of Ki-67 antigen is now possible on archival material collected in histopathology departments all over the world.

Pituitary adenomas may remain intrasellar or infiltrate dura and bone. Invasive adenomas are not considered to be malignant; in biological behavior they are between non-infiltrative adenomas and pituitary carcinomas. The latter are defined as tumors with subarachnoid, brain, or systemic metastasis. Invasion may be defined radiologically, operatively, or histologically. On the basis of operatively assessed tumor size and gross invasion of dura and bone as well as immunocytochemical and ultrastructural analysis of 365 pituitary adenomas, the following data were obtained. There were 23 growth hormone (GH)-cell adenomas: 14% microadenomas and 86% macroadenomas; their overall frequency of invasion was 50%. There were 24 prolactin (PRL)-cell adenomas: 33% microadenomas and 67% macroadenomas, with an overall frequency of invasion of 52%. Mixed GH-cell and PRL-cell adenomas were found in 35 cases; 26% were microadenomas and 74% were macroadenomas, and the overall frequency of invasion was 31%. Sixty patients had adrenocorticotropic hormone (ACTH)-cell adenomas (Cushing's disease): 87% microadenomas and 13% macroadenomas; the overall frequency of invasion was 25% (in 8% of microadenomas and 62% of macroadenomas). Twenty patients had ACTH-cell adenomas (Nelson's syndrome): 30% microadenomas and 70% macroadenomas; the overall frequency of invasion in these cases was 50% (in 17% of microadenomas and 64% of macroadenomas). Silent ACTH-cell adenomas, 100% macroadenomas, were found in 11 patients, with an 82% frequency of invasion. There were 32 follicle-stimulating and luteinizing hormone adenomas, all macroadenomas, with a frequency of invasion of 21%. Four patients had thyroid-stimulating hormone adenomas, all macroadenomas, with a 75% frequency of invasion. Null-cell adenomas were found in 93 cases: 2% microadenomas and 98% macroadenomas, with a frequency of invasion of 42%. There were 63 plurihormonal adenomas (GH, PRL, glycoprotein): 25% microadenomas and 75% macroadenomas, with a 50% overall frequency of invasion. Based on this study, and on their usual frequency of occurrence, the estimated rate of gross invasion by pituitary adenomas of all types is approximately 35%. It is concluded that immunocytochemical and ultrastructural characteristics of pituitary adenomas reflect the tendency of these tumors to infiltrate and hence may be of prognostic significance.

Monoclonal antibody Ki-67 is a reliable and easy means of accurately assessing the growth fraction of human neoplasms. Although the number of long-term follow-up studies is limited, it does appear to provide valuable prognostic information particularly in lymphoproliferative disease. Since the estimation of growth fraction is only one factor influencing tumour behaviour it would be naive to believe that measurement of this parameter alone, no matter how accurately, would provide the clinician with definitive prognostic information for all tumours. The antibody is also of use in research, providing a means of measuring proliferative activity in a variety of conditions besides malignancy, and may prove of value in monitoring tumour response to established and trial therapies.