Immunohistochemical Evaluation of Canine Pituitary Adenomas Obtained by Transsphenoidal Hypophysectomy

Hypophysectomy specimens from 16 dogs with pituitary adenoma were evaluated with periodic acid-Schiff (PAS), reticulin, and immunohistochemistry for adrenocorticotrophic hormone (ACTH), melanocyte stimulating hormone (MSH), growth hormone (GH), and Ki-67. The reticulin network was obliterated in all adenomas. One adenoma expressed ACTH and GH. Eight corticotroph adenomas were basophilic to chromophobic, and PAS- and ACTH-positive. Seven melanotroph adenomas were distinguished from corticotroph adenomas by expression of MSH. Pituitary-dependent hypercortisolism was diagnosed in 5 of 8 dogs with corticotroph and 4 of 7 with melanotroph adenoma. Pituitary height/brain area (P/B) ratio was elevated in all dogs. Previous canine hypophysectomy studies suggested that melanotroph adenomas were larger and carried a worse prognosis than corticotroph adenomas; however, in this study, corticotroph adenomas in comparison to melanotroph adenomas were larger (median P/B ratio: 1.06 versus 0.76), more proliferative (median Ki-67 index: 9.47% versus 1.99%), and associated with shorter survival (median: 300 versus 793 days). Recommended immunohistochemistry for PAS-positive pituitary adenomas includes ACTH and MSH to distinguish corticotrophs from melanotrophs and Ki-67 for proliferation index.