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tvpjournal.com | November/December 2015 | ToDay's VeTeriNary PracTice caNiNe PiTuiTary DePeNDeNT HyPeraDreNocorTicism series Peer reviewed 39 Canine pituitary dependent hyperadrenocorticism (PDH), also known as Cushing's disease, is a common endocrine disorder in older dogs. This disorder is caused by a pituitary adenoma (PA) that secretes inappropriate amounts of adrenocorticotropic hormone (ACTH), which results in bilateral adrenal hyperplasia and disorderly and excessive production of cortisol by the adrenal gland. CLASSIFICATION Basics of Classifcation As with all central nervous system tumors, the Tumor–Node–Metastasis system used by the World Health Organization (who.int) does not apply. Current classifcation systems for PAs in veterinary patients are based primarily on secretory characteristics of the tumor. However, in humans, PAs are currently classifed based upon: • Tumor size and degree of invasiveness (Table 1 ) 1 • Tumor endocrine activity (hormone secretion), or functional classification based on immunohistologic findings, such as ACTH and thyroid- and follicle- stimulating hormones. In both humans and dogs, pituitary corticotroph adenomas that are responsible for Cushing's disease (ie, PDH in dogs) are classifed as functional ACTH-secreting PAs (ACTH-PAs). Further Classifcation The World Health Organization classifcation system for PAs in humans has been refned to include designations for benign adenoma, atypical adenoma, and pituitary carcinoma on the basis of proliferation indices (p53 immunoreactivity, MIB-I Index, mitotic activity) and the absence/presence of metastases. 2 More comprehensive molecular classifcation systems based on relevant gene expression have not been systematically used to further characterize pituitary tumors. Similar work to classify canine pituitary tumors both morphologically and functionally is currently underway. Canine Pituitary DePenDent HyPeraDrenoCortiCism series Part 1: Comparative Epidemiology & Etiology in Dogs & Humans David Bruyette, DVM, Diplomate ACVIM VCA West Los Angeles Animal Hospital, Los Angeles, California, and Veterinary Diagnostic Investigation and Consultation, Woodland Hills, California Table 1. Classifcation of Pituitary Tumors in Humans by Size & Anatomic Location CLASSIFICATION BY SIZE BASED ON RADIOLOGIC FINDINGS Microadenomas Less than 10 mm diameter Macroadenomas Equal to or greater than 10 mm diameter CLASSIFICATION BASED ON RADIOANATOMIC FINDINGS Stage I Microadenomas (< 1 cm) without sella expansion Stage II Macroadenomas (≥ 1 cm); may extend above the sella Stage III Macroadenomas with enlargement and invasion of the foor or suprasellar extension Stage IV Destruction of the sella Peer Reviewed 39