Defining Variations in the Size of Normal Pituitary Glands and Pituitary Macrotumors Based on Canine Skull Morphology

Computed tomography (CT) is a common imaging modality used to evaluate for a pituitary macrotumor in dogs. However, a standard definition of a pituitary macrotumor based on imaging characteristics has not yet been established. The human definition of a pituitary mass that measures >1 cm in diameter has previously been adopted for dogs but fails to account for variability in size, breed, and skull conformation. We hypothesized that normal pituitary gland size and macrotumor size vary by skull morphology. We also hypothesized that canine pituitary macrotumors may be smaller than 1 cm. In this retrospective, case-control analytical study, contrast CT scans from 89 dogs with imaging-diagnosed pituitary macrotumors and 89 dogs with normal pituitary glands were compared. The height, length, width, and volume of the pituitary gland, sella turcica, and brain were measured along with the brain area at the level of the pituitary gland. Of the pituitary macrotumors, 21.3% (19/89) were smaller than 1 cm in height. Data support that a pituitary gland height >0.60 cm or a pituitary volume of 0.17 cm³ may represent a macrotumor in brachycephalic dogs, and a pituitary gland height >0.65 cm or a pituitary volume of 0.31 cm³ may represent a macrotumor in mesocephalic dogs; small numbers of dolichocephalic dogs precluded determination of cutoffs. These data provide a foundation for future studies to classify pituitary macrotumors on CT imaging according to skull conformation, thereby aiding in the detection, treatment, and response assessment of dogs with pituitary neoplasms.