Page 587 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

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Tumors of the Endocrine System

KATHARINE F. LUNN AND SARAH E. BOSTON

Pituitary Tumors
Pituitary-dependent hypercortisolism (PDH) is the most com-
Primary tumors of the pituitary gland can arise from several mon form of spontaneous HAC in dogs and cats, accounting for
different cell types, including corticotrophs, somatotrophs, thy- 80% to 85% of cases in these species. This disorder is a conse-
rotrophs, gonadotrophs, and lactotrophs. The clinical signs asso- quence of autonomous synthesis and secretion of ACTH from a
ciated with these tumors depend on the tumor’s size and secretory pituitary tumor. The secretion of ACTH from the pituitary tumor
properties. In a recent study of pituitary glands collected from is chronically excessive, leading to bilateral adrenal cortical hyper-
136 dogs and 65 cats during routine necropsy, pituitary neopla- plasia and hypercortisolemia. The pituitary tumor is relatively
sia was detected in 14% of the middle-aged and old dogs; how- insensitive to negative feedback by cortisol, and a loss of hypotha-
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ever, in most cases the lesions were considered to be incidental. lamic control over ACTH release occurs because corticotropin-
Pituitary tumors also may be detected incidentally when brain releasing hormone (CRH) secretion is suppressed by the chronic
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imaging is performed for unrelated reasons. The most clini- hypercortisolemia. 10
cally important pituitary tumor in the dog is the corticotroph Pituitary tumors that secrete ACTH are derived from pitu-
adenoma. This tumor produces chronically excessive amounts of itary corticotroph cells; approximately 70% to 80% arise from
adrenocorticotrophic hormone (ACTH) and is associated with the pars distalis of the pituitary gland, which is mainly under the
clinical signs of hypercortisolism. In the cat, the most clinically control of CRH. The remainder of these tumors arise from the
significant pituitary tumor is the growth hormone (GH)–secret- pars intermedia, which is predominantly under dopaminergic and
ing somatotroph adenoma, which causes hypersomatotropism serotoninergic control, with less regulation by CRH. This distinc-
and acromegaly. tion between pars intermedia and pars distalis tumors typically
Nonfunctional pituitary tumors become clinically significant is not made clinically or from the results of diagnostic testing,
when they are large enough to cause neurologic signs, including but it does potentially help explain why some pituitary-directed
obtundation, stupor, behavioral changes, decreased appetite, gait medical therapies are not efficacious in all dogs with PDH. The
abnormalities, seizures, blindness, narcolepsy-cataplexy, and other specific molecular defects that lead to the development of canine
cranial nerve abnormalities. 3–7 The pituitary gland may also be corticotroph tumors continue to be investigated, and it gener-
affected by secondary tumors, either through direct extension ally is believed that these tumors result from somatic mutations
or by metastatic spread from a distant site. Locally invasive or that occur within the corticotroph cells, rather than from chronic
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compressive primary or secondary pituitary tumors also have the stimulation of the corticotrophs by hypothalamic factors in com-
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potential to cause loss of pituitary function, resulting in hypo- bination with decreased dopaminergic tone. There is only one
thyroidism, hypocortisolism, gonadal atrophy, or central diabetes clearly documented report of ectopic ACTH secretion in the
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insipidus. dog, which may reflect the fact that it is very difficult to prove
this diagnosis.
Pituitary tumors may be described as macrotumors or micro-
Pituitary Corticotroph Tumors: tumors. The latter distinction is derived from human medicine:
Hyperadrenocorticism microtumors are less than 1 cm in diameter, and macrotumors
are 1 cm or larger in diameter. The use of this size-based classifica-
Pathogenesis tion is controversial in veterinary medicine, at least partly because
of variability in patient size and conformation. Pituitary tumors
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Hyperadrenocorticism (HAC), also termed hypercortisolism or may also be classified as noninvasive adenomas, invasive adeno-
Cushing’s syndrome, is a common endocrine disease of middle-aged mas, or adenocarcinomas. The latter term is reserved for tumors
and older dogs. It is uncommon in cats. This clinical syndrome in which there is demonstrated evidence of metastatic disease.
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results from chronic exposure to excessive blood levels of gluco- Canine pituitary adenocarcinomas are uncommon. In a study of
corticoids. Naturally occurring canine and feline HAC is usually 33 dogs with pituitary tumors that underwent necropsy evalua-
either pituitary dependent or a result of excessive glucocorticoid tion after brain imaging, 61% had a pituitary adenoma, 33% had
secretion from an adrenocortical tumor. an invasive adenoma, and 6% had an adenocarcinoma. 3

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