Page 593 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 26 Tumors of the Endocrine System 571

acromegaly 103 ; the same group reported the outcome of surgical for up to 4 weeks with no apparent improvement in GH levels. 81,112
hypophysectomy in 21 diabetic cats in abstract form. 104 These two However, another study showed that GH levels were significantly
reduced for up to 120 minutes postinjection in five cats with acro-
publications provide relatively little detail but in summary: 3 of
VetBooks.ir 21 cats died postoperatively; all 18 surviving cats experienced a megaly that received a single dose of octreotide. These studies used
113
reduction in serum IGF-1 concentrations; and 14 of these cats
the short-acting form of octreotide and were performed over a very
achieved diabetic remission. 104 As was noted previously transsphe- short time without assessment of the clinical response. Both short-
noidal hypophysectomy is technically challenging, and it requires and long-acting pasireotide have been studied in acromegalic cats,
an experienced surgeon and coordinated team of specialists. with encouraging results. 114,115 Treated cats experienced improved
RT rarely is used as a first-line therapy in human medicine. 105 insulin sensitivity, and serum IGF-1 concentrations were reduced.
In contrast, RT is the treatment for feline acromegaly that has Diarrhea was a common side effect, and the expense of this medica-
a
been most widely reported in the veterinary literature. Conven- tion is a significant impediment for most cat owners.
tional fractionated RT protocols range from five fractions given For many cats with acromegaly, insulin therapy is the only
weekly to as many as 20 fractions given over a period of 4 weeks. treatment available or acceptable to the owner. In general these
Improved diabetic control is one potential outcome of RT ther- patients should receive the amount of insulin necessary to control
apy, although IGF-1 concentrations do not appear to correlate their diabetes, although adequate blood glucose regulation can be
with this improved control. 76,109 Few reports are available on the difficult to achieve in many cases. In-home blood glucose moni-
long-term follow-up of acromegalic cats receiving conventional toring, with close cooperation between the owner and the vet-
RT; therefore it is difficult to assess the risk of complications of erinarian, is strongly recommended. Feeding a low-carbohydrate
this modality in this species. However, published case reports and diet also may be beneficial. These patients should be expected to
case series of cats receiving conventional RT suggest that short- receive insulin doses in the range of 10 to 20 units per dose or
and long-term adverse effects are relatively uncommon. more. Concurrent illnesses and complications of acromegaly and
Disadvantages of RT as a treatment for feline acromegaly diabetes mellitus also should be addressed. The short-term prog-
include cost, availability, and the necessity for repeated anesthetic nosis for cats diagnosed with acromegaly generally is fair to good,
83
events. The latter disadvantage can be lessened by the use of SRT, but the long-term prognosis is poor without specific therapy.
which is widely used in the treatment of pituitary tumors in Patients may succumb to cardiac or renal failure, neurologic dis-
human patients. In an early case series in veterinary medicine, cats ease, or complications of poorly regulated diabetes mellitus. In
81
with pituitary tumors received treatment with a linear accelerator- one early case series the MST was 20.5 months.
76
based modified radiosurgical approach. Cats received a single
large dose of radiation, but it was delivered in a nonconformal Adrenal Gland Neoplasia
fashion. The technique was reported to be safe and effective. A
recent study documented the outcome of SRT, delivered in 3 or The prevalence of primary adrenal gland tumors in the dog
4 doses, for the treatment of acromegaly in 53 cats. 110 The overall and cat is difficult to discern from the literature. For dogs and
MST was 1072 days, with no association noted between tumor cats undergoing necropsy or adrenal surgery, tumors of the
size or serum IGF-1 concentration and survival. Causes of death adrenal cortex appear to be more common than those of the
included neurologic disease, heart disease, kidney disease, and medulla. A number of case series over the past 15 years have
unrelated diseases. Ten cats experienced acute radiation effects, all documented the outcome of adrenal surgery in dogs. 20,22,116–120
of which responded to prednisolone therapy. Seven of 50 cats in When the data from these cases was combined, a histopathologic
which serum T concentrations were monitored developed hypo- diagnosis was reported for a total of 304 adrenal tumors, with 227
4
thyroidism months to years after SRT. Diabetic regulation was (75%) arising from the adrenal cortex and 72 (23%) from the
followed in 41 of the 53 cats, and 39 experienced a reduction medulla. The remaining tumors included two myelolipomas and
in insulin dose, with 13 (32%) cats going into diabetic remis- one each of fibrosarcoma, lymphoma, and leiomyosarcoma. For
sion. 110 SRT, therefore, appears to be a safe and effective therapy the adrenocortical tumors that were further classified, 114 of 199
for feline acromegaly. This therapy is becoming more widely avail- (57%) were carcinomas, 77 of 199 (39%) were adenomas, and 8
able, at least compared to hypophysectomy, and compared to con- of 199 (4%) were described as hyperplastic lesions. It is important
ventional RT, it offers considerable advantages in terms of owner to note the bias inherent in this data because only dogs that under-
time commitment and the risks of anesthesia and hospitalization went surgery were included. Functional cortisol-secreting tumors
in elderly diabetic cats. of the adrenal cortex are responsible for 15% to 20% of canine
Medical therapy for acromegaly commonly is used in humans and feline cases of naturally occurring HAC, with PDH account-
either as a first-line treatment or as an adjunct to surgery or RT. ing for 80% to 85%. Functional adrenocortical tumors in dogs
The classes of drugs used are somatostatin analogs, GH-receptor and cats also can secrete one or more sex hormones, including
antagonists, and dopamine agonists. GH-receptor antagonists androstenedione, progesterone, 17-hydroxyprogesterone, testos-
have not been evaluated in cats, and dopamine agonists do not terone, and estradiol. These tumors may or may not secrete gluco-
appear to be useful in this species. 111 corticoids, and some patients show signs of HAC in the absence of
Somatostatin analogs, also termed somatostatin receptor ligands elevated cortisol levels on typical screening tests. Signs of sex hor-
(SRLs), bind to somatostatin receptors, suppressing the release of GH mone excess with sex hormone–secreting adrenal tumors appear
from the pituitary gland. These medications are available as long- or to be more common in cats than dogs.
short-acting preparations, and the response to SRLs is assessed by Imaging techniques such as ultrasonography, CT, and MRI
measurement of IGF-1 and GH levels and tumor size, and evaluation have greatly enhanced the ability to identify both clinical and
of clinical signs. Octreotide has been evaluated in a small number subclinical adrenal abnormalities; it appears that the adrenal
of cats with acromegaly. In five cats short-acting octreotide was used gland is affected with neoplasia more commonly than previ-
ously suspected. The ability to detect these adrenal lesions also
a References 62, 75, 76, 81, and 106–109. leads to diagnostic dilemmas as the clinician attempts to elucidate