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Feline pituitary adenomas

© B. P. Meij, R. H. van der Vlugt-Meijer, T. S. G. A. M. van den Ingh, G. Flik and A. Rijnberk Melanotroph Pituitary Adenoma in a Cat with Diabetes Mellitus Vet Pathol 42:92-97 (2005). http://www.vetpathology.org/cgi/content/full/42/1/92

In dogs, pituitary-dependent hyperadrenocorticism (PDH) is a common disease. The causative adrenocorticotropic hormone (ACTH)–producing tumour in the adenohypophysis may arise not only from the anterior lobe (AL) but also from the pars intermedia (PI).16,23 In cats, PDH is a rare disease and is usually associated with diabetes mellitus.

In both dogs and cats the melanotroph cell is the abundant cell type of the PI. Corticotropic cells are a minority of the parenchymal cells of the canine PI, this being even more pronounced in the feline PI.7,24 Functional and morphologic studies have provided evidence that in dogs PI tumours may originate from both corticotroph and melanotroph cells.22,28 Also, in cats with PDH PI tumors have been found, staining positively for both ACTH and alpha-melanocyte–stimulating hormone (alpha-MSH).15

Pituitary adenomas are considered benign tumors. However, in humans they may invade surrounding tissues such as dura mater, cavernous sinus, and the sphenoid sinus.12 On microscopic examination, the incidence of dural invasion by pituitary adenomas has been reported to be as high as 45%.12 The so-called giant invasive pituitary adenomas are characterized by one or more of the following features: 1) extensive invasion of dura mater and cavernous sinus, 2) extensive suprasellar extension with lateralization, and 3) extensive erosion of the skull base and invasion of sphenoid sinus.9 These tumors look histologically benign, but because of their extension and infiltration of regional surrounding structures, they have an aggressive and rather malignant clinical course (high rate of recurrence after surgical resection).9 The authors realize that there is a contradiction in the term ‘invasive adenoma.’ Nevertheless, for comparative reasons the authors feel that the use of this terminology in the cat in this study as well is justified.

Figs. 2, 3. Fig. 2. Contrast-enhanced computed tomography in a 13-year-old male, castrated, domestic shorthair cat with a melanotroph pituitary adenoma. A transverse image revealed an enlarged, uniformly enhancing pituitary gland (arrow) measuring 6.2 mm in height and 6.8 mm in width. Fig. 3. Dynamic contrast-enhanced computed tomography in a 13-year-old male, castrated, domestic shorthair cat with a melanotroph pituitary adenoma. A strongly enhancing area (arrow) in the right-central part of the pituitary indicated the presence of an adenoma.
   

Pituitary carcinoma is a diagnosis that is best made on both pathologic and clinical criteria. Although histologic features may show cellular pleomorphism, mitotic figures, and nuclear atypia, which are suggestive of a high proliferating potential, the clinical course may still be relative benign. For this reason, the diagnosis of pituitary carcinoma has been reserved for tumors with cerebrospinal or extracranial metastasis.10 Invasive pituitary adenomas are not considered pituitary carcinomas.10 In the cat in this study, the adenoma was found to infiltrate the neural lobe, and histologically the tumor was an adenoma. There were no characteristics of a pituitary carcinoma.

In cats presenting with insulin-resistant diabetes mellitus, one should consider GH excess (acromegaly) and hyperadrenocorticism. In addition, there is the possibility of a double adenoma, i.e., somatotroph and corticotroph adenoma leading to GH excess and hypercortisolism.13

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