Pheochromocytoma in dogs undergoing adrenalectomy: an exploratory investigation of 24 cases.

Pheochromocytoma is relatively common in dogs and carries a guarded prognosis. Outcome of adrenalectomy is the most important predictor of survival, although younger age and absence of vascular invasion may also be associated with a favourable prognosis. Current histological criteria do not predict a malignant behaviour in dogs, similar to humans. In the latter, the characterization of tumors has been refined by the “Pheochromocytoma of the Adrenal gland Scaled Score” (PASS), which includes 12 microscopic features, and by immunohistochemical analysis. The aim of the study was to investigate PASS and several immunohistochemical markers used in human medicine in dogs with pheochromocytoma that underwent adrenalectomy. Pathology records of dogs with pheochromocytomas were reviewed to identify those treated surgically and tumors were collected. Sections were stained with haematoxylin and eosin to apply the PASS and were single-labelled for chromogranin A, proliferation marker Ki-67, cyclooxygenase-2 (COX-2), tumor suppressor gene product p53, proto-oncogene products BCL-2 and c-erbB-2, vascular endothelial growth factor (VEGF), protein S100. The results of PASS and of immunohistochemistry were compared for surgical outcome, age and frequency of capsular and vascular invasion caused by the tumor. In addition, PASS was assessed in relation to immunohistochemical findings. Twenty-four dogs with pheochromocytomas were included. PASS did not differ based on surgical outcome, age, vascular and capsular invasion. Immunohistochemistry was also not different for any variable. Tumors showing BCL-2 expression in >50% cells had lower PASS than those with lower expression (7±2 vs. 9±2; P=0.011). Tumors positive for S100 had higher PASS than those negative (10±2 vs. 7±2; P=0.001). Associations between immunohistochemical markers were not identified. In conclusion, in dogs with pheochromocytoma, survival, age, vascular and capsular invasion are not linked to the results of PASS and of the selected immunohistochemical markers. The lower PASS in dogs with elevated BCL-2 expression suggests that pheochromocytomas with high anti-apoptotic rate have few morphologic changes. The higher PASS in S100-positive tumors may indicate that pheochromocytomas developing morphologic changes acquire S100 expression. Overall, the use of PASS and of the present immunohistochemical markers is of limited clinical value in dogs with pheochromocytoma.