Regarding tumors. Mast cell tumors when removed release histamines. Always ask your vet if they are going to administer antihistamines "prior" to surgery. Sadly many pets have survived the surgery only to succumb to shock due to the rapid release of histamines because they were not given antihistamines prior to surgery.
Mast cell tumors can be malignant and benign. So don't think they have to be cancerous for this to happen.
"Because of the histamine, heparin, and enzymes present in mast cell tumors, they can create problems when damaged or removed. Large amounts of these substances can be released into the body and have significant effects on heart rate, blood pressure, and other body functions."http://www.peteducation.com/article.cfm?c=2+2087&aid=461