Has she confirmed malignancy through any testing (such as fine needle aspiration to look at cells)? Usually malignant tumors are solid and can wrap around organs, etc; whereas benign tend to be more 'moveable' beneath the skin. I've also found, just from my own experience, that malignant tumors tend to be not only more solid in appearance, but often a darker color, almost a dark red/purple and often very venous. Yes, mammary tumors can and often do grow back, and yes, there is always some risk with anesthesia. It's a tough call. With any open wound, there's always some risk for infection, short of keeping her on very clean surfaces (newspaper, clean fleece, etc. changed daily) and an antibiotic to counter any infections (like amoxicillin or your bactrim). Maybe this is a 'keep an eye on things over the next week or two) to see how things go. If you can keep the area clean and bacteria-free to some extent, then perhaps she can go with it 'as is', but if things worsen 'as is' then the surgery may be the only other option. My heart is with you; very hard situation because all you want is for your little one to be okay. Sending many healing wishes your way; please keep us updated.