tumor?

Question
Im am guessing my question is regarding the skin since I recently found a hard, odd shaped, less than 5 cm lump on my dogs underbelly by her uppermost left mammary.  It doesn't seem to hurt her at all and she does not have any other symptoms at all.  Appetite is fine and she is as playful as ever.  She is an unspayed, 11 year old maltese who never had a litter.  Her vet saw the lump and said it was a mammary nodule with a high risk of being malignant.  She also found another smaller lump between her bottom mammaries. She recommended immediate surgery to remove the lump and to spay her at the same time. My question is basically one of asking for a second opinion before having her go into surgery.  Is the lump removal and spay surgery the correct next step to take here?  Is there any chance its a fatty lump or blocked milk duct that would go away on its own?  I am just so afraid of having something bad happen while under anesthesia, since she has never been under.  Please help.
Thank you so much for your time and help.
Lily A.  

Answer
I agree with your vet.  You can have the lumps sent off to see what they are, but in an 11 year old unspayed bitch with a mammary nodule, it is probably a mammary tumor.  Many of these are only locally invasive, and they do not spread to other parts of the body. A small number of the softer mammary tumors do spread.

Spaying her will remove the source of the hormones that keep the tumors growing. It will also prevent her from getting pyometra, which is pretty devastating to older bitches.

A healthy 11 year old Maltese is young enough that surgery is not all that risky.