Mysterious lump on my rat

Question
I have a female pet rat named Cinnamon. I think she's about three -- I'm not sure about that, but I got her two years ago and her previous owner thought she was about a year old. As far as I know she's never been spayed. She's never had any health problems before and has always been lively and frisky in spite of her age.

A few minutes ago I noticed Cinnamon was walking kind of funny and I had a look at her and discovered a large, flattish lump on her underside, near one of her back legs. It is soft and jiggles a bit, and Cinnamon doesn't seem to be bothered when I touch it. She doesn't seem bothered by the lump at all, actually, except it makes it a little harder for her to walk. She is behaving normally and has a good appetite. I'm pretty sure this lump only appeared within the past few days, and it's already quite big.

My research on the internet suggests a mammary tumor, but would a mammary tumor grow that fast, and appear on a rat this old? Since it doesn't seem to be bothering her, should I take the trouble to have it removed?

Answer

What a sweet old girl!  I love older rats. I have three elderly rats that are over three years old and close to three and they are just so laid back and sweet. We just want to do what is best for them and worry if we are making the right decision, often based on their age.

For starters, if your rat is a candidate for surgery, do not let age stand in her way.  Age is a number, not a disease, so being three doesnt mean she is more of a risk for surgery than a younger rat.  As long as she is healthy and you have a very skilled surgeon that is certified in exotics, her chances are just as good as a younger rat in having a successful surgery.

That said, I will say that for starters, she is not too old for a mammary tumor.  In fact, mammary tumors are usually not seen in rats that are younger.  Intact female rats usually develop mammary tumors after they go through menopause (end of their estrus cycle) usually between the ages of 18 months and 2 years old. Sometimes older depending on the rat herself.  Some people think its a sure thing their older rat has gone through it and will allow the female with males and much to their surprise the female ends up pregnant, so there is no real number as to when they will end their estrus cycle, but it is usually sometime after 18 months of age.   With this, it means their hormone levels sky rocket causing estrogen levels to soar. Estrogen fuels growth and development of mammary tumors.  
It sounds like the prime location for a mammary tumor that is on Cinnamon.  I am hoping its not though and instead, maybe its an abscess.  
If its an abscess, you will notice it possibly have a crater in the center or a scab on it which is really necrotic tissue.  If you notice these things, let me know right away.

If not, and I have my doubts, it is probably a mammary tumor.
The next thing we do is ask if you want to leave it or remove it. Do you have a qualified exotic Vet? I dont mean one that sees exotics but is a general DVM, but one that is devoted to just exotics and have all the proper implements including being able to keep the rat overnight for observation after surgery.
Mammary tumors are usually benign, but this still doesnt mean they dont cause problems and often problems that can end the rats life.

Please check my website on tumors here for info and photos:
http://www.freewebs.com/crittercity/allabouttumors.htm

LEts just start with the chance its an abscess and go from there.