Tumor questions

So my largest rat has developed a tumor just above one of her back legs. I am very financially strapped right now, but if surgery would be worth it I'm going to try my best.

I guess my first question would be how I would tell if this is a fatty or cancerous tumor? I'm thinking fatty, but I could just be seeing what I want to. Plus, she is a very chunky rat. She's only between a year and a half and two years old... that just seems so young for this.

Any tips on dietc change or anything else that might slow the growth or be generally healthy would be greatly appreciated.

Does the tumor seem to be just under her skin and not attached? or does it seem to be attached and deep?
Is she spayed?
Pictures?

Nothing with added sugar because sugar feeds cancer
Feed a very healthy diet, good quality block, vegs every day and occasional healthy treats like a piece of fruit, berries, cooked grains (such as steel cut oats, quinoa etc), organic pumpkin seeds, etc
The tumor will steal the nutrition from the food she is fed, leaving her with little.

It seems just under the skin. Like I can kind of move it around.

If she is not spayed, then it is likely a mammary tumour
http://ratguide.com/health/neoplasia/mammary_tumor.php

and http://www.ratbehavior.org/SpayTumorIncidencePetRat.html