Nicki I understand how you feel.  Did your doctor order a diagnostic ultrasound when you had your mammo?  The ultrasound determines if a lump is solid or if it is fluid filled like a cyst....which could play a part in their reasoning for it being benign - otherwise you could request that one be done or you could ask to see your films.  A cancerous spot has a pretty distant look.  When your films are held up to light your lump will show as a bright white spot, the not so bright white white, gray and black areas are just the density of your breast tissue.  If your lump or bright white spot is perfectly shaped chances is that it is benign.  If the white spot has crackle edges, looks like sun-rays or resembles a crab (anything with lines coming out of the edges of the white spot) runs the risk of being cancerous.

You can see a copy of my mammogram films that I scanned and posted on my blog here.  Click on each photo and it will enlarge.  All 4 of my films are available and you can note the differences between each breast.  You'll see the red circle I placed around my cancerous lump.  I knew as soon as I saw my films that I had cancer because it looked exactly like the mammogram pamplet they give you - the example in the pamplet and my films were near identical....though I swear it was more easily seen in the originals, as the scans aren't as large or as filled with detail as the originals.

You can also type in a google search for examples of images of a benign and malignant tumor of breast cancer.

I'm only suggesting you see for yourself so you can rest assured.  If you don't like what you see definitely get a second opinion - take your films with you to the new doctor as (s)he will want to see them - but I am hoping once you see them yourself you will feel better with your diagnosis.

Best of luck!

Angi