Help with mysterious mass- response needed soon

[reanie]

I am 33 years old and have a moderate amound of breast cancer on my mother's side.  My general surgeon and I have found a mass in my right breast.  He said that it was very asymmetric to the left breast, and that it was of concern to him.  We did a core biopsy, and it came back negative for cancer.  I have also had a mammogram and an ultrasound done, the results were no abnormal findings. My question is how can I have a palpable mass that both the surgeon and I can feel and it not show up on mammogram or ultrasound?  What type of tissue can that be?  Also, what is it about asymmetry that is spooking my doc so much?  I take it that is a bad thing?  Next week, I am having an MRI as a last resort to try to keep me out of surgery for excisional biopsy.  I had a fibroid tumor removed in March, and do not want to have it again.
Sorry to sound so green, I am just so uneducated, I thought you nice people may be able to shed some light on my situation.
Thank you for any response

[CCF-RN,MSN-rf]

Dear reanie:  There are lumps and bumps that do not show up on mammogram or ultrasound.  These can be both benign and malignant.  It may be that you are feeling some tissue that is just irregular or scar tissue from your fibroid removal.  Often, breasts do not feel the same.  Since your doctor is concerned, it may be that your only recourse (to be certain) is to do an excisional biopsy, if the MRI does not add helpful information.

[surgeon]

Not every woman has breasts that are mirror images of each other: it's not uncommon to have more glandular tissue on one side, causing "asymmetrical" masses. Many women have one breast significantly larger than the other (for some reason, it's much more common that the left is larger.) So when there's an asymmetric mass, it may be simply that it's more glandularity on one side compared to the other. The question is how far to go to prove it. It sounds as if, between the mammogram, ultrasound, and biopsy, you have a lot of info that says it's ok. Whether to go further is a matter of your comfort, and that of your surgeon.