False Positive Biopsy

Hello,
I am 40 years old.  When I was 29 I discovered that I had a 12cm (13x8) lump in my breast.  I was told that it was a fibroadenoma with refined borders and that I should not worry.  I then had routine check ups annually until I was 34 and the lump remained the same size and shape.
2 months ago, as a result of recurring breast pain, I decided to do a mammography.  The result was that the lump was the same size but they concluded that now there are also some calcifications.  They recommended a fine needle biopsy.
The result of this first biopsy revealed that I had some malignant cells and the lump was a ductal carcinoma G2.  They soon after did a core biopsy.  To my surprise, this second biopsy revealed that the lump was a fibroadenoma and calcifications.
Now I don't know how to interpret these results.  One doctor said that I'm clear and I should just remove the lump for peace of mind.  Another said that she still believes that there are some malignant cells and she was lucky to have seen them from the first biopsy since they didn't appear in the second.
My question:
1. How is it possible that in one biopsy there was a presence of malignant cells and in the second biopsy (the one that should be more accurate) there are none?
2. Is it possible for a biopsy to falsely detect malignant cells?  If so, how?  Are there some cells that look like malignant cells and then with the second biopsy it clarifies that they are not?

[japdip]

Fine needle biopsy obtains such a small amount of specimen that they are not always 100% reliable. I really don't believe a ductal tumor would be that large. The malignant cells may have come from the area of microcalcifications. I would definitely opt for removal of this mass since it is quite large and the Pathology report on the tissue removed would certainly be the most accurate. This would also include removal of the area of microcalcifications. Removal for whatever reason I feel is the best choice under these circumstances. A Breast MRI may be valuable since there seems to be some measure of confusion.     Regards ....

[MariP]

Thank you very much for responding.  I'm so sorry, I meant to write that the lump is 12mm.  I read this so many times and I didn't even realize my mistake until my brother pointed it out.  12 cm would indeed be enormous!  Does this change your response/opinion?  Again, sorry for the mistake.

[japdip]

Glad it was just mm ... and not cm. Doesn't change my response though.... no problem; we all make those type of mistakes .... even without realizing it. My thing is leaving out a word which changes the entire picture ... we have an excuse; we're HUMAN.