Can sonograms miss bc?

Hi- I'm back again with questions:
I'm one year out from Stage 2 diagnosis (invasive ductal-lumpectomy, chemo, radiation).  Felt small lump (pea sized) at about 5 o'clock in my unaffected breastabout a month ago.  My rad oncologist felt it also and sent me last week for mammo, nothing showed (expected this as I have extremely dense breast tissue and the original bc didn't show either).  Immmediately sent for sonogram.  The doc who did the sonogram could feel the lump as well but the sonogram showed nothing.  I was told to follow-up with my surgeon "just in case".
So my questions--What's the possibility this little lump, which can be felt by 3 of us but does not show up on Mamo or sonograms, is bc?  Does this happen very often (having a small palpable lump which does not show up on mam/sono)?  And, if a biopsy is recommended, I want to skip the FNA and go directly to a core--how many cores are standard for a good biopsy of a little lump?  Thanks so much.  Your info has always helped me to put things into perspective!

[CCF-RN,MSN-JS]

Dear vermontgirl, It is impossible to say if this lump is cancer or not without doing a biopsy and checking the sample under the microscope.  The mammogram and sonogram are done as evaluation, and may increase or decrease the suspicion based on the results.  A sonogram is helpful in distinguishing between a fluid filled or a solid structure.  A fluid-filled lump is more likely to be seen on a sonogram.

Usually core biopsy is done with guidance of sonogram or mammogram.  If neither is visualizing the lump, obtaining the sample would rely on feel.  That might best be done by needle, or removal of the entire lump after needle localization.  However, discuss with the surgeon the options, they would be recommending type of biopsy with the least amount of trauma that would give the most information, and discuss with you the pros and cons for the different procedures.