4mm cyst

I just had my first 6 mos f/u mammogram after having been dianosed last year with early stage bc. I also had an u/s because I was having some pain in the left breast (the one that had the microcalifications). The mammo came out clear. The u/s however shows a 4mm cyst that the surgeon wants me to come in and get checked.  Why would the cyst show up on u/s and not the mammo? At this time I am not concerned about this. My next question is should I be?  Thanks

[japdip]

Different tests show different things ... US is used mainly to show cysts. Some do show on mammograms but not clearly enough to be diagnosed definitely as fluid filled or solid mass. An Ultrasound is often ordered following a mammogram to clarify.