How much can be determined from a mammogram?

I'm 32 and had my first mammogram.  I've have fibrocystic breasts since being a teenager, so my doctor and I decided we should begin to monitor the changes (two of my dad's sisters and two of my mom's aunts had breast cancer), I'm still unclear on if this counts as "history").  I also was given an ultrasound during the same appointment.  The ultrasound reflected a number of fibroadenomas and cysts that were not of a concern per my report.  The mammogram didn't show much due to the density of my breast tissue, but there was a cluster of heterogeneous microcalcifications in one breast that "indicated malignancy."  There was no associated lump with these calcifications, so I am waiting for a call back to schedule a stereotactic biopsy.  My general practitioner, who delivered the news, was *very* concerned and l was left with a feeling of impending doom. After doing a marathon of internet research, I was left with the impression that this type of calcification cluster did not indicate malignancy, but rather this type (heterogeneous cluster) did not provide enough information to say one way or the other, with 25-30% being malignant. I also read that, without an associated tumor, the calcifications most likely pointed to a non-invasive diagnosis rather than level 4 or level 5.  I was really given no information or statistics at all.
(1) Can a radiologist make a good "guess" that my heterogeneous microcalcification cluster is *likely* malignant just by looking at it?
(2) How often does this type of calcification turn out to be an invasive form of cancer?

When I left the doctor's office Friday, I felt like I should be making "arrangements."  

Thank you in advance for your comments.

[japdip]

The only arrangement you should be making is to have your recommended biopsy. Any time calcifications are observed in clusters or groups they are an indication that the breast is reacting to some abnormality within the tissue. This abnormality is NOT necessarily cancer ! A great number of BIRAD 4 scores do indeed come back with a benign finding. I don't approve of all the internet research ... it's much to general and only serves to confuse the issue. When the Radiologist "sees" a cluster of calcifications it is automaticly a recommendation for some type of biopsy. The BIRAD 4 score indicates that the area is suspicious and need further investigation. A BIRAD 5 score is highly suggestive of a malignancy. Invasive or otherwise .... doesn't depend upon the nature of the calcifications IF the result is not benign. The two types of imagery(mammogram & ultrasound) see things differently and see different things. They are often used together for a complete picture and both are valuable tools. Relax and don't be making those "arrangements" that you spoke about ... PLEASE !   Regards ....