Can it spread without being in the other breast?

I'm glad to hear that your mom got a PET.  They want to make sure the BC has stayed in the breast region (only affecting her breast and lymph nodes by the breast). Having a PET, CT or CT/PET is a standard step in the assessment.

Be aware, however, that PET scans are notorious for false positives.  The indicate excess glucose activity which is often associated with cancer since cancer cells are highly active.  But there can be other reasons for areas of "high uptake".  Sometimes a different scan is called for to further understand what was see in the PET. 

And since most of us are human our organs reflect our imperfections.  Just like we have scars on the outside we can get them on the inside, too.  I have a couple of nodules on the lungs, for instance.  Wasn't fun to hear that the first time but here it is two years later and they haven't changed.  So we know that they are not a problem.

Some women who were candidates for a double mast have asked the question of leaving the "good" breast with the theory that any cancer left in the body would settle in that breast and be easily detected.  It's a good theory but the reality is that if there are cancer cells already in the system then they will probably find a new home in the bones, liver, lungs or brain before they make it back to another breast.

Concern of cancer cells having already left the breast region is why stage III women get chemo.  And why we often get chemo first, before surgery, so those cells won't have a chance to start developing elsewhere while we're healing from surgery.

They always told me no test was 100% but I think a CT or MRI are the more accurate ones.  PET's are funny with the metobolic uptake thingy, if cancer is too small it won't light up at all and then areas where there are inflammation (no cancer) can light up like a Christmas tree giving you some false postives.  CT are sometimes done after PET's to rule out or confirm a questionable area.

As for your original question, yes it can & call me crazy but I'm more scared of mets to other areas than I am to my other breast.  I hope all goes well with your mom and whatever tx is used, just keep in mind the beginning is a very hard place....it's the unknown.  Once all test are done and a tx plan is in place things seem in control again.

The combination of PET and CT tries to bring together the best of both worlds.  The PET shows the areas of high glucose activity and the CT shows the actual "slices" of the soft tissue.  So in a worse case scenario the PET show uptake and the CT shows a mass -- that is bad.  But if the PET shows high glucose activity and the CT doesn't show a mass then it might not be a concern.  However, it the CT shows a mass but the PET doesn't indicate a lot of activity then it's probably just a nodule or lesion that is non-cancerous.

 PET/CT is a good scan for a baseline.