During our visit with the surgeon today, he made some comment about the fact that the tumor has probably been there for three or four years. Sherrie asked him why it wouldn't have shown up in previous mammograms. He then talked about the geometric growth rate of cancerous cells--they double about every three months or so (according to him). This means that the tumor would grow about an order of magnitude in size every year (my calculations; not his: double every three months = 2^4 increase in size per year, or 16x per year). So, the tumor was probably about 16x smaller a year ago, or one or two millimeters in diameter, which may be why it was missed. Perhaps it was even smaller.