Hi, this research study into Invasive Lobular Carcinoma could hold some answers for you.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1357735/
It says that ILBC tumors are much harder to image clearly with ultrasound than the more common IDC and that they are prone to come in multiples in different parts of the breast, so there can be more tumors spread around the breast, some of which they may not see on the ultrasound. Also ILBC tumors often turn out to be bigger than they appeared on ultrasound. All of which adds to surgeons prefering to do a mastectomy not a lumpectomy to be sure they have got all the tumors and not missed any.
