Generally not every bit of tissue is looked at: the area where the cancer was is examined in detail, as well as the tissues very nearby. In most cases, the rest of the tissue is examined by taking fairly random samples, after first carefully examining it by hand and eye. The tissues are saved for a while, so that if something were to show up on the other samples, more sections could be taken and looked at.
Dear pattsy: A mastectomy is a procedure that removes all of the breast tissue. A mastectomy is not a biopsy - although all the tissue would be examined by pathology to determine the extent of the cancer and the margins. A lumpectomy would remove only the area of cancer. A lumpectomy can be done as a biopsy or as a treatment.