In toto means that all of the tissue sent for pathologic examination was put on a glass slide and examined under the microscope.
A tissue sample sent to pathology needs to be placed on one or more glass slides before it can be examined by a pathologist under a microscope. The first step in this process involves deciding how much of the tissue should be put on the glass slides.
If the tissue is small enough, for example a biopsy, all the tissue can be put onto glass slides. If the tissue is very large, for example a mastectomy specimen (where the entire breast is removed and sent for pathological examination), only some parts of the tissue will be placed on glass slides.