When breast cancer is removed during a surgical biopsy or surgery, such as a lumpectomy or mastectomy, the tissue is sent to a pathologist, who preserves and examines it under a microscope. In the U.S., tissue samples are usually preserved by treating them with formalin, which is a substance that hardens the tissue and prevents it from breaking down. The tissue is then placed into a block of paraffin wax. This process is called formalin- fixed, paraffin-embedded tissue. The tissue is stored at the hospital where the biopsy was done. In some cases, people may request that it be taken out of storage and re-examined for a second opinion. At other times, a new test may become available that can be done on the tissue and affect treatment options. In some surgical biopsies, part of the tissue sample is frozen in a lab.

The pathologist looks at thin sections of this frozen tissue under a microscope. If cancer cells are found close to the edges of the sample, the surgeon may remove more tissue during the same procedure. Results from frozen samples always need to be confirmed by other laboratory tests. For more information about preserving breast tissue samples, visit the Diagnosis section on komen.org.