When breast cancer is surgically removed (during a surgical biopsy, lumpectomy or mastectomy), the tissue removed is sent to a pathologist.
The pathologist preserves tissue samples. Then, he or she studies the samples under a microscope.
Formalin fixed paraffin embedded tissue
In the U.S., the standard way to preserve a tissue sample is called formalin fixed paraffin embedded tissue.
The sample is treated with a substance called formalin, which hardens the tissue and prevents it from breaking down over time. Then, the sample is embedded in a block of paraffin (wax).
In some cases, part of the biopsy sample is frozen in the pathology lab during a surgical biopsy.
The pathologist examines thin sections of this frozen sample under a microscope to check for cancer cells or to assess the margins (see how close the cancer cells are to the edges of the sample).
Although a frozen section can give a quick check of the tissue sample, it may show false negative or false positive results. A false negative result suggests cancer is not present when in fact, it is. A false positive result suggests cancer is present when it is not.
So, the results from a frozen sample always need to be confirmed by other methods. This can take several days.