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Preserving Breast Tissue Samples

When breast cancer is surgically removed (during a surgical biopsy, lumpectomy or mastectomy), the tissue removed is sent to a pathologist.

The pathologist preserves the tissue samples, then studies them under a microscope.  

Learn more about pathology exams

Formalin fixed paraffin embedded tissue  

In the U.S., the standard way to preserve a tissue sample is called formalin fixed paraffin embedded tissue.

First, the sample is treated with a substance called formalin, which hardens the tissue and prevents it from breaking down over time.

Then, thin slices of the sample are embedded in blocks of paraffin (wax).

The preserved tissue is stored at the hospital where the biopsy was done in case it’s needed later. For example, you may wish to have your biopsy tissue re-examined for a second opinion, or a new test may come out in the future that can be done on the tissue and could affect your treatment.

Frozen sections

Sometimes, part of the biopsy sample is frozen in the pathology lab during a surgical biopsy.

Assessing margins during surgery

The pathologist examines thin sections of this frozen sample under a microscope to assess the margins (to see how close the cancer cells are to the edges of the sample) during the surgery.

If there are cancer cells in the margins, more tissue may be removed during the same surgery.


Sometimes a surgical biopsy (instead of a core needle biopsy) is done to check whether or not there’s cancer in the breast. During the surgery, the pathologist may check to see if the frozen sections of tissue contain cancer cells.

Although a frozen section can give a quick check of the tissue sample, it may not give an accurate result. It may show false negative results (the results show no cancer when cancer is present) or false positive results (the results show cancer, but cancer is not present) [11].

The results from a frozen sample always need to be confirmed by other diagnostic methods (results from a formalin fixed paraffin embedded tissue sample). This can take several days.

Updated 12/19/22