Page "Metastasis" Paragraph 39

For instance, breast cancer cells look the same whether they are found in the breast or have spread to another part of the body.

So, if a tissue sample taken from a tumor in the lung contains cells that look like breast cells, the doctor determines that the lung tumor is a secondary tumor.

Still, the determination of the primary tumor can often be very difficult, and the pathologist may have to use several adjuvant techniques, such as immunohistochemistry, FISH ( fluorescent in situ hybridization ), and others.

Despite the use of techniques, in some cases the primary tumor remains unidentified.