Pathologists use the word undifferentiated to describe a tumour made up of cells that do not look anything like the normal, non-cancerous cells found in the area of the body where the tumour started. Because undifferentiated cancer cells are so abnormal looking when examined under the microscope, your pathologist may not be able to determine the specific tumour type or exactly where the tumour started.
Undifferentiated tumours are often aggressive tumours that quickly spread to other parts of the body such as lymph nodes, lungs, liver, and bones. The movement of tumour cells to another part of the body is called a metastasis.
Undifferentiated is on one end of a scale that pathologists call differentiation. This scale is used to compare cancer cells to normal cells. In most areas of the body, the scale includes 4 levels of differentiation.
Your pathologist may order additional tests, such as immunohistochemistry or next generation sequencing (NGS) to better understand an undifferentiated cancer and to try to determine where the cancer started.