Anaplastic is a word pathologists use to describe very abnormal looking cancer cells. While most cancer cells share some features (shape or size) with normal cells, anaplastic cells do not look anything like a normal cell. Another word for anaplastic is anaplasia.
Because anaplastic cancer cells look nothing like normal cells, pathologists will often perform additional tests such as immunohistochemistry to determine where the cancer started.
Unfortunately, cancers with anaplastic cells tend to be very aggressive and are often associated with poor prognosis.