These cells are said to be immortal because, in contrast to normal cells, they appear to have an unlimited ability to reproduce. Their growth is out of control. As a result, they tend to reproduce at a rate which exceeds the rate needed to replace dying cells. Cancers are classified by both the tissue of origin and the cell type. For example, osteo in osteosarcoma means the cancer arose in bone.
Sarcoma is a term used for tumour cells that resemble embryonic connective tissue cells.
Some tumour cells continue to modify their properties as they grow. Cancer in children can be difficult to diagnose. In the majority of children with cancer, the early symptoms, such as fatigue, irritability, aches and fever, are non-specific and mimic those of common childhood illnesses. Blood tests and various types of X rays or scans are ordered to find the problem. However, the definitive diagnosis of cancer is usually made by microscopic and other sophisticated examinations of either a small piece of the tumour obtained by biopsy or a sample of bone marrow.