A tumor is a mass of abnormally dividing cells. There are two types of tumors:
Benign tumors are slow-growing, well-defined tumors that do not spread. In rare cases, untreated benign tumors may be life-threatening if they affect a vital organ. Because they do not invade the surrounding tissue, benign tumors are relatively easy to remove surgically, depending on their location. In some cases, benign tumors may become malignant, though this usually takes a long time (sometimes decades) if it happens at all. Benign tumors generally do not cause death or serious illness unless they are very large (such as some benign ovarian tumors) or if they affect a critical organ that makes it difficult to operate on and remove them (such as tumors in the brain).
The term cancer is usually reserved for malignant tumors, which, like benign tumors, are masses of structurally abnormal cells growing uncontrollably. They differ, however, in their ability to invade the surrounding tissue and, in many cases,
(spread) to different sites of the body via the lymphatic system or the bloodstream. This is one reason why surgical treatment alone for more advanced malignancies is often unsuccessful. Because malignant cells have a tendency to leave their site of origin, surgeons may leave them behind, which may result in new tumors in other locations. This is often treated by chemotherapy (systematic drugs aimed at killing deposits of cancer cells in sites other than the primary), or by radiation therapy applied over a broader area than that involved by the main tumor itself. To learn more about the process of metastasis, see the
Cancerous Cell Growth and Development
section of this classroom. Image 1 illustrates the difference between benign and malignant cancer cells.
Image 1: Benign versus malignant tumors