A tumor is a concentration of cancer cells that have developed abnormally.
Tumors are divided into two categories:
1. Benign tumors
These tumors, which are formed by proliferating cells, are usually the result of an irritation. This phenomenon is called benign neoplasia. However, when the irritation is no longer there, the neoplasia is disrupted, the tumor does not grow any further, it does not destroy surrounding healthy tissues (although it may exert a detrimental effect upon them) and it does not cause the formation of new tumors in other parts of the body. Similarly, when a benign tumor is surgically removed, there is no re-growth. Benign tumors rarely cause serious problems in an organism and are usually easy to treat.
2. Malignant tumors
These tumors are also known as neoplasms. They are formed in a manner similar to benign tumors, but they work quite differently. The proliferating cells have a pathogen that causes a continuous and unregulated growth. The malignant tumor (read how malignant tumors are formed here) remains active even if the original tumor has been removed from the body by forming a new tumor in its place. At the same time, it has a devastating effect on neighboring tissues, which it invades (a process called infiltration). Malignant tumors also have the ability to form new tumors in other parts of the body where cancer cells travel through the blood or lymph to the rest of the body, after they have been detached from the original tumor. This process is called metastasis (relocation of cancer cells). Malignant tumors are classified, depending on their nature, as either carcinomas or sarcomas and are potentially lethal, especially if they are not diagnosed early.
Not all cancer types are tumors. A very good example of that is leukemia, a type of blood cancer in which cancer cells are constantly moving inside the body through the circulatory system. Therefore, a key criterion for the diagnosis of cancer, is not the existence of malignant tumors, but the kind of virulence that causes cell proliferation.