Cancer starts when a cell is somehow altered so that it multiplies out of control. A tumor is a mass composed of a cluster of such abnormal cells. Benign, or noncancerous, tumors do not spread to other parts of thCancers continue to grow and spread by direct extension or through a process called metastasis, whereby the malignant cells travel through the lymphatic or blood vessels -- eventually forming new tumors in other parts of the body.e body, and do not create new tumors. Malignant, or cancerous, tumors crowd out healthy cells, interfere with body functions, and draw nutrients from body tissues. Cancers continue to grow and spread by direct extension or through a process called metastasis, whereby the malignant cells travel through the lymphatic or blood vessels -- eventually forming new tumors in other parts of the body. The major types of cancer are carcinoma, sarcoma, melanoma, lymphoma, and leukemia. Carcinomas -- the most commonly diagnosed...