Introduction

A cancer (malignancy) is a cell or a group of cells that become abnormal and grow without any of the body's usual controls. Cancers move into normal tissues and can spread to adjacent tissues (invade locally) or spread through blood vessels or lymph vessels (part of the body's immune system) to more distant tissues (metastasize). Cancers can develop from any tissue within any organ.

Leukemias and lymphomas are cancers of blood and blood-forming tissues. With leukemias, most of the cancerous cells are in the bone marrow and blood. With lymphomas, the cancerous cells are in lymph nodes, the bone marrow, the spleen, and the liver. Lymphoma cells usually form clumps as they grow, resulting in a solid mass. A collection of cancerous cells is called a tumor.

Carcinomas and sarcomas are types of cancers that always form solid masses. Carcinomas are cancers of epithelial cells, cells that cover the surface of the body, produce hormones, or make up glands. Cancers of the skin, lung, colon, stomach, breast, prostate, and thyroid gland are carcinomas. Carcinomas occur more often in older than in younger people. Sarcomas are cancers of mesodermal cells, which are cells that form muscles and connective tissue. Examples of sarcomas are leiomyosarcoma (cancer of smooth muscle that is found in the walls of digestive organs) and osteosarcoma (bone cancer). Sarcomas occur more often in younger than in older people.

Some of the most common cancers in older people are breast cancer, chronic lymphocytic leukemia, colorectal cancers (cancers of the colon and the rectum), lung cancer, mouth, head, and neck cancers, multiple myeloma, prostate cancer, skin cancer, and vulvar cancer.