Cancer is not a single disease rather it is a general term used to describe various malignant tumours that affect all forms of higher organisms including plants and animals. More than a hundred types and sub types of cancer are known to affect human beings. Cancer can be defined as an abnormal growth of cells in any tissue or organ of the body. Cancer cells have the potential to spread and grow in other parts of the body. Cancer preys on the host and continues to grow indefinitely competing with normal cells of the body for nutrition.
Cancer had its origin since the evolution of multicellular organisms. Our knowledge of cancer goes back to the dawn of civilisation. The evidence of cancer has been found in skeletons of prehistoric animals and Egyptian mummies. The earliest written records on cancer have been traced to the ancient Egyptian, Greek and Indian writings. The word Cancer has its origin from the Latin word Cancrum (Greek: Karkinos), which means crab. Ancient Indian surgeon, Sushruta, who used to cauterise the tumours with red hot iron rods, described various types of tumour in his text ‘Sushruta Samhita’ written in circa 600 B.C. Ancient Egyptians tried fire drills (insertion of heated metallic sticks into the tumour) to treat cancer. It is believed that the Greek surgeon, Leonides, was the first to operate upon cancer with a knife.
‘What causes Cancer?’ was the most debatable topic during the twentieth century. After a prolonged era of confusion over the genesis of cancer, this was finally established by the end of the century that cancer is caused by mutations in the growth regulatory genes & pathways including oncogenes and tumour suppressor genes.
Robert Weinberg & Douglas Hanahan, both cancer biologists, published an article “The Hallmarks of Cancer” in January 2000 that explains how a normal cell is transformed into a cancer cell by activation of oncogenes (ras, N-myc, c-myc, HER-2/neu, etc); inactivation of tumour suppressor genes (p53, Rb, Ret, WT-1, APC, etc); dysregulation of certain pathways (ras, Rb, myc, etc), evasion of apoptosis; acquisition of tumour angiogenesis; acquisition of ability to migrate, invade and colonise in other tissues and organs (metastasis); and activation of specific pathways that make cancer cells immortal.
Normal cell division (mitosis) in our body is a highly regulated mechanism, controlled by genes (made up of DNA) through growth regulatory pathways. A prolonged exposure to carcinogens damages the DNA and induces mutations in growth regulatory genes including oncogenes and tumour suppressor genes and pathways leading to loss of control over normal cell division. The mutated cells go haywire and proliferate indiscriminately (pathological mitosis), usually forming a mass, known as a neoplasm or a malignant tumour or in simple words, a Cancer.
As the time passes, the cancer cells go on accumulating further mutations and acquire more evil characteristics such as ability to invade & move into the adjoining tissues, travel through lymph and blood vessels, lodge and grow in other parts of the body to form colonies (metastasis), create their own blood vessels (tumour angiogenesis) for their nutrition, evade the process of programmed cell death (apoptosis) and acquire the ability of limitless replication, making the cancer cells immortal. By the time most of the cancers are finally diagnosed, they have already added many mutations, for example ALL (a type of blood cancer) has been found to have 5 to 10 mutations at the time of diagnosis. Pancreatic cancer has shown 50 to 60 mutations while Breast & Colon cancers have 50 to 80 mutations at the time of diagnosis. Similarly most of the cancers have 11 to 15 aberrant (mutated) pathways at the time of diagnosis.
Further exposure to radiation emitted by X-rays, CT scans, PET scans, Bone scans, etc. during investigation may induce a few more mutations in the cancer cells making them more aggressive. Similarly radiotherapy, chemotherapy, targeted chemotherapy, hormonal therapy during treatment may induce further mutations in the cancer cells making them resistant or refractory to the therapy, which leads to progression or recurrence of cancer.