A tumour, or neoplasm, is a tissue growth with no physiological function. The first sign of tumour growth in a tissue is the appearance of a small number of cells which multiply with uncontrolled division and lose lose their capacity for differentiation. The tumour enlarges solely as a result of multiplication of its own cells. Tumour growth proceeds through stages of disorderly increase in the number of cells, focal growth, benign growth and malignant growth; the stages immediately preceding malignancy are called precancerous.
Cancer is a disease of the genome, which means it’s characterised and caused by changes in our genes that can drive a healthy cell to mutate into a cancerous one. Cancer remains difficult to treat because each cancer is different, even within the same cancer type, such as breast or bowel. Each tumour has a genetic code that makes it unique, but there are also genetic differences within the tumours themselves. And tumours can evolve over time to become resistant to treatment.
Up to 90% of cancer is preventable. "Environmental or nutritional factors probably account for up to 90% of human cancers. These factors include smoking; diet; and exposure to sunlight, chemicals, and drugs. Genetic, viral, and radiation factors may cause the rest." (The Merck Manual, 17th edition, pp. 2591-2592)