About Cancer - A Beginner's Guide to Solid Tumour Growth
What is cancer?
Cancer is a disorder of cells in which growth and development are abnormal. It affects not only humans, but also higher mammals and plants.
What causes cancer?
A variety of factors are known to play a role in carcinogenesis, the development of tumours. These include carcinogens, for example chemicals used in the rubber industry and those present in cigarette smoke. In addition, some viruses (e.g. Epstein-Barr virus and human papillomavirus) are associated with particular tumours. Physical factors such as exposure to sunlight and radiation are also known to promote cancer. Age is another important factor that influences the type of tumour that develops; for example certain leukaemias and bone tumours are more prevalent in young children while there is a clear correlation between increasing age and the incidence of cancers of the prostate, colon and skin.
How do tumours grow?
The development of a cancer into a life-threatening disease is a multistage process. While the details are typically tumour-specific, they usually start with a mutation that causes an individual cell, and its offspring, to divide in an uncontrolled manner. The resulting avascular tumour receives nutrients (and excretes waste products) from the surrounding tissue via diffusion. Consequently the size of avascular tumours is usually limited to a few millimetres in diameter; thereafter, they remain dormant unless they acquire a new blood supply by the process of angiogenesis. During angiogenesis, the tumour cells secrete a range of diffusible chemicals known collectively as angiogenic factors. These chemicals stimulate neighbouring blood vessels to proliferate, form new capillary sprouts and migrate towards the tumour, eventually furnishing it with a circulating blood supply so that vascular tumour growth may commence. Once vascularised, the tumour has access to an almost limitless supply of nutrients and hence growth is rapid. Additionally, tumour fragments that break away from the primary tumour and enter the vasculature may be transported to other parts of the body where, under favourable conditions, they establish second tumours or metastases that further compromise the host.