How cancer spreads

An initial, or “primary”, cancer tumour forms when cells become modified to allow them to grow uncontrollably, either by excessive division or by extended cell life. At the time of diagnosis, i.e. when it is large enough to detect, a typical primary tumour contains several thousand million such cells. It only needs one of these to escape and lodge in another part of the body for a further, or “secondary”, tumour to develop, a process known as metastasis. The primary tumour can be treated by surgery or by radiotherapy. Disseminated (i.e. escaped) tumour cells may be killed by chemotherapy, but if these cancerous cells survive then little can be done until any secondary tumour is large enough to be detected. By this time, several other secondaries may have established and it may be too late for effective treatment. In fact, the majority of deaths from cancer arise from secondary tumours affecting vital organs. The organ ceases to function effectively and the patient dies.