The question the reader will ask at this point is ‘Given all this epidemiological study, do we know the causes of cancer?’ Broadly the answer is ‘yes’ in many circumstances and for many cancers, and the opportunities for prevention that this understanding generates are there to be taken. We do not always know how the factors that have been identified by the epidemiological studies discussed in this chapter link up to what is being learned in the laboratories of the molecular biologists. This connection is being made rapidly and will be increasingly clear by the end of the century. Epidemiology has been very successful in discovering or confirming which features of our lives in the Western world can be now identified as causes of cancer.
Very few medicines have been implicated as causing cancers but there are three groups of drugs where cancer is probably an important and often unavoidable side effect. Hormones have already been mentioned. The very drugs that are used for treating cancers by chemotherapy include some (particularly those known as alkylating agents) which interfere with DNA and, hence, with some genes. Cases of leukaemia and other cancers are being discovered as a delayed after-effect of such drugs in patients who have been cured of their first cancer by such chemotherapy. Not all the drugs used in chemotherapy have this effect and modem treatments appear to have reduced the risks considerably. The third group which may put people at risk are those drugs which are used to suppress the body’s immune function. These are used for patients who have had transplants and in such patients, particularly those with kidney transplants, certain rare kinds of cancer, including those known as lymphoma, have been found. As a result of the risks, these patients have to be monitored very carefully.