Cancer is caused by the abnormal proliferation of cells. There are two types of tumor. The first is benign. Although such tumors may grow fairly rapidly, they remain localized to their site of origin and do not invade adjacent organs. Their symptoms are caused by local pressure and treatment is by surgical removal. Complete recovery of the patient is the rule. Malignant tumors, on the other hand, are characterized by their ability to grow rapidly, to invade adjacent organs and tissues, and to spread throughout the blood or lymphatic system to establish distant colonies of tumor known as metastases. It is the presence or absence of these metastases which often determines whether patients suffering from malignant tumors will live or die. Such metastases may in themselves give rise to further spread of the tumor. Although the primary tumor may be eradicated by surgery or radiotherapy, the metastases will remain unaffected. It is the metastatic tumor that ultimately causes treatment failure and kills the patient.
To the clinician treating patients with malignant tumors it is clearly of great importance to establish whether or not an individual patient has metastatic disease. This is true, both at the time of diagnosis when treatment is being planned, and also afterwards when the patient is being regularly reviewed by his physician. Many investigations are currently available to help in the follow up of cancer patients. These include the measurement of enzymes and protein products in the blood, which will determine liver and kidney function; and conventional X-rays and computerized tomography scans (CT), which are capable of detecting areas of abnormality greater than at least 0.5 cm in diameter. Similarly, nuclear medicine has contributed greatly to the management of patients with malignant disease by the development of techniques such as isotope bone and liver scanning. Here a radioactive material is given which is taken up by the organ and areas of abnormality due to the presence of tumor will show up either as increased or decreased uptake on a scan of radioactivity distribution in the patients. These scans detect areas of abnormality with great precision and reproducibility. The limitation with all these methods is that the abnormalities detected are non-specific. Similar X-ray or scan appearances may be produced by tumors, abscesses, cysts and certain congenital malformations. What is required in clinical practice is the development of techniques which specifically reveal if the patient has evidence of active malignant disease. Such specific substances are usually called tumor markers and, in many cases, their place in clinical medicine is already well established. An example is carcinoembryonic antigen (CEA) in colon cancer. CEA is shed by the cancer cells into the blood and a rise in its level in a blood sample heralds the recurrence of malignant disease in a particular patient. The detection of such circulating tumor markers has been greatly facilitated by the development of specific MCA'S. In this chapter we review the use of MCA's to tumor-associated antigens to locate malignant cells within patients suffering from certain types of cancer.
