What is the purpose of radiopharmaceutical therapy?
In sufficient dosages, radiation will kill cells. In many cases, radiation can be delivered directly to sites of disease with external beam techniques. An alternative means of delivering radiation to sites of disease is radiopharmaceutical therapy, wherein a radioactive compound is administered to the patient. Through a combination of the route of administration and the physical and biological properties of the radiopharmaceutical, the radiation dose will be preferentially delivered to sites of disease. The biggest potential advantage of radiopharmaceutical therapy is the ability to deliver therapeutic radiation regardless of the number and location of disease sites.
What is the therapeutic ratio?
In the context of radiotherapy, the therapeutic ratio is the treatment effect in the sites of disease (the desirable effect of radiation) divided by the radiation effects in normal tissues (the side effects of therapy). Ideally, this ratio is as high as possible, meaning that a very high radiation dose is delivered to sites of disease with very little radiation dose to normal tissues. However, it is important to understand that one cannot simply compare absolute radiation doses because different tissues and different diseases have different sensitivities to the effect of radiation. For example, if a given treatment delivers the exact same radiation dose to an organ and a tumor, the therapeutic ratio would be extremely favorable if the disease in question is lymphoma (very radiation sensitive) and the organ in question is the liver (fairly radiation resistant). The situation is reversed if the disease in question is thyroid cancer (quite radiation resistant) and the organ in question is the bone marrow (very sensitive).