Imaging in Radiation Therapy

Radiation therapy, often called radiotherapy, can be defined as the use of high-energy radiation such as x-rays, gamma rays, electrons, protons and neutrons to kill cancer cells and reduce the size of tumors. Shortly after the discovery of the x-ray by Wilhelm Conrad Roentgen and radioactivity by Marie and Pierre Curie about 100 years ago, the mysterious and powerful radiations were being used to treat cancer.[1-4] Continuous improvement of technology in related areas such as radiation biology, biomedical engineering and medical physics have made radiation therapy an important part of cancer treatment. At present, about half of all cancer patients are subject to radiation therapy during their cancer care treatment.[5] When radiation particles are introduced to patient body, they start to interact with body cells and deposit energy in them. With enough energy absorbed, cells can be seriously damaged and even be dead.[6] Based on this principle, radiation therapy mainly attacks cancer cells, but it can also affect normal cells. The damage to normal cells is what causes side effects. Whenever radiation therapy is given it involves a balance between destroying the cancer cells and sparing the normal cells. Therefore, the goal of radiation therapy is simply giving the necessary radiation dose to cancer cells while keeping dose to normal cells as low as possible. In general, the more dose to the tumor, the better outcome obtained. However, as indicated in the goal of radiation therapy, the amount of dose can be delivered to the tumor is very often limited by tolerance dose to surrounding normal tissues. Radiation therapy can be categorized in two types, external beam therapy and brachytherapy.[7] Radiation is introduced from outside to patient body in external therapy. In brachytherapy, radiation source is placed typically inside patient body, very close to the tumor lesion. This review is focused on external beam therapy that is more popular. The most popular external beam machine is medical linear accelerator, called LINAC. A conventional commercial LINAC is shown in Fig. 1. Major parts of LINAC are gantry, collimator and table. All of three parts can be rotated with respect to a point in the space and this point is called isocenter. Both gantry and table determine the incident angle of any beam to the patient while collimator does