Safety Margins for Geometrical Uncertainties in Radiotherapy

Cancer is the unrestricted growth of cells in an organism, which can eventually destroy organs that are needed for survival of the organism. Throughout human history, cancer has been one of the major medical causes of death. At the moment, there are about 840.000 cancer fatalities in Europe per year. It is estimated that in the westem world approximately 1 in 3 people will develop some kind of tumor during their lifetime, and more than 1 in 5 will die of it. Although significant progress has been achieved in the fight against cancer in the last decades, still about half of the cancers cannot be cured. Currently, the three main therapies for cancer are surgery (removing the tumor), radiotherapy (killing the tumor cells with radiation), and chemotherapy (the use of anti-cancer mugs). The first two are especially used for tumors that are well localized. Surgety is straightforward but catmot always be applied, for instance when the tumor is localized in or close to a vital organ. Futthermore, invisible, microscopic extensions of the tumor might be trtissed. In radiotherapy, those tumor extensions can be treated more easily. Moreover, this therapy may be less demanding on the patient and hospitalization is usually not necessaty. When microscopic tumor cells have spread from the primary tumor site to different parts of the body, chemotherapy can be applied. The blood circulation is used to transpoti the drugs are transported through the body. In many cases, the difterent therapies are combined to improve the treatment outcome. Radiotherapy is used for about half of all cancer patients. With radiotherapy, ionizing radiation in the fotm of high energy photons, electrons or protons is aimed at the tumor. These particles deposit some of their energy in the tumor cells, which can cause ionization of DNA or surrounding molecules. This can induce irreparable genetic damages in the tumor cells that either kill the cell directly or result in the socalled apoptosis, i.e. cellular suicide. However, since radiation may kill healthy cells as well, one has to be careful to deliver the radiation dose in the right place. Basically, there are two ways the radiation can be delivered: by brachytherapy or by external beam radiotherapy.