Radiotherapy is one of the mainstays of cancer therapy as tumor and healthy cells respond differently to ionizing radiation. Irradiation with electrons (6-12 MeV) is currently limited to superficial tumors because of their steep dose gradient in water/tissue, however using ultra-high energy electrons (UHEE) deep-seated tumors can be reached and sparing of healthy tissue is possible as well. Furthermore, it has been shown in recent studies, that the application of high doses within an extremely short time (>40 Gy/s; termed FLASH irradiation) is sparing healthy tissue from radiotoxicity, while at the same time providing similar antitumor efficacy.
The Electron Stretcher Accelerator (ELSA) at the University of Bonn, is able to accelerate and store UHEEs (up to 3.2 GeV) and release them in a very short pulse (250 ns), yielding dose rates of > 40 Gy/s. Currently ELSA operates with 1.2 GeV electrons for FLASH experiments, where one pulse holds around 2nC electrons. The extraction of short and intense electron pulses from the booster synchrotron, using the stretcher ring as a transfer line, is a newly designed extraction mode at ELSA. To evaluate the process, dosimetry is currently performed using GafchromicTM-films and computer simulated depth dose curves (GEANT4). Within a collaboration with PTW (a dosimetry company), different detectors are tested to quantify the applied dose as well.
First in vitro cells from the type MDA MB 231were successfully irradiated with different doses at the dose maximum in a water phantom at ELSA and compared to experiments using a clinical LINAC, the Varian TrueBeam® STx, with 12 MeV electrons, located at the University Hospital Bonn. The resulting cell surviving curves were determined and give information about the relative biological effectiveness (RBE).
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Maike Hansen, Tatjana Lenz, Saime Gürbüz