Document Type : Original Paper
Assistant Professor, Medical Physics Dept., Tabriz University of Medical sciences, Tabriz, Iran
Introduction: Heavy ions are nucleus of elements of iron, argon, carbon and neon that all carry positive electrical charges. For these particles to be useful in radiotherapy they need to accelerated to high energy by more than thousand mega volts. Also the cosmic environment is considered to be a complicated mixture of highly energetic photons and heavy ions such as iron. Therefore, the health risks to astronauts during long mission should be considered.
Materials and Methods: The induction of interphase death was tested on Chinese hamster ovary cells by exposing them to accelerated heavy ions (carbon, neon, argon and iron) of 10-2000 linear energy transfers (LETs). The fraction of cells that underwent interphase death was determined by observing individual cells with time-lapse photography (direct method) as well as by the indirect method of counting cells undergoing interphase death made visible by the addition of caffeine (indirect method).
Results: The interphase death due to the exposure to X- rays is increased linearly as the dose exceeds the threshold dose of 10 Gy. Whereas the interphase death increases at a higher rate due to the exposure to high LET heavy ions and no threshold dose was observed. The range of LET values corresponding to the maximum RBE for the interphase death is 120-230 keV/µm. The probability of inducing the interphase death by a single heavy ion traversing through the nucleus is about 0.04-0.08.
Discussion and Conclusion: The relative biological effectiveness (RBE) of heavy ions as compared to X- rays as determined at the 50% level of induction is increased with LET. It reached a maximum value at a LET of approximately 230 keV/µm and then decreased with further increase in LET. The range of LET values corresponding to the maximum RBE appears to be narrower for interphase death than for reproductive death.