Beam optimization of a heavy ion microbeam for targeted irradiation of mitochondria in human cells

Abstract

Mitochondria, organelles of the cytoplasm, are the power plants of the cell and thus their function is important for the survival of cells. Mitochondria are known to depolarize after targeted irradiation, but the effects on cells are still unclear. The aim of this work is to investigate the effects of mitochondrial depolarization on growth and survival of cells. We performed targeted irradiation with 55 MeV C5+-ions at the ion-microbeam SNAKE at the 14 MV tandem accelerator in Garching near Munich, with a beam spot size of ∼1 µm. Approx. 6% of the mitochondrial area was irradiated in 74 cells with 5,120 carbon ions homogenously distributed over a square area of 13.2 µm2. Cell growth was investigated by observing the cells for 3.5 days via live-cell phase-contrast microscopy and evaluating the number of vital cells. While the number of irradiated cells remained constant during the observation, the unirradiated control group showed exponential growth. An additional particle track detector test with polycarbonate revealed that 4% parasitic ions hit the cells up to 500 µm away from the target, forming a so-called halo and inducing a mean parasitic dose of (2 ± 2) Gy on the cells. This dose alone, when applied in cell nuclei, is large enough to reduce the survival and growth of cells significantly and overrides any effects caused by targeted irradiation of mitochondria. Subsequently, several methods to reduce the halo were investigated. A significant reduction in halo size and number of ions in the halo could be achieved by using C6+-ions instead of C5+-ions. The slit openings, correction of lens errors, and the beam spot size had minor influence on the halo size but could achieve a reduction in halo dose. Overall, a 97% reduction in halo area and a halving of halo dose were achieved.