Analysis of SPICE microbeam profile using fluorescent nuclear track detector (FNTD)

Single Particle Irradiation system to Cell (SPICE) facility at the National Institute of Radiological Sciences (NIRS) provides a magnetically focused 3.4 MeV proton microbeam. Throughout the radiobiological studies using SPICE over the years, we have realized that biological effect can be affected by the highly localized dose distribution of the microbeam and results may differ from that of broadbeam irradiation. Previously, we have reported that the beam size of the SPICE microbeam was approximately 2 um; the proton traversal was identified as etch pits produced on plastic nuclear track detectors, CR-39. However, this widely used method cannot distinguish individual proton tracks of highly focused microbeam due to the overlapping of etch pits, which was major limitation for beam profile analysis as well as estimation of localized dose distribution in the cells. To overcome this limitation, we used fluorescent nuclear track detectors (FNTD; Al2O3: C, Mg) to investigate the beam size and its proton-number dependence (7-1,000 protons per position). Detail analysis of Z-stacks of fluorescent images of the FNTD with a confocal laser scanning microscope clearly demonstrated that FNTD has better position resolution and detection sensitivity compared to CR-39. The overall beam size (±2σ, σ: standard deviation) was measured to be in the range of 2.0 (1.5) to 3.1 (2.4) m in the long (short) axis direction of ellipse-shape depending on the number of protons per position, suggesting that out-focused fraction of the incident protons contributes the broadening of the beam size. Localized dose distribution in a cell nucleus was numerically simulated using the measured beam size for 500 protons per position and amorphous track structure models. Calculated dose distribution consists of sharp peaks due to core and a broad convex profile due to penumbra, and owing to the short penumbra range of 3.4 MeV proton, the SPICE microbeam was found to have a sharply defined dose distribution without low dose tails. On the other hand, targeted nuclei of WI-38 human normal fibroblast cells were irradiated with the same number of protons per position and subsequently fixed and immunostained against γ-H2AX, which is an antibody to detect DNA double strand breaks (DSB) in cells. The size of localized DSB identified as fluorescent spots of γ-H2AX was observed to be rather larger than that of the dose distribution, indicating induction of DSB in the targeted cell nuclei was enhanced beyond the beam size.16th International conference on nuclear microscope technology and application