Towards high dose rate proton radiation in zebrafish

Abstract

Cancer remains one of the leading causes of death in today's world. Radiation is a widely used in the treatment of cancer. One of the relatively newer methods is proton therapy, which can spare healthy tissues better than traditional photon treatments due to the unique Bragg peak of protons. A promising improvement is FLASH proton therapy, which utilises ultra-high dose rates (40-100x higher than conventionally used). However, a lot is still unknown about this method. This research took first steps towards experimentally investigating FLASH proton radiation in living tissues. To achieve this, three types of experiments were performed. The first consisted of simulation with proton radiation, the second used proton to irradiation zebrafish, and the last experiment irradiated zebrafish with a gamma source. First, a virtual model of the irradiation conditions was built with TopasMC, using an 140 MeV proton beam propagating in a water tank containing model cells. The flux and number of protons were tallied at various positions. A new beam was created for each positing using the tallied data resembling the original beam. The new beam was used for small scale irradiation of a model cell. The damage was assessed by calculating DNA strand breaks.
The simulation showed a Bragg peak for dose measurements, in accordance with literature. The damages to DNA significantly increased as the LET increased, as expected. This forms a basis for future simulation into FLASH. Next an experimental setup was realised and a first experiment was preformed. The setup showed a field homogeneity with a maximum difference of 3\%, as well as a maximum dose difference of 2\% in the region of interest. This provided a solid foundation for accurate measurements of the experiment with living samples. Subsequently, zebrafish embryos were irradiated with protons with either 5 Gy or 20 Gy and a dose rate of approximately 1 Gy/min. A different batch of zebrafish embryos received gamma radiation with a dose rate of around 10 Gy/min, also to 5 Gy and 20 Gy dosages. The damage to the embryos was estimated via the detection of heart beats, and their length, directly after receiving radiation and after a day of growth. The results of the proton irradiation of the embryos showed no significant differences between the separate groups. For the gamma irradiation also no significant difference were found, either for the 0-day measurements. After a day of growth, a tangible difference was found, but this was with a control group that was not transported. Therefore, no direct link could be made to radiation as a sole source of the influence. Additionally, temperature had a large effect on the development of the embryos. The simulations showed the difference in biological effect of different LET coefficients of protons. The practical irradiation's do appear to reveal some discrepancy between the different samples, but the samples sizes are too small to draw definitive conclusions from, especially went comparing the results after a day of growth. Nevertheless, a solid base has has been build for future research into FLASH. A simulation capable of doing physical and biological measurement was constructed. Also, an accurate physical setup for proton irradiation was made. In addition, a design for the containment of the zebrafish during transport, irradiation, and processing was devised. Lastly, protocols for the transportation, handling, processing, and analysis of the samples were created.