Characterization of the biological benefits of ultra-high dose rate FLASH proton therapy.

Cancer is a condition characterized by the development of abnormal cells that grow uncontrollably and have the ability to invade and destroy normal tissue. The most commonly used cancer treatment modalities are surgery, chemotherapy and radiotherapy (RT). During their course of illness, approximately 50 % of all cancer patients receive RT, which uses ionizing radiation to kill cancer cells. Proton therapy (PT) is an advanced RT method that allows more precise targeting of the tumor compared to conventional photon RT, thereby reducing side effects in healthy tissues and organs. In recent years, ultra-high dose rate (above 100 Gy/sec) or ‘FLASH’ RT has attracted much attention due to its ability to further protect healthy tissues while maintaining anti-tumor activity. The FLASH effect has been demonstrated in several pre-clinical studies for electrons. Dose rates for potentially enabling the FLASH effect have also recently been achieved on clinical PT systems, which are more suitable for the treatment of deeply located tumors as opposed to electrons. However, more studies are needed to firmly establish the conditions of occurrence of the FLASH effect with protons, as some of the few available studies show controversial results. The main goal of this project is to characterize the FLASH effect in both normal and cancerous tissues using a PT system with a pulsed pencil beam structure and thus bring this treatment modality one step closer to clinical practice. First of all, we will investigate the degree of early normal tissue toxicity induced by proton FLASH versus conventional dose rate PT in zebrafish embryos and mice, two well-established models for RT toxicity. In zebrafish embryos we will focus on the effect on overall survival and morphological abnormalities. Mice, which are a more relevant pre-clinical model, will be exposed to whole abdominal proton irradiation at conventional and FLASH dose rates and the degree of early radiation enteropathy will be evaluated based on the survival of intestinal epithelial stem cells. Secondly, the capability of PT at FLASH dose rates to insure at least similar tumor control compared to conventional dose rate will be assessed. To this end, pancreatic cancer will be used as a tumor model as this is an appealing indication for PT due to the high rates of severe acute toxicity during and after (chemo)radiation which have a detrimental effect on quality of life in this poor prognosis patient group. Both an indirect xenograft model (cancer cells derived from the KPC autochthonous mouse model of pancreatic cancer will be transplanted heterotopically into immunocompetent mice) and a patient-derived xenograft model (cells from the patient tumor samples will be transplanted orthotopically or heterotopically into immunodeficient mice) will be used to monitor tumor response to RT as well as long-term normal tissue effects by evaluating the development of intestinal fibrosis. Lastly, the mechanistic background of the potential FLASH effect will be studied in the tissue and blood samples collected from the irradiated healthy and tumor-bearing mice with a particular focus on inflammation. The results obtained in the present project will provide solid evidence of the presence of the FLASH effect in ultra-high dose proton irradiation as well as provide valuable insights into the underlying biological mechanisms and thus bring the FLASH PT one step closer to the clinical practice.