Late Side Effects in Normal Mouse Brain Tissue After Proton Irradiation

Abstract

Radiation-induced late side effects such as cognitive decline and normal tissue complications can severely affect quality of life and outcome in long-term survivors of brain tumors. Proton therapy offers a favorable depth-dose deposition with the potential to spare tumor-surrounding normal tissue, thus potentially reducing such side effects. In this study, we describe a preclinical model to reveal underlying biological mechanisms caused by precise high-dose proton irradiation of a brain subvolume. We studied the dose- and time-dependent radiation response of mouse brain tissue, using a high-precision image-guided proton irradiation setup for small animals established at the University Proton Therapy Dresden (UPTD). The right hippocampal area of ten C57BL/6 and ten C3H/He mice was irradiated. Both strains contained four groups (nirradiated = 3, ncontrol = 1) treated with increasing doses (0 Gy, 45 Gy, 65 Gy or 85 Gy and 0 Gy, 40 Gy, 60 Gy or 80 Gy, respectively). Follow-up examinations were performed for up to six months, including longitudinal monitoring of general health status and regular contrast-enhanced magnetic resonance imaging (MRI) of mouse brains. These findings were related to comprehensive histological analysis. In all mice of the highest dose group, first symptoms of blood-brain barrier (BBB) damage appeared one week after irradiation, while a dose-dependent delay in onset was observed for lower doses. MRI contrast agent leakage occurred in the irradiated brain areas and was progressive in the higher dose groups. Mouse health status and survival corresponded to the extent of contrast agent leakage. Histological analysis revealed tissue changes such as vessel abnormalities, gliosis, and granule cell dispersion, which also partly affected the non-irradiated contralateral hippocampus in the higher dose groups. All observed effects depended strongly on the prescribed radiation dose and the outcome, i.e. survival, image changes, and tissue alterations, were very consistent within an experimental dose cohort. The derived dose–response model will determine endpoint-specific dose levels for future experiments and may support generating clinical hypotheses on brain toxicity after proton therapy.

Models for Translational Proton Radiobiology—From Bench to Bedside and Back

“…Experiments with new radiation qualities, such as FLASH radiotherapy, laser-driven sources or proton mini-beams, demand for adapted solutions with respect to sample positioning and dosimetry [54][55][56][57]. Moreover, point-like measurements with small dosimeters can be supported by simulations [58] to resolve proton dose distributions [59,60]. Standardized 3D phantoms of rodents could clearly improve preclinical proton dosimetry, but are currently only available for orthovoltage X-rays [61].…”

“…Common specifics of these models as well as some representative endpoints are shown in Figure 3. Irradiated zebrafish embryo [68], C57BL/6 mouse after proton brain irradiation [59], rat proton irradiation setup at Institut Curie [32], rabbit photon treatment plan of a mini-pig brain [69].…”

“…Another strength of in vivo models is reverse translation, i.e., replicating clinical observations in a preclinical setting. For example, recent studies have established proton irradiation of mouse brain subvolumes [42,59] to investigate the underlying causes of radiation-induced brain lesions appearing after proton therapy [10,18]. Overall, central nervous system (CNS) toxicity studies in rodent models are of increasing interest and performed from both a behavioral [213][214][215] and histological perspective [59,216,217].…”

“…For example, recent studies have established proton irradiation of mouse brain subvolumes [42,59] to investigate the underlying causes of radiation-induced brain lesions appearing after proton therapy [10,18]. Overall, central nervous system (CNS) toxicity studies in rodent models are of increasing interest and performed from both a behavioral [213][214][215] and histological perspective [59,216,217]. Using clinically relevant settings is crucial in developing protocols that reduce normal tissue complications.…”

“…Overview of in vivo models for proton radiobiology experiments. Images (left to right):Irradiated zebrafish embryo[68], C57BL/6 mouse after proton brain irradiation[59], rat proton irradiation setup at Institut Curie[32], rabbit photon treatment plan of a mini-pig brain[69].…”