Improving preclinical proton radiation dosimetry using a biologically relevant murine dosimetry phantom

The goal of radiotherapy (RT) in the treatment of cancer is to deliver as much radiation dose to the tumour, whilst minimising the amount of dose delivered to the surrounding normal tissue. Proton beam therapy (PBT) is an alternate form of RT, delivering high energy particles (protons) rather than conventional x-rays (photons) to a tumour. The favourable dose deposition of protons follows a low entrance dose culminating in a steep dose gradient and rapid fall off (Bragg peak), increasing the precision of the dose delivery to the tumour. Due to the overall cost and technology required to generate protons, experiments are often performed in dedicated research rooms within a clinical department, using equipment designed for human treatment. The significant size difference between a human and a mouse makes it difficult to utilise the Bragg peak dose distribution within the small animal so often the initial plateau region of the beam is used to irradiate the target. However, this means the beam extends beyond the target as the beam energy is too high to be attenuated by the small animal and, consequently, healthy tissue is irradiated increasing toxicity. To improve the precision of the irradiation, and scale down the beam to the small animal size, a variety of techniques and equipment across institutions are used, including range shifters, energy degraders, shielding or collimators.
The aim of this study is to adapt a previously developed 3D printed mouse-shaped tool (phantom) to measure proton dose using radiochromic film to ensure accurate proton dose delivery at the small animal scale in our laboratory. We aim to validate this against measurements of DNA damage in 3D matrices of tumour cells encapsulated in a hydrogel bead. Following this, the phantom will be delivered to multiple international institutions as part of a dose measurement audit, testing the techniques used and comparing the planned and delivered doses. This will be facilitated through our existing collaborations with the Inspire project and the Particle Therapy Co-operative Group (PTCOG).
Currently, a small number of animals (~200) are used in pilot studies to test the suitability of different equipment to reduce the beam size to suit specific experiments. Through the multicentre audit we aim to show direct users that the phantom we have developed can be a suitable replacement for these animals. The phantom can also be used as a mouse substitute to refine experiments and streamline the set-up, reducing the time the animals are under anaesthesia or immobilised. According to a literature search, in the last 5 years ~1300 mice were used in proton research but this number is set to rise as proton beam technology becomes more widely available. Regular use of the tool to measure the dose output and identify any problems will increase confidence in the results, therefore reducing the numbers of animal required to achieve statistically significant data and refining the dose delivery to minimise potential toxicity.

Proton beam therapy is an alternate form of radiotherapy, delivering high energy particles to a tumour. The favourable dose deposition of protons follows a low entrance dose ending in a steep dose gradient and rapid fall off (Bragg peak), increasing the precision of the dose delivery. The size difference between a human and a mouse make it difficult to utilise the Bragg peak distribution within the small animal so often the initial plateau region of the beam is used to irradiate the target. Consequently, healthy tissue beyond the target is irradiated, increasing toxicity. The beam energy can be reduced to deposit the Bragg peak within the animal but this is a complicated process so often range shifters, energy degraders, shielding or collimators are used to attenuate and shape the beam before it reaches the animal.
The aim of this study is to adapt a 3D printed murine dosimetry phantom to ensure accurate proton dose delivery in our proton research laboratory. Once validated, this model will be sent to international institutions to perform an audit of the different techniques implemented and the accuracy of delivered dose. This will be facilitated through our existing collaborations with the Inspire project and the Particle Therapy Co-operative Group. Furthermore, 3D cellular models in a hydrogel matrix will be combined with markers of DNA damage to validate the dose distribution.
Since 2016, ~1300 mice have been used worldwide in proton research. The phantom could be used to refine research by streamlining experiments to reduce the time the animals are under anaesthesia or immobilised. Increasing confidence in delivered doses will minimise the risk of unnecessary radiation toxicity and reduce the numbers required for statistically significant data. ~200 of these animals were used in pilot studies to validate irradiation techniques and equipment. This 3D printed phantom contains tissue mimicking materials, therefore is a suitable replacement for these animals.