Proton radiography for integrating positioning and treatment planning of small animals at an experimental proton beam

Integrated small animal X-ray imaging and irradiation devices have facilitated image-guided pre-clinical radiooncological experiments. Here, a proton-based radiography method is proposed for in-line small animal treatment planning and positioning verification at an experimental proton beamline.
A double-scattered 125 MeV proton beam (10 x 10 cm² field size) and a flat-panel, scintillation-based dose detector (Lynx, IBA Dosimetry) were used to obtain planar scans of deceased mice positioned in a closed multi-modality bed. Two modes of image acquisition were used: absorption and scattering. In absorption mode, slabs of plastic with a water equivalent path length (WEPL) of 7.08 g/cm² were placed between object and Lynx detector. In scattering mode, all plastic slabs were removed. Digital, pixel-wise image processing was applied to combine the information. A WEPL calibration curve was obtained using plastic slices of known WEPL as objects. For verification, planar X-ray images of the same objects were acquired.
The proposed radiography technique allowed for detection of contours (e.g., mouse body, bed and background) and reliable automated (rigid) registration of proton radiography images to planar X-ray scans. Planar 2D radiological thickness maps of mice were obtained. Specific structures could be visualized with a spatial resolution of less than 1 mm, especially regions with high local density differences including lung, hind leg, jaws and skull.
The method appears suitable to position small animals for precise irradiation of subcutaneous (hind leg) and orthotopic (lung, brain) tumor models. Currently extended image analysis may enhance visualization of the animal’s internal structure to facilitate in-line treatment planning.