First proton radiography of an animal patient.

The purpose of this work is to show the feasibility of proton radiography in terms of radiation dose, imaging speed, image quality (density and spatial resolution), and image content under clinical conditions. Protons with 214 MeV energy can penetrate through most patients and were used for imaging. The measured residual range (or energy) of the protons behind the patient was subtracted from the range without an object in the beam path and used to create a projected image. The image content is therefore proportional to the range that protons have lost in the patient. We took proton images of the head of a dog after it received proton radiotherapy treatment of a nasal tumor. The spatial resolution by measuring for each proton separately its coordinate in front of and behind the patient was approximately 1 mm. The acquisition time was on the order of several seconds and was limited by the patient table movement. The range sensitivity of the images was approximately 0.6 mm, which is good enough to use the images for therapy range verification. The dose that the dog received during exposure was 0.03 mGy, which is approximately a factor 50-100 smaller than for a comparable x-ray image. The potential to obtain quantitative images of proton ranges with satisfying spatial and range resolution and low dose to the patient suggests that proton radiography should be applied to patients who are under proton radiotherapy treatment.