Steps in producing a proton CT of a human phantom head

Montage showing different stages (top left to bottom right) in the CT reconstruction of a single slice (bird's eye view; transverse section) through an adult phantom head. The phantom is a real human skull that has been filled with a material that simulates the average density of brain tissue. The first panel (top left) shows the projection of the broad proton beam through the skull at one angle. The false-coloured stripes are residual energies of the protons as they exit the side of the head. More projections are then obtained by changing projection angles. Here, the head is rotated but in practice the patient would be stationary and the beam rotated around the patient. The final panel (bottom right) shows the final CT reconstruction, with the bony skull and nasal voids clearly visible. A clinical proton CT has not yet been produced to date. This is a simulation using X-ray data of a patient receiving radiotherapy where every X-ray event has been converted to its proton equivalent. 180 image projections were then processed using a new algorithm to reconstruct one slice through the phantom head. Proton beam radiotherapy is rapidly becoming a recognised treatment for certain cancers, especially in children and young people. To target the tumour site while minimising the exposure to healthy surrounding tissue it is necessary to image using the same particles (protons) that are used for therapy. The advantage of using high-energy treatment protons to construct the CT scan instead of much lower energy X-ray photons is that registration errors are reduced from ±4% to less than ±1%. Proton therapy could be used in many more cases and with better outcomes.