Proton radiation therapy is one of the most precise forms of non-invasive image-guided cancer therapy. It is based on the well defined range of protons in material, with low entrance dose, a dose maximum ("Bragg peak") and a rapid distal dose fall-off, providing better sparing of healthy tissue and allowing higher tumor doses than conventional radiation therapy with photons.
It is possible to generate volumetric representations of a patient using measurements at a variety of angles. X-rays are currently in common use for generating images of internal organs for treatment of a variety of conditions.
At present, the potentials of proton therapy cannot be fully exploited because the conversion of Hounsfield values, measured with x-ray computed tomography (CT), to relative electron density values is not always accurate. The resulting uncertainties can lead to range errors from several millimeters up to more than 1 cm depending on the anatomical region treated. Additional uncertainties exist with respect to the target position relative to normal tissues in the treatment room that could be minimized by using proton CT for guiding the therapy.