Is it possible to verify directly a proton-treatment plan using positron emission tomography?

A PET camera is used to visualize the positron activity induced during protonbeam therapy in order to verify directly the proton-treatment plans. The positron emitters created are predominantly the O-15 and C-11, whose total activity amounts to 12 MBq after an irradiation with 85 MeV protons, delivering 3 Gy in a volume of approximately 300 cm 3. Although this method is a useful verification of patient set-up, care must be taken when deriving dose distributions from activity distributions. Correlation between both quantities is difficult, moreover at the last millimeters of their range, protons will no longer activate tissue. Due to the short half-lives the PET camera must be located close to the treatment facility.