Breathing interplay effects during proton radiation therapy and development of repainting solutions*

The movement from passive scattering to active spot scanning in proton radiation therapy introduces the problem of interplay effects when elements of beam motion have a similar time scale to periodic tumor motion, as in a lung tumor. This can lead to significant deviations from the planned radiation dose. Although the repetition of a field over many treatment sessions tends to average out these inhomogeneities, the usual 30 fractions may still leave sizeable errors. These errors are characterized via computer simulation, and field `repainting' methods are developed to reduce them through increased dose averaging.