Electron dose rate and photon contamination in electron arc therapy

The electron dose rate at the depth of dose maximum d max and the photon contamination are discussed as a function of several parameters of the rotational electron beam. A pseudoarc technique with an angular increment of 10° and a constant number of monitor units per each stationary electron field was used in our experiments. The electron dose rate is defined as the electron dose at a given point in phantom divided by the number of monitor units given for any one stationary electron beam. For a given depth of isocenter d i the electron dose rates at d max are linearly dependent on the nominal field width w , while for a given w the dose rates are inversely proportional to d i . The dose rates for rotational electron beams with different d i are related through the inverse square law provided that the two beams have ( d i , w ) combinations which give the same characteristic angle β. The photon dose at the isocenter depends on the arc angle α, field width w , and isocenter depth d i . For constant w and d i the photon dose at isocenter is proportional to α, for constant α and w it is proportional to d i , and for constant α and d i it is inversely proportional to w . The w and d i dependence implies that for the same α the photon dose at the isocenter is inversely proportional to the electron dose rate at d max .