Influence of an Air Gap between Bolus and Patient Skin on Electron Beam Dose Characteristics

Introduction In electron beam therapy bolus is often used to conform as closely as possible to the target volume while avoiding critical structures in the depth. In most cases a flexible bolus can cover the skin without air gap. However, under certain conditions it is difficult –or even not possible– to avoid air gaps between bolus and patient without reducing treatment quality. In a special clinical case in our hospital (USB) bolus was used in the neck region in combination with a head fixation. The fixation grid was not in contact with the patient skin in the treatment region but the bolus was attached outside the grid resulting in an air gap behind the bolus. Such an air gap can affect dose and dose distribution [1]. The aim of the current investigation is to evaluate the effects of an air gap behind a bolus on dose and dose distribution characteristics.