EFFECT OF THE AIR GAP BETWEEN THE PATIENT AND BOLUS TO THE SURFACE DOSE AT PERPENDICULAR AND OBLIQUE ELECTRON TREATMENTS

When bolus is used in electron beam treatments on a Saturne-42 linear accelerator, the airgap effect between the bolus and skin and the effect of perpendicular and oblique incidence of the beam to the surface doses were studied. Dose measurements were made relatively using a PTW Marcus parallel plate ionization chamber and thermoluminescence dosimeters on a solid water phantom at 100 cm source to skin distance for different air gaps and gantry angles with 0.5 and 1.0 cm thick boluses. Measurements performed for different electron energies, bolus thickness, field sizes and air gaps showed that in the case of the presence of air gap under the bolus, the surface dose decreased with decreasing field size. During the oblique incidence of the beam the air gap under the bolus caused a decrease in the surface dose but an increase in the surface dose with gantry angle was observed. However, when we used 1.0 cm thick bolus there was a decrease in the surface dose for oblique incidence at 4.5 MeV. The result is that it should be taken into account that during oblique and perpendicular electron irradiation the air gap under the bolus might result in lower surface doses than expected.

Keywords: electron, bolus, surface dose