Influence of Air Gap Under Bolus on Dose Calculation Accuracy In the Clinical 6 MV Photon Beam
Abstract
Purpose
To investigate the effect of bolus–surface air gap on surface dose calculation for a clinical 6 MV photon beam.
Methods
Surface dose was measured in a solid water phantom at an effective depth of 0.3 mm using a diode detector for square fields from 5×5 to 25×25 cm². Bolus thicknesses of 0.5, 1, and 2 cm were evaluated with bolus–surface air gaps ranging from 0 to 6 cm. Corresponding plans were calculated using the AcurosXB algorithm (Varian Medical Systems). Surface dose was normalized to the zero–air-gap condition, and an exponential model was used to describe the non-linear dependence on field size and air gap.
Results
Both calculations and diode measurements showed field-size and bolus-thickness dependent sensitivity to bolus surface air gaps, with surface dose decreasing with increasing air gap and decreasing field size. For small fields (5x5-10x10 cm2), Acuros XB predicted surface dose within 5% for air gaps of 4, 3, 1 cm for bolus thickness of 0.5, 1, and 2cm respectively.
Conclusion
Bolus–surface air gaps significantly reduce surface dose, particularly for small photon fields, and increasing bolus thickness alone does not fully mitigate this effect. The accuracy of AcurosXB decreases with increasing air gap and bolus thickness. Future work includes evaluation of modulation fields.