Calculation of air gaps between bolus and skin on the dose received from the skin

    Introduction: In the last few decades, a lot of monte carlo codes have been introduced for medical applications. For close - to - surface tumors, the surface of the skin can be used to increase the surface dose so that all or part of the bunch area can fall into bolus, thereby increasing the dose of the skin. The air gaps between bolus and skin can affect the dose of skin, especially if the skin is only the target of treatment. For this purpose, the effect of energy, bolus thickness, the amount of air gap, field dimensions and angle of field contact angle will be evaluated. Materials and Methods: In this study, the MCNPX calculation code is used to simulate the linear accelerator of Siemens to determine the effects of air gaps between bolus and skin. The studied fields were chosen in this study with dimensions 4×4 and 10×10 cm2. it also simulated a water phantom at SSD=100 cm and bolus thickness 0.5,1 cm in distances 0, 0.5 cm. Results: For field sizes 4×4 cm2 with 6 photon beam The bolus was as thick as 0.5 cm in the distance of 0 and 0.5 cm from the phantom level with a percent of surface dose to the maximum dose in the absence of the air gaps with %104.615 in the presence of air gaps %101.866 and the bolus thickness of 1cm in a same distance from the phantom level with a percent of surface dose to the maximum dose in the absence of the air gaps with %106.89 in the presence of air gaps %100.47. Also for field sizes 10×10 cm2, thickness of 0.5 cm in distance of 0 and 0.5 cm, from the phantom level with a percent of surface dose to the maximum dose in the absence of the air gaps with %99.62 in the presence of air gaps %97.74. Conclusion: For field size 10×10 cm2 Dsurf is largely unaffected by air gaps. However, smaller air gap results in shallower dmax for 6 MV photon beams at all fields sizes. Special consideration should be taken to reduce air gaps between bolus and skin for field sizes smaller than 10×10 cm2 or when surface contour variations are greater or when the bolus covers small area and at the border of the field.