Evaluation of the Effects of Different Filters and Helium Bag on the Reduction of Electron Contamination in Photon Beam of Neptun Linac

Abstract

Introduction: Skin sparing is one of the most desirable characteristics of high energy photon <br /> beams. However, the photons emerging from the target of linacs are contaminated by secondary <br /> electrons as a result of their interactions with air, collimators, flattening filter and any other <br /> objects in their path. This phenomenon tends to increase the skin dose received by the patients. A <br /> practical and simple way to reduce the contribution of electron contamination is to place a sheet of <br /> medium to high Z material just after the secondary collimator. In this study, filters having <br /> different thickness and atomic number were applied and their effectiveness on the reduction of <br /> skin dose was evaluated. <br /> Materials and Methods: The filters of different thickness and atomic number were applied. The <br /> percent depth dose values were determined by the direct measurements made in a Scanditronix <br /> water phantom using a PTW 31006 Pin Point chamber having a sensitive volume of 0.015 cm3. A <br /> Perspex filter holder was made to be installed on the accessory slot. A plastic bag containing <br /> helium was also made using thin plastic sheet to study the effect of the helium bag when it <br /> replaces the air column between the head of the linac and the phantom. All of the measurements <br /> were carried out for the three field sizes of 10×10, 20×20 and 25×25 cm2. The setups were <br /> adjusted for SSD = 100 cm. The ratio of the surface dose to maximum dose (Ds) was used as the <br /> criterion to determine the optimum filter. <br /> Results: The dosimetry results obtained in the water phantom indicated that a 0.4 mm thick Pb <br /> filter is the most effective one. This filter reduces the Ds for the field sizes of 10×10, 20×20 and <br /> 25×25 cm2 by 5.7, 7.9 and 9.6%, respectively. Also the simultaneous use of the optimum filter and <br /> He bag is more effective than the filter alone. It reduces the Ds by 6.3, 10.1 and 12.3% for the <br /> field sizes of 10×10, 20×20 and 25×25 cm2, respectively. <br /> Discussion and Conclusion: Based on the results of this work it is evident that the contribution <br /> of contaminant electrons to dose from the air column between the head and the phantom is much <br /> smaller than it from the secondary electrons arising from the head of the linac. On the other hand, <br /> the electron contamination originating from the air column is almost independent of the field size. <br /> But the surface dose arising from the secondary electrons produced by the head of the linac <br /> depends on the field size, which is increased by increasing the field size.