Formation and surface dose determination of natural rubber bolus for a 9-MeV therapeutic electron beam
Keywords:
Bolus, Depth-scaling factor, Gafchromic EBT3, Natural rubberAbstract
Backgrounds: Bolus is a device used in electron beam radiotherapy to increase the surface dose. It may be made from natural or synthetic materials that have a density close to human tissue. Because of an expensive price, commercial bolus cannot be cut to a suitable size for each patient in clinical practice.
Objectives: The aims of this study were to form the in-house bolus with natural rubber and to determine the doses at surface of our bolus sheet.
Materials and Methods: The natural rubber and additional materials were mixed to mold the in-house bolus. The computed tomography (CT) number and physical density of our bolus have been obtained from the CT images and CT calibration curve, respectively. The surface doses were measured using Gafchromic EBT3 film for the 9-MeV electron beam from a Varian Clinac 23EX linear accelerator. The correction factors for scaling thickness of bolus were introduced.
Results: The natural rubber was successfully fabricated as a bolus sheets with the thickness of 0.32 and 0.52 cm. The physical densities of two bolus sheets were consistent to each other which equal to 0.87 g/cm3. The natural rubber bolus gave more measured surface dose than the commercial bolus that made of synthetic oil gel. The high differences of percentage surface dose were observed in the 0.5 cm thickness. For both the ~0.3 and ~0.5 cm thickness, the correction factors were closed to 1. After the correction, we found excellent agreement between the measured and calculated surface dose.
Conclusion: Natural rubber can be used as an alternative material to form the bolus sheet, because of its suitable property and density. Based on the correction factor from scaling thickness by its density, the surface doses equipped with the natural rubber bolus were equivalent to that with the synthetic gel bolus.
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