An investigation of backscatter factors for kilovoltage x-rays: a comparison between Monte Carlo simulations and Gafchromic EBT film measurements
Abstract
Backscatter factors are important parameters in the determination of dose for kilovoltage x-ray beams. However, backscatter factors are difficult to measure experimentally, and tabulated values are based largely on Monte Carlo calculations. In this study we have determined new backscatter factors by both experimental and Monte Carlo methods, and compared them with existing backscatter factors published in the AAPM TG-61 protocol. The purpose of this study is twofold: (1) to evaluate the overall effectiveness of using Gafchromic EBT film for backscatter factor measurements and (2) to determine whether existing Monte Carlo-calculated backscatter factors need to be updated. We measured backscatter factors using Gafchromic EBT film for three field sizes (2, 4 and 6 cm diameter cones) and three kilovoltage beam qualities, including 280 kVp for which similar measurements have not previously been reported. We also present new Monte Carlo-calculated backscatter factors obtained using the EGSnrc/BEAMnrc code system to simulate the Pantak kilovoltage x-ray unit used in our measurements. The results were compared with backscatter factors tabulated in the AAPM TG-61 protocol for kilovoltage x-ray dosimetry. The largest difference between our measured and calculated backscatter factors and the AAPM TG-61 values was found to be 2.5%. This agreement is remarkably good, considering that the AAPM TG-61 values consist of a combination of experimental and Monte Carlo calculations obtained over…
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