The measurement of backscatter factors of kilovoltage X-ray beams using Gafchromic™ EBT2 film

Abstract

Backscatter factors are essential in the determination of radiation dose for kilovoltage X-ray beams. The accurate measurement of backscatter factors in water (B (W)) is difficult and published values are based largely on Monte Carlo calculations. A number of studies have found that the measurement of B (W) in the energy range from 50 to 300kVp is possible using Gafchromic EBT film, but this film is no longer commercially available. In this work, we evaluated whether the newer Gafchromic EBT2 film is suitable for the determination of B (W) for kilovoltage X-ray beams. B (W) were measured with Gafchromic EBT2 film for beam qualities of 50, 100 and 280kVp and field sizes of 2, 3, 4 and 6cm diameter and compared with tabulated values published in the AAPM TG-61 protocol. We found that rotation of the film position during readout had a small but non-negligible effect on the optical density readings. The agreement between measured and published B (W) was better than 3%, with the largest difference of occurring for the 2cm diameter field with the 50kVp X-ray beam. However, these differences are consistent with the total estimated uncertainty for the measurements, as calculated by the ISO GUM. Our results demonstrate that Gafchromic EBT2 film is a suitable dosimeter for B (W) measurements for clinical kilovoltage X-ray beams.