AbstractID: 8026 Title: Measurement of backscatter factors in clinical kilovoltage x-ray beams using
GafChromic MD-55-2 film
Backscatter factors for water (Bw) are important for absorbed dose determination at the phantom surface in kilovoltage x ray beams
using the in-air calibration method. The Bw-data in protocols (e.g., AAPM TG-61) are based on Monte Carlo calculations which
assume an open, scatter-free beam. We investigated the suitability of GafChromic MD-55 film to measure Bw for clinical kilovoltage
X-ray beams. We devised an irradiation protocol by which four values of Bw could be inferred from measurements on four pieces of
film which had each been irradiated three times, free-in-air or on a phantom, in a specific order. We measured Bw for four x-ray
applicators for 150 kVp and 300 kVp x-rays. We compared measured values to recent tabulations from the TG-61 report. For 150 kVp,
measured Bw is 0.2% to 3.8% smaller than tabulated values whereas for 300 kVp values range from 0.6% greater to 2.3% smaller than
tabulated values. The agreement was systematically better for the smaller applicators. Using Monte Carlo calculations, we studied
correction factors that account for the energy dependence of the response of the film when using it for measurements of Bw. For
applicators larger than 2 cm diameter, the correction factors ranged from 1.010 at low energy (80 kVp) to less than 1.005 at high
energy (250 kVp); for the 2 cm diameter applicator, they were only slightly larger (up to 1.017) demonstrating that GafChromic MD55 film is a suitable dosimeter for the measurement of backscatter factors in kilovoltage beams.