Measured and Monte Carlo simulated surface dose reduction for superficial X-rays incident on tissue with underlying air or bone
Abstract
PURPOSE: Measurement of surface dose reduction effects for superficial x-rays incident on tissue with underlying air or bone and comparison with Monte Carlo simulations of such effects. Further to investigate the correlation between surface dose reduction and changes in Compton backscatter spectra with tissue-bone separation.
METHODS: An Advanced Markus chamber with entrance window facing downstream on the surface of a solid water phantom was used to investigate changes in surface dose with an underlying air or bone interface located at various depths below the surface. Chamber readings were obtained for interface depths ranging from 1 to 100 mm using the 50 kV, 100 kV and 150 kV beams of an Xstrahl 150 x-ray unit, with field diameters (ϕ) = 2.5 cm and 5 cm. For each beam quality and field size the dose correction factor, DCF(t), namely the ratio of measured dose (t) to dose (t = 100 mm) was determined. Monte Carlo simulations of DCF(t) for air and bone interfaces in tissue are used to validate corresponding measured data. For a given beam and field size, the difference between simulated spectra with an air or bone interface at t = 3 mm was used to determine the Compton backscatter from bone at the surface.
RESULTS: For air, DCF(t < 40 mm) is less than unity and for a given t DCF(t) decreases as beam energy increases from 50 kV to 150 kV. Conversely for bone DCF(t < 40 mm) increases for a given t with increasing beam energy. In particular for t = 1 mm, ϕ = 5 cm, DCF for…
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