Measurement of high energy x-ray beam penumbra with Gafchromic™ EBT radiochromic film

High energy x-ray beam penumbra are measured using Gafchromic(TM) EBT film. Gafchromic(TM) EBT, due to its limited energy dependence and high spatial resolution provide a high level of accuracy for dose assessment in penumbral regions. The spatial resolution of film detector systems is normally limited by the scanning resolution of the densitometer. Penumbral widths (80%/20%) measured at D-max were found to be 2.8, 3.0, 3.2, and 3.4 mm (+/- 0.2 mm) using 5, 10, 20, and 30 cm square field sizes, respectively, for a 6 MV linear accelerator produced x-ray beam. This is compared to 3.2 mm +/- 0.2 mm (Kodak EDR2) and 3.6 mm +/- 0.2 mm (Kodak X-Omat V) at 10 cm X 10 cm measured using radiographic film. Using a zero volume extrapolation technique for ionization chamber measurements, the 10 cm x 10 cm field penumbra at D-max was measured to be 3.1 mm, a close match to Gafchromic(TM) EBT results. Penumbral measurements can also be made at other depths, including the surface, as the film does not suffer significantly from dosimetric variations caused by changing x-ray energy spectra. Gafchromic(TM) EBT film provides an adequate measure of penumbral dose for high energy x-ray beams. (C) 2006 American Association of Physicists in Medicine.