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

High energy x-ray beam penumbra are measured using Gafchromic™ EBT film. Gafchromic™ 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™ 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™ EBT film provides an adequate measure of penumbral dose for high energy x-ray beams. © 2006 American Association of Physicists in Medicine.