Dosimetry of 6-MV x-ray beam penumbra

The measurement of x-ray beam dose profiles in the penumbral region, using silicon diode, ionization chamber, TLD, and film dosimetry, has been investigated for a 6-MV beam defined by independent collimators. Penumbral width (80%-20%) at dmax, as measured by diode, film, and TLD was found to be 3.6, 3.6, and 3.4 mm, respectively. These results reflect the relative sensitive widths of each of the measurement systems (2.5, 2.0, and 1.0 mm, respectively). An empirical forming function was used to relate the penumbral shape measured with a finite-sized detector to that which would be measured with a point detector, the width of the point detector penumbra calculated from the diode penumbra is 3.4 mm, indicating that the TLD rods are a good approximation to a point detector. An alternative method of determining the width of a point detector penumbra is to extrapolate the penumbral widths obtained using two or more detectors of sensitive width. With this method, using Farmer and RK ionization chambers, a point detector penumbra width of 3.1 mm is obtained. An EGS4 Monte Carlo simulation, where a point source was assumed, gave a penumbral width of 2.8 mm. Negligible differences between the penumbra of beams defined by symmetric and asymmetric collimators was observed.