Doses on the central axes of narrow 6-MV x-ray beams

The absorbed doses on the central axes of narrow beams (radii 0.07-2.5 cm) of 6-MV x rays have been studied by experiments and Monte Carlo simulations. The measurements were made in a geometry used for irradiation of intracranial lesions. For radii less than 1.0 cm the dose on the central axis is progressively reduced due to electron disequilibrium. This leads to measurement artifacts when the detector is too large, as was readily observed with ionization chambers. Radiographic and radiochromic films were used with densitometric evaluation to provide the resolution necessary to measure absorbed doses for the narrowest beams. The contribution by phantom-scattered photons is significant even at small field sizes, and scatter factors were determined from the experimental results. Photons scattered by the auxiliary collimator did not add appreciably to the dose on the central axis. The data were used to characterize the dose-to-kerma ratio as a function of beam radius. Differences between experimental results and those from Monte Carlo calculations were observed.