No abstract

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.

The head-scatter factors H were examined for four different linear accelerators and were found to be similar at field sizes larger than 3 x 3 cm2. Sharply reduced values for small collimator openings were observed for all the accelerators. It is concluded that the head-scatter (or collimator-scatter) factor has two major components. Scatter of photons in various structures in the beam path, especially the flattening filter, causes a slow (about 10%) increase with increased collimator opening. Insertion of a built-in wedge may double this number. When the collimators are closed, they ultimately block photons from the periphery of the source. This may cause a considerable reduction of the primary photon fluence and typically affects fields smaller than 3 x 3 cm2. The effect can be used to estimate the source size, with results that correlate with the design of the bending magnet.

Attenuation factors in water have been measured by a narrow-beam technique in various portions of x-ray beams with nominal energies of 6 and 25 MV, with and without a wedge in the beam. The results were analyzed in terms of an attenuation coefficient mu for small water thicknesses and a beam-hardening coefficient eta that describes the change in attenuation per unit depth. The variation of mu within the field was significant, about 0.5% per centimeter at 6 MV and 0.8% per centimeter at 25 MV for open beams. The heavy wedge used in these experiments caused significant (about 10%) beam hardening at 6 MV, softened the beam somewhat at 25 MV, and increased the variation of mu within the field to 3%-5%. These effects should be taken into account in dose calculations, and correction factors can be designed based on the variation of mu with off-axis radius for open beams and with off-axis position for wedged beams. The experimental technique, based on two measurements with the beam going through a water tank with either 26- or 50-cm path length, was simple and highly reproducible. The beam hardening with depth in water, i.e., the value of eta, was readily determined but found to be clinically insignificant.