Output factors of multileaf-collimator (MLC) shaped radiation fields were measured for a commercial linear accelerator whose MLC leaves form parts of the upper collimator system. The approach of taking into account the reduced phantom scatter due to the MLC shaping on the output factor has previously been shown to be inadequate for this type of machine because of the effect of the MLC leaves on the collimator factor [Palta et al., Med. Phys. 23, 1219-1224(1996)]. In this article, we present two forms of the collimator factor that give satisfactory agreement with measured values of the output factors of MLC-shaped fields. The present method should be directly applicable to other linacs of similar MLC configuration. For clinical treatment planning, we believe the method is practical and accurate enough to be satisfactory. The equation for calculating the output factor requires only peak scatter and output factors of the machine. These are normally measured during machine commissioning.
Luminescent centers involving Ag impurities were introduced into CdS single crystals through doping with 109Cd radioisotopes. Thus, the Ag concentration increases with time as more 109Cd decays. This enables a study of photoluminescence versus Ag concentration in a given crystal without changing the concentrations of other impurities.A new emission band at 5600 Å results in addition to the 6100 Å band present in Ag-doped crystals using conventional techniques. This new emission is quenched with increasing Ag concentration at high concentrations. Also concentration quenching by the Ag impurities occurs for the green edge, and the bound-exciton emissions I1, and I2. The quenching is explained by assuming a donor–acceptor recombination process.The new emission probably arises from the recombination of a bound electron with a bound hole at a distant donor–acceptor pair, with Ag as the acceptor. The acceptor role of Ag is supported by electrical conductivity measurements on 109Cd-doped crystals. Estimates are obtained for the acceptor binding energy, the donor concentrations, and the separations of pairs responsible for the new 5600 Å emission and the green-edge emission. The 6100 Å emission is attributed to Ag closely associated with other impurities. These conclusions are verified by our temperature annealing and quenching experiments.
The Batho equation gives a satisfactory method to correct the dose for points in the electronic equilibrium region for a uniform slab of inhomogeneity in a photon beam. In spite of the many investigations, we believe no simple and adequate method has been found for routine clinical dose calculations which require dose correction of a small-volume inhomogeneity in an arbitrary location. In the present report, we combine the values of the two calculation types of the differential Batho method, which we have developed previously, to give a new calculated value for the scatter perturbation due to an annulus of inhomogeneity. The coefficients in the combination, which we derived from a detailed analysis of the scatter perturbation, are simple geometrical ratios. The new calculated values are in good agreement with measured values. We believe this application of the differential Batho method can provide a practical and accurate method of correcting for inhomogeneities of any size and shape in clinical dose calculations.
For a uniform slab of inhomogeneity in a supervoltage beam, correction factors can be calculated from the Batho equation. In this report, we present a method for calculating the effect of an annular inhomogeneity, concentric about the beam axis, upon the dose at a point on the axis and below the annulus. A derivation of the equation required in the calculation for supervoltage radiation is given. Results from measurements made in 60Co beams for polystyrene foam, cedar, and aluminum annuli, all having 3.0 x 2.0 cm2 in cross section but with different inside diameters, are compared with correction values calculated by the method. For situations where the annulus is just submerged in the phantom, measured and calculated values are in good agreement. For a general situation, two calculation types are proposed and the data show that in general the measured scatter perturbation lies between the calculated values of the two types. Application of our technique predicts a sign reversal in the scatter perturbation due to an inhomogeneity. This reversal has previously been observed and reported and is also demonstrated in our measurements.
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