title: A differential method for inhomogeneity correction on dose in a photon beam.
abstract: 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.
mesh_terms: D003037:Cobalt Radioisotopes / Q000627:therapeutic use*; D004601:Elementary Particles; D008961:Models, Structural; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy; D012542:Scattering, Radiation
chemical_list: D003037:Cobalt Radioisotopes