Relative electron beam measurements: scaling depths in clear polystyrene to equivalent depths in water.

For purposes of mean incident energy determination and the accrual of consistent treatment planning data, measurements of relative ionization or dose made in clear polystyrene must be scaled in depth to produce depth-ionization or depth-dose curves equivalent to what would have been measured in water. Recommendations from various protocols for clear polystyrene to water depth scaling factors differ by as much as 5%. Here, central axis measurements of relative ionization as a function of depth have been made with parallel-plate chambers both in a popular clear polystyrene phantom of density 1.045 g/cm3 and in water. Perturbation and displacement corrections were thus minimized. Both depth-ionization and depth-dose curves were formed for electron beams with nominal incident energies between 6 and 20 MeV and field sizes from 6 cm X 6 cm to 15 cm X 15 cm. Comparisons of the depths for 50% relative reading and practical range between the two phantoms yield average empirical scaling factors of 0.990 and 1.002, respectively.