Experimental determination of depth-scaling factors and central axis depth dose for clinical electron beams

Depth-scaling factors rho(eff) for clear polystyrene and polymethylmethacrylate (PMMA) phantoms have been determined experimentally as a function of nominal electron-beam energy in the range 6 to 22 MeV. Values of rho(eff) have been calculated from the ratio rho(eff) = R(wat)(50) / R(med)(50), where R(wat)(50) and R(med)(50) are the measured depths of 50% ionization in electron solid water and plastic (clear polystyrene and PMMA) phantoms, respectively. Measurements were made using an Attix chamber in an electron solid water phantom, a Holt chamber in a clear polystyrene phantom, and a Markus chamber in a PMMA phantom. The average value of measured rho(poly)(eff) was found to be 0.999 +/- 0.009. This is higher than the value of 0.975 recommended by Task Group 25 (TG-25) of the American Association of Physicists in Medicine (AAPM) by 2.5%. Depending on energy, the maximum differences between the AAPM TG-25-recommended and the measured values lie in the range 1% to 3.5%. Similarly, the average value of measured rho(PMMA)(eff) was found to be 1.168 +/- 0.023. This is higher than the AAPM TG-25-recommended value of 1.115, by 5%. Depending on energy, the maximum differences between the AAPM TG-25-recommended and the measured values lie in the range 3% to 8%. Central axis depth dose curves in water were generated for 6, 15, and 20 MeV electron beams from measured depth-ionization data in PMMA and clear polystyrene phantoms following the recommendations of the AAPM TG-25 report and using both TG-25-recommended and experimentally determined values of depth-scaling factors rho(eff). For both phantoms, either the TG-25-recommended value or the experimentally determined values of rho(eff) yielded agreement to within about 2 mm among all depth doses in water at the depths of clinical relevance.