Evaluation of water-equivalent plastics as phantom material for electron-beam dosimetry.

This investigation evaluated samples of three phantom materials designed as substitutes for water for electron beam calibration and depth ionization measurements. Two of the materials are commercially available (photon-electron Solid Water and Plastic Water), while the third (Homat) is not. Applying the values for water for all factors used in the calibration protocol of the American Association of Physicists in Medicine [Task Group 21, Med. Phys. 10, 741-771 (1983)] results in a discrepancy in calculated peak dose rates. Eliminating this discrepancy requires the additional inclusion of a multiplicative correction factor of approximately 1.015 for beam energies below 10 MeV, 1.01 for beam energies between 10 and 12 MeV, and 1.005 for beam energies above 12 MeV. Measurements for R50 and extrapolated range may be made in these materials with no corrections. Some improvement can be made in the performance of the phantom material by optimizing the match to water specifically for electron beams without regard for photon beam response. As with all radiation oncology apparatus, calibration phantoms need acceptance testing before routine use.