Transmission Properties of Aluminum and Brass Used for Solid IMRT Compensators in Megavoltage Photon Beams
Wayne M. Butler, Nick Cyr, Brian S. Kurko, Richard L. Anderson, Gregory S. Merrick
Schiffler Cancer Center, Wheeling Hospital, Wheeling, WV
Purpose. The design of solid IMRT compensators requires the accurate determination of the linear attenuation coefficient, m, of the compensating material. For clinical applications, m is not a single value for a given material. Here we determine m as a function of beam quality, field size, measurement geometry, and material thickness for two common compensator materials.
Methods. The attenuation of brass and aluminum compensator blanks was measured for ten thicknesses ranging from 6 mm to 51 mm and for eleven field sizes ranging from 3x3 cm² to 25x25 cm² for each of three photon energies, 6, 10 and 18 MV from an Elekta Synergy linear accelerator. To minimize scatter contributions from structures other than the collimator, compensator, and mini-phantom, the gantry was rotated to orient the beam horizontally. To ensure complete coverage of the mini-phantom at small field sizes, all primary measurements were made at 200 cm source to chamber distance (SCD) followed by measurements at each energy for selected field sizes at 100 cm SCD to verify there were no significant deviations from inverse square dependence. All measurements were acquired using an farmer-type ionization chamber (Exradin model A12) covered by a buildup cap of appropriate thickness for the given energy. Measurements of charge on a Standard Imaging MAX-4000 electrometer were corrected for temperature and pressure variations.
Results. The linear attenuation coefficient decreases with photon energy and is lower for aluminum than for brass. As a function of field size, m attains a maximum value at intermediate field sizes between 5x5 cm² and 10x10 cm² and decreases toward smaller and larger field sizes with the lowest values at 25x25 cm². The range of m across the spectrum of field sizes spans 14.8% for aluminum and 17.4% for brass. Beam hardening becomes more pronounced as the metal thickness increases so that greater thickness results in a lower m; however, this effect is less pronounced than the field size effect and varies up to 7.0% for aluminum and 12.2% for brass over the range of thicknesses studied.
Conclusion. An approach that designs solid metal compensators using linear attenuation coefficients that reflect typical field sizes (and to a lesser extent the average thickness of the compensator material) for various anatomic sites will reduce the variation between calculated and measured doses when performing quality assurance checks on the compensator.