Photon spectral characteristics of dissimilar 6MV linear accelerators

• Published in: Medical physics (Lancaster) 2008-05, Vol.35 (5), p.1698-1702
• Main Authors: Hinson, William H., Kearns, William T., deGuzman, Allan F., Bourland, J. Daniel
• Format: Article
• Language: English
• Subjects: Aluminium; biomedical equipment; Biomedical modeling; Dose‐volume analysis; dosimetry; Dosimetry/exposure assessment; Field size; Gas‐filled counters: ionization chambers, proportional, and avalanche counters; Graphite; high energy photon spectrum; ionisation chambers; iterative methods; Linear accelerators; Magnets; Medical treatment planning; Photons; quality assurance; Quality assurance in radiotherapy; radiation therapy; Spectral properties; Therapeutic applications, including brachytherapy; Transmission measurement; transmission measurements; treatment planning modeling; waveguides
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.2900001

This work measures and compares the energy spectra of four dosimetrically matched 6MV beams, generated from four physically different linear accelerators. The goal of this work is twofold. First, this study determines whether the spectra of dosimetrically matched beams are measurably different. This study also demonstrates that the spectra of clinical photon beams can be measured as a part of the beam data collection process for input to a three‐dimensional (3D) treatment planning system. The spectra of 6MV beams that are dosimetrically matched for clinical use were studied to determine if the beam spectra are similarly matched. Each of the four accelerators examined had a standing waveguide, but with different physical designs. The four accelerators were two Varian 2100C/Ds (one 6MV∕18MV waveguide and one 6MV∕10MV waveguide), one Varian 600C with a vertically mounted waveguide and no bending magnet, and one Siemens MD 6740 with a 6MV∕10MV waveguide. All four accelerators had percent depth dose curves for the 6MV beam that were matched within 1.3%. Beam spectra were determined from narrow beam transmission measurements through successive thicknesses of pure aluminum along the central axis of the accelerator, made with a graphite Farmer ion chamber with a Lucite buildup cap. An iterative nonlinear fit using a Marquardt algorithm was used to find each spectrum. Reconstructed spectra show that all four beams have similar energy distributions with only subtle differences, despite the differences in accelerator design. The measured spectra of different 6MV beams are similar regardless of accelerator design. The measured spectra show excellent agreement with those found by the auto‐modeling algorithm in a commercial 3D treatment planning system that uses a convolution dose calculation algorithm. Thus, beam spectra can be acquired in a clinical setting at the time of commissioning as a part of the routine beam data collection.