title: Comprehensive analysis of electron beam central axis dose for a radiotherapy linear accelerator.
abstract: This work evaluates the application of AAPM task group 25 (TG25) methodology for determination of central axis depth dose for a radiotherapy linear accelerator, whose dual scattering foil system and applicators were recently modified. The percent depth dose (%DD) and the dose output factor have been measured for square and rectangular fields at 100- and 110-cm source-to-surface distance (SSDs). At 100-cm SSD, results showed that %DD for a specific energy and field size can vary with applicator, the largest variation being for the 20-MeV, 10 x 10-cm field where a spread of +/- 2.5% or +/- 3 mm about the mean %DD is observed. The square-root method determines rectangular field %DD within 1%. Output factors for rectangular fields are calculated from square field values more accurately using a square-root method than the equivalent-square method recommended by TG25. At 110-cm SSD, the %DD calculated from that at 100-cm SSD using an inverse square factor does not agree with measured values for all fields. The maximum difference observed for the 20-MeV, 6 x 6-cm field was 5.5% or 10 mm. Output data at the 110-cm SSD show that the square-root method is suitable for determination of the air-gap correction factors of rectangular fields. In summary, the recommendations of TG25 work reasonably well for central axis electron beam dosimetry for this version of a radiotherapy linear accelerator, except in limited cases where applicator-scattered electrons apparently cause minor but clinically significant discrepancies.
authors.affiliation: Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston 77030.
authors.forename: A S
authors.initials: AS
authors.lastname: Shiu
authors.forename: S S
authors.initials: SS
authors.lastname: Tung
authors.forename: C E
authors.initials: CE
authors.lastname: Nyerick
authors.forename: T G
authors.initials: TG
authors.lastname: Ochran
authors.forename: V A
authors.initials: VA
authors.lastname: Otte
authors.forename: A L
authors.initials: AL
authors.lastname: Boyer
authors.forename: K R
authors.initials: KR
authors.lastname: Hogstrom
mesh_terms: D055592:Biophysical Phenomena; D001703:Biophysics; D004583:Electrons; D005069:Evaluation Studies as Topic; D006801:Humans; D010315:Particle Accelerators; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011882:Radiotherapy, High-Energy / Q000379:methods