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Comprehensive analysis of electron beam central axis dose for a radiotherapy linear accelerator
Author(s) -
Shiu Almon S.,
Tung Samuel S.,
Nyerick Carl E.,
Ochran Timothy G.,
Otte Victor A.,
Boyer Arthur L.,
Hogstrom Kenneth R.
Publication year - 1994
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.597313
Subject(s) - linear particle accelerator , radiation therapy , particle accelerator , beam (structure) , physics , cathode ray , dosimetry , electron , nuclear medicine , external beam radiotherapy , medical physics , nuclear physics , optics , medicine , radiology , brachytherapy
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, 10×10‐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, 6×6‐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.