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Small‐beam calibration by 0.6‐ and 0.2‐cm 3 ionization chambers
Author(s) -
Engler Mark J.,
Jones Gary L.
Publication year - 1984
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 - 10.1118/1.595570
Subject(s) - ionization chamber , beam (structure) , dosimetry , flatness (cosmology) , ionization , laser beam quality , detector , electron , fluence , photon , optics , cathode ray , irradiation , physics , radiation , calibration , materials science , atomic physics , nuclear medicine , nuclear physics , laser beams , ion , laser , medicine , cosmology , quantum mechanics
Small beams are often applied in radiotherapy, e.g., in shrinking field and high‐dose techniques with curative intent. For a given beam, measured field size factors (FSF) may vary with responses of different detectors to nonuniform radiation fluence. Dose rates of small photon and electron beams with diverse profiles were measured in polystyrene with 0.6‐ and 0.2‐cm 3 Farmer ionization chambers. FSF of 6 0 Co, 4‐, 8‐, and 16‐MV photon beams, and of 6‐, 12‐, and 20‐MeV electron beams, were determined as ratios of dose rates of beams with differing field dimensions to dose rates of 10×10 cm beams. 6 0 Co and 4‐MV photon beam FSF were also determined in air using acrylic buildup caps. FSF obtained via 0.6‐ and 0.2‐cm 3 chambers are compared as functions of beam flatness and quality. It is shown that notable discrepancies can exist between FSF for the same field obtained with different detectors, even when chamber volumes are well within nominal beam dimensions. Possible dose delivery errors arising from use of the 0.6‐cm 3 chamber were found to be from 0% to 2% for 4×4 to 5×5 cm photon beams, and from −1% to 5% for electron beams 4 cm in diameter. Possible errors >5% were noted for most beams <3.5×3.5 cm. Consequently, it is recommended that detectors smaller than the Farmer 0.6‐cm 3 chamber be employed in determining FSF of beams ≤5×5 cm. For facilities where smaller detectors are unavailable, beam profile averaging and extrapolation of rectangular FSF are examined as corrective techniques for calibration of small beams by a relatively large detector.