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Intercomparison of normalized head‐scatter factor measurement techniques
Author(s) -
Frye Douglas M. D.,
Paliwal Bhudatt R.,
Thomadsen Bruce R.,
Jursinic Paul
Publication year - 1995
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 - 10.1118/1.597602
Subject(s) - imaging phantom , electron , head (geology) , photon , ionization , optics , coaxial , beam (structure) , materials science , physics , nuclear physics , ion , electrical engineering , engineering , quantum mechanics , geomorphology , geology
Normalized head‐scatter factors were measured with cylindrical beam coaxial miniphantoms and high purity graphite buildup caps for 4‐, 6‐, 10‐, and 24‐MV photon beams at field sizes from 4×4 to 40×40 cm 2 . The normalized head‐scatter factors determined by the two methods matched well for 4‐ and 6‐MV photon beams. The miniphantom technique produced normalized head‐scatter factors 1.5% and 4.8% lower than the buildup caps for the 10‐ and 24‐MV beams for large field sizes, respectively. At small field sizes, the miniphantom technique produced larger normalized head‐scatter factors than the buildup caps. Measurements made with an electromagnet indicate that a significant portion of the ionization measured in the buildup cap at 24 MV arises from contamination electrons. Measurements made with the miniphantom and magnet found no contamination electron contribution. The miniphantom technique may exclude such contamination electrons, potentially leading to inaccuracies in tissue‐maximum ratios and phantom scatter factors, as well as inaccuracies in monitor unit calculations.