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Use of electron filters to improve the buildup characteristics of large fields from Cobalt‐60 beams
Author(s) -
Leung Philip M. K.,
Johns H. E.
Publication year - 1977
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 - 10.1118/1.594314
Subject(s) - collimator , dosimetry , filter (signal processing) , cobalt 60 , materials science , optics , contamination , electron , cobalt , physics , nuclear medicine , irradiation , nuclear physics , electrical engineering , medicine , engineering , ecology , metallurgy , biology
It has been shown that for Cobalt‐60 units capable of producing large fields (up to 40×40 cm 2 ) at 80 cm, the dose distribution in the buildup region is quite different from that observed with small fields. First, the maximum dose occurs at 1–2 mm instead of 5 mm; and second, the maximum dose in some extreme cases is 15%–20% higher than the dose at 5 mm. Thus, the dose is decreased by more than 15%–20% by 3 mm of tissue, suggesting severe electron contamination. In this report, it is shown that this contamination can be best removed by a filter of thickness 0.4 g/cm 2 of medium atomic number, placed some 25 cm from the source inside the collimator. For existing units, positioning the filter inside the collimator is not practical. Acceptable results, however, can be obtained by placing the filter just below the collimator.