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Technical Note: On Cerrobend shielding for 18 – 22 MeV electron beams
Author(s) -
Wojcicka Jadwiga B.,
Yankelevich Rafael,
Werner Barry Leonard,
Lasher Donette E.
Publication year - 2008
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 - 10.1118/1.2977801
Subject(s) - optics , ionization chamber , beam (structure) , physics , ionization , electromagnetic shielding , cathode ray , percentage depth dose curve , electron , materials science , atomic physics , nuclear physics , ion , quantum mechanics
The purpose of this study is to investigate (1) the depth at which the measurement of the block transmission factor should be made, and (2) the level of the transmission of 18 and 22 MeV electron beams through conventional Cerrobend. We measured the block transmission in water phantom as ionization profiles across the beam and as ionization distributions along the central axis of the beam for 18 and 22 MeV electron beams, for cone sizes ranging from 6 × 10cm 2to 25 × 25cm 2 . In our analysis, we separated the bremsstrahlung component produced in the Cerrobend block from the component originating in the head in the transmitted dose under the standard Cerrobend block. The block transmission for both beam energies and cone sizes was maximum on the central axis of the beam at depths between 0.4 and 0.7 cm . For the 18 MeV beam, the maximum transmission was 6.2% for the 6 × 10cm 2cone, and 7.4% for the 25 × 25cm 2cone. For the 22 MeV beam, it was 9.5% for the 6 × 10cm 2cone, and 11.3% for the 25 × 25cm 2cone. For the 22 MeV beam and 15 × 15cm 2cone, it takes 2.95 and 1.4 cm of Cerrobend to reduce the maximum block transmission to 5% and 10%, respectively. The maximum dose under a blocked electron beam occurs on the central axis closer to the surface than it does for the open beam, and the block transmission factor should be defined at this shallower depth. To decrease the block transmission factor to the level of 5% on the central axis, electron beams with energy 18 MeV and greater require additional shielding.