Technical Note: On Cerrobend shielding for 18–22MeV electron beams

• Published in: Medical physics (Lancaster) 2008-10, Vol.35 (10), p.4625-4629
• Main Authors: Wojcicka, Jadwiga B., Yankelevich, Rafael, Werner, Barry Leonard, Lasher, Donette E.
• Format: Article
• Language: English
• Subjects: Ancillary equipment; block transmission; Cerrobend; Data analysis; dosimetry; Dosimetry/exposure assessment; Electrodeposition; Electron and positron beams; electron beams; electron shielding; Ionization; Ionization chambers; Lipowitz's metal; metals; phantoms; Photons; Physicists; Radiation monitoring, control, and safety; radiation protection; radiation therapy; Transmission measurement
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.2977801

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 22MeV 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 22MeV electron beams, for cone sizes ranging from 6×10cm2to25×25cm2. 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 0.7cm. For the 18MeV beam, the maximum transmission was 6.2% for the 6×10cm2 cone, and 7.4% for the 25×25cm2 cone. For the 22MeV beam, it was 9.5% for the 6×10cm2 cone, and 11.3% for the 25×25cm2 cone. For the 22MeV beam and 15×15cm2 cone, it takes 2.95 and 1.4cm 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 18MeV and greater require additional shielding.