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Surface dose measurements for highly oblique electron beams

Journal Article


Abstract


  • Clinical applications of electrons may involve oblique incidence of beams, and although dose variations for angles up to 60° from normal incidence are well documented, no results are available for highly oblique beams. Surface dose measurements in highly oblique beams were made using parallel-plate ion chambers and both standard LiF:Mg, Ti and carbon-loaded LiF Thermoluminescent Dosimeters (TLD). Obliquity factors (OBF) or surface dose at an oblique angle divided by the surface dose at perpendicular incidence, were obtained for electron energies between 4 and 20 MeV. Measurements were performed on a flat solid water phantom without a collimator at 100 cm SSD. Comparisons were also made to collimated beams. The OBFs of surface doses plotted against the angle of incidence increased to a maximum dose followed by a rapid dropoff in dose. The increase in OBF was more rapid for higher energies. The maximum OBF occurred at larger angles for higher-energy beams and ranged from 73° for 4 MeV to 84° for 20 MeV. At the dose maximum, OBFs were between 130% and 160% of direct beam doses, yielding surface doses of up to 150% of D(max) for the 20 MeV beam. At 2 mm depth the dose ratio was found to increase initially with angle and then decrease as D(max) moved closer to the surface. A higher maximum dose was measured at 2 mm depth than at the surface. A comparison of ion chamber types showed that a chamber with a small electrode spacing and large guard ring is required for oblique dose measurement. A semiempirical equation was used to model the dose increase at the surface with different energy electron beams.

UOW Authors


  •   Kron, Tomas (external author)

Publication Date


  • 1996

Citation


  • Ostwald, P. M., & Kron, T. (1996). Surface dose measurements for highly oblique electron beams. Medical Physics, 23(8), 1413-1420. doi:10.1118/1.597873

Scopus Eid


  • 2-s2.0-0029789530

Start Page


  • 1413

End Page


  • 1420

Volume


  • 23

Issue


  • 8

Abstract


  • Clinical applications of electrons may involve oblique incidence of beams, and although dose variations for angles up to 60° from normal incidence are well documented, no results are available for highly oblique beams. Surface dose measurements in highly oblique beams were made using parallel-plate ion chambers and both standard LiF:Mg, Ti and carbon-loaded LiF Thermoluminescent Dosimeters (TLD). Obliquity factors (OBF) or surface dose at an oblique angle divided by the surface dose at perpendicular incidence, were obtained for electron energies between 4 and 20 MeV. Measurements were performed on a flat solid water phantom without a collimator at 100 cm SSD. Comparisons were also made to collimated beams. The OBFs of surface doses plotted against the angle of incidence increased to a maximum dose followed by a rapid dropoff in dose. The increase in OBF was more rapid for higher energies. The maximum OBF occurred at larger angles for higher-energy beams and ranged from 73° for 4 MeV to 84° for 20 MeV. At the dose maximum, OBFs were between 130% and 160% of direct beam doses, yielding surface doses of up to 150% of D(max) for the 20 MeV beam. At 2 mm depth the dose ratio was found to increase initially with angle and then decrease as D(max) moved closer to the surface. A higher maximum dose was measured at 2 mm depth than at the surface. A comparison of ion chamber types showed that a chamber with a small electrode spacing and large guard ring is required for oblique dose measurement. A semiempirical equation was used to model the dose increase at the surface with different energy electron beams.

UOW Authors


  •   Kron, Tomas (external author)

Publication Date


  • 1996

Citation


  • Ostwald, P. M., & Kron, T. (1996). Surface dose measurements for highly oblique electron beams. Medical Physics, 23(8), 1413-1420. doi:10.1118/1.597873

Scopus Eid


  • 2-s2.0-0029789530

Start Page


  • 1413

End Page


  • 1420

Volume


  • 23

Issue


  • 8