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Surface dose measurements for highly oblique electron beams
Author(s) -
Ostwald P. M.,
Kron T.
Publication year - 1996
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 - 10.1118/1.597873
Subject(s) - collimated light , percentage depth dose curve , collimator , thermoluminescent dosimeter , materials science , dose profile , dosimeter , optics , dosimetry , imaging phantom , ionization chamber , beam (structure) , electron , perpendicular , atomic physics , nuclear medicine , radiation , ion , physics , nuclear physics , laser , ionization , medicine , geometry , mathematics , quantum mechanics
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.