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The response of prototype plane-parallel ionization chambers in small megavoltage x-ray fields

Journal Article


Abstract


  • Accurate small-field dosimetry has become important with the use of multiple small fields in modern radiotherapy treatments such as IMRT and stereotactic radiosurgery. In this study, we investigate the response of a set of prototype plane-parallel ionization chambers, based upon the Exradin T11 chamber, with active volume diameters of 2, 4, 10, and 20 mm, exposed to 6 MV stereotactic radiotherapy x-ray fields. Our goal was to assess their usefulness for accurate small x-ray field dose measurements. The relative ionization response was measured in circular fields (0.5 to 4 cm diameter) as compared to a 10×10 cm2 reference field. A large discrepancy (∼40%) was found between the relative response in the smallest plane-parallel chamber and other small volume dosimeters (radiochromic film, micro-metal-oxide- semiconductor field-effect transistor and diode) used for comparison. Monte Carlo BEAMnrc simulations were used to simulate the experimental setup in order to investigate the cause of the under-response and to calculate appropriate correction factors that could be applied to experimental measurements. It was found that in small fields, the air cavity of these custom-made research chambers perturbed the secondary electron fluence profile significantly, resulting in decreased fluence within the active volume, which in turn produces a chamber under-response. It is demonstrated that a large correction to the pfl correction factor would be required to improve dosimetric accuracy in small fields, and that these factors could be derived using Monte Carlo simulations. © 2006 American Association of Physicists in Medicine.

UOW Authors


  •   Kron, Tomas (external author)

Publication Date


  • 2006

Citation


  • McNiven, A. L., Mulligan, M., Kron, T., & Battista, J. J. (2006). The response of prototype plane-parallel ionization chambers in small megavoltage x-ray fields. Medical Physics, 33(11), 3997-4004. doi:10.1118/1.2356650

Scopus Eid


  • 2-s2.0-33750560217

Start Page


  • 3997

End Page


  • 4004

Volume


  • 33

Issue


  • 11

Abstract


  • Accurate small-field dosimetry has become important with the use of multiple small fields in modern radiotherapy treatments such as IMRT and stereotactic radiosurgery. In this study, we investigate the response of a set of prototype plane-parallel ionization chambers, based upon the Exradin T11 chamber, with active volume diameters of 2, 4, 10, and 20 mm, exposed to 6 MV stereotactic radiotherapy x-ray fields. Our goal was to assess their usefulness for accurate small x-ray field dose measurements. The relative ionization response was measured in circular fields (0.5 to 4 cm diameter) as compared to a 10×10 cm2 reference field. A large discrepancy (∼40%) was found between the relative response in the smallest plane-parallel chamber and other small volume dosimeters (radiochromic film, micro-metal-oxide- semiconductor field-effect transistor and diode) used for comparison. Monte Carlo BEAMnrc simulations were used to simulate the experimental setup in order to investigate the cause of the under-response and to calculate appropriate correction factors that could be applied to experimental measurements. It was found that in small fields, the air cavity of these custom-made research chambers perturbed the secondary electron fluence profile significantly, resulting in decreased fluence within the active volume, which in turn produces a chamber under-response. It is demonstrated that a large correction to the pfl correction factor would be required to improve dosimetric accuracy in small fields, and that these factors could be derived using Monte Carlo simulations. © 2006 American Association of Physicists in Medicine.

UOW Authors


  •   Kron, Tomas (external author)

Publication Date


  • 2006

Citation


  • McNiven, A. L., Mulligan, M., Kron, T., & Battista, J. J. (2006). The response of prototype plane-parallel ionization chambers in small megavoltage x-ray fields. Medical Physics, 33(11), 3997-4004. doi:10.1118/1.2356650

Scopus Eid


  • 2-s2.0-33750560217

Start Page


  • 3997

End Page


  • 4004

Volume


  • 33

Issue


  • 11