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Dosimetric changes induced by positional uncertaintyof cutout in electron radiotherapy

Published online by Cambridge University Press:  01 September 2008

James C.L. Chow ,
Grigor N. Grigorov  and
Kathryn Ross
James C.L. Chow*
Affiliation:
Radiation Medicine Program, Princess Margaret Hospital and Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada
Grigor N. Grigorov
Affiliation:
Medical Physics Department, Grand River Regional Cancer Center, Kitchener, Ontario, Canada
Kathryn Ross
Affiliation:
Medical Physics Department, Grand River Regional Cancer Center, Kitchener, Ontario, Canada
*
Correspondence to: James C.L. Chow. E-mail: James.Chow@rmp.uhn.on.ca
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Abstract

Dosimetric changes caused by the positional uncertainty of centring a small electron cutout to the machine central beam axis (CAX) of the linear accelerator (linac) were investigated. First, six circular cutouts with 4 cm diameter were made with their centres shifted off from the machine CAX for 0, 2, 4, 6, 8 and 10 mm using the 6 × 6 cm2 applicator. Then, the percentage depth doses (PDDs) at the machine CAX and cutout centre were measured using the 4, 9 and 16 MeV clinical electron beams produced by a Varian 21 EX linac. The cross- and in-line axis beam profiles were measured at depth of maximum dose (dm) and source-to-surface distance equal to 100 cm using a scanning water tank system and diode detector. When the cutout centre was shifted away from machine CAX for the electron beam with low energy of 4 MeV, the dm, depths of the 80 (R80) and 90% (R90) depth dose at the machine CAX had no significant change (<0.1 mm). For higher energies of 9 and 16 MeV beams, the dm were reduced with 0.45 and 1.63 mm per mm off-axis shift between the cutout centre and the machine CAX, respectively. The R80 and R90 were reduced with 0.7 mm per mm off-axis shift for both energies. When there was a 4 mm off-axis shift, the relative output factors for the 4, 9 and 16 MeV beams were reduced with 0.8, 1.6 and 0.5%, respectively. The isodose coverage of the in-line axis beam profile was reduced when the cutout centre was shifted away from machine CAX. It is important for radiation oncologists, dosimetrists, therapists and physicists to note such dosimetric changes in the electron radiotherapy to the patient, because such positional uncertainty is unavoidable in fabricating an electron cutout in the mould room.

Keywords

Electron cutoutelectron dosimetryelectron radiotherapypositional uncertainty
Type
Original Article
Information
Journal of Radiotherapy in Practice , Volume 7 , Issue 3 , September 2008 , pp. 133 - 140
Copyright
Copyright © Cambridge University Press 2008

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