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Dosimetric changes induced by positional uncertainty of 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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References

Chow, JCL, Newman, S.Experimental verification of the application of lateral build-up ratio on the 4 MeV electron beam. J Appl Clin Med Phys 2006; 7:35–41.Google Scholar
Chen, JZ, VanDyk, J, Lewis, C, Battista, JJ.A two-source model for electron beams: calculation of relative output factors. Med Phys 2001; 28:1735–1745.CrossRefGoogle ScholarPubMed
Chow, JCL, Grigorov, GN, MacGregor, C.A graphical user interface for an electron monitor unit calculator using a sector-integration algorithm and exponential curve fitting method. J Appl Clin Med Phys 2006; 7:52–64.Google ScholarPubMed
Hogstrom, KR, Mills, D, Almond, PR.Electron beam dose calculations. Phys Med Biol 1981;26:445–459.CrossRefGoogle ScholarPubMed
Brahme, A, Lax, I, Andreo, P.Electron beam dose planning using discrete Gaussian beams:mathematical background. Acta Radiol Oncol 1981; 20:147–158.CrossRefGoogle ScholarPubMed
Glegg, MM.Electron dose calculations: a comparison of two commercial treatment planning computers. Med Dosim 2002; 28:99–105.CrossRefGoogle Scholar
Rustgi, SN, Working, KR.Dosimetry of small field electron beams. Med Dosim 1992; 17:107–110.CrossRefGoogle ScholarPubMed
Zhang, GG, Rogers, DW, Cygler, JE, Mackie, TR.Monte Carlo investigation of electron beam output factors versus size of square cutout. Med Phys 1999; 26:743–750.CrossRefGoogle ScholarPubMed
Boyd, RA, Hogstrom, KR, White, RA, Starkschall, G.Modeling pencil-beam divergence with the electron pencil-beam redefinition algorithm. Phys Med Biol 2001; 46:2841–2856.CrossRefGoogle ScholarPubMed
Radiation Dosimetry: Electron beams with energies between 1 and 50 MeV, ICRU Report No. 35:91–92. Bethesda, MD: International Commission on Radiation Units and Measurement, 1984.Google Scholar
Ding, GX, Yu, CW.Determination of percentage depth dose curves for electron beams using different types of detectors. Med Phys 2001; 28:298–302.CrossRefGoogle ScholarPubMed
Sharma, AK, Supe, SS, Anantha, N, Subbarangaiah, K.Physical characteristics of photon and electron beams from a dual energy linear accelerator. Med Dosim 1995; 20:55–66.CrossRefGoogle ScholarPubMed
Almond, PR, Biggs, PJ, Coursey, BM, Hanson, WF, Huq, MS, Nath, R, Rogers, DW.AAPM’s TG-51 protocol for clinical reference dosimetry of high energy photon and electron beams. Med Phys 1999; 26:1847–1870.CrossRefGoogle ScholarPubMed
Cho, SH, Lowenstein, JR, Balter, PA, Wells, NH, Hanson, WF.Comparison between TG-51 and TG-21: calibration of photon and electron beams in water using cylindrical chambers. J Appl Clin Med Phys 2000; 1:108–115.Google ScholarPubMed
Brahme, A et al. Accuracy requirements and quality assurance of external beam therapy with photons and electrons. Acta Oncol 1988: 1–76(Suppl 1).Google Scholar
Van Dyk, J, Barnett, RB, Cygler, JE, Shragge, PC.Commissioning and quality assurance of treatment planning computers. Int J Rad Oncol Biol Phys 1992; 26:261–273.Google Scholar