Characteristics of secondary electrons produced by 6, 10 and 24 MV x-ray beams.
Megavoltage x-ray beams generated by linear accelerators (linacs) deliver their maximum dose a few centimetres below the treatment or phantom surface. This skin-sparing effect is degraded by the generation of secondary electrons as the x-ray beam passes to the patient or phantom. This work measures the characteristics of these electrons. A light-weight electromagnet was constructed that could be mounted in the block-tray position, 58 cm from the x-ray source of a Varian Clinac 2100C or 2500 linac. A field strength as high as 0.1500 T was generated, which was strong enough to sweep secondary electrons out of a 10 cm x 10 cm field. For 6, 10 and 24 MV x-ray beams, secondary contamination electrons produced 18, 38 and 65% of the surface dose, corresponding to 3, 5 and 12% of the maximum dose, respectively. A parameterized depth-dose curve for the contamination electrons was produced and was valid for all the x-ray energies studied.
Jursinic
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PA
Medical Physics Department, University of Wisconsin, Madison 53706, USA.
Mackie
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TR
D055592:Biophysical Phenomena; D001703:Biophysics; D055590:Electromagnetic Phenomena; D004583:Electrons / Q000627:therapeutic use*; D006801:Humans; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy