Broad beam attenuation of kilovoltage photon beams: effect of ion chambers

The British Journal of Radiology
I J Das

Abstract

In kilovoltage X-ray treatment, beam shaping and shielding normal tissue are accomplished by thin sheets of lead cutout, the thickness of which is selected based upon either published data or measurements. Available broad beam attenuation (BBA) data are found to be unsatisfactory and are the subject of this investigation. BBA is defined as the ratio of intensity with (I) and without (I0) attenuating medium for a large field in a phantom. BBA = I(x,t,E)/I0(x,0,E), where x is the depth of measurement, t is the thickness of attenuator, and E is the beam energy. The depth x should be zero for kilovoltage beams and dmax for megavoltage beams. Unfortunately, x is limited by the window thickness which is the core of this study. A Farmer-type cylindrical ion chamber and three parallel plate ion chambers (Capintec, PS-033; Markus; and Holt) were used to measure BBA for kilovoltage beams from a Siemens Stabilipan unit. Results indicate that attenuation is strongly dependent on the window thickness. For the 240 kVp beam, the thickness of lead for 5% and 1% transmissions are 3.1 mm, and 5.2 mm, respectively, with the Capintec chamber. The corresponding values of lead thickness for the Markus chamber are 2.3 mm and 4.0 mm; for the Holt cham...Continue Reading

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Citations

Oct 2, 2008·Zeitschrift für medizinische Physik·Syed F AkberThan S Kehwar
Jul 11, 2003·Medical Physics·Mohammad BahmaidJatinder R Palta
Mar 4, 2014·Physics in Medicine and Biology·Robin HillClive Baldock
Feb 8, 2000·Physics in Medicine and Biology·C H Lee, K K Chan

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