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British Journal of Radiology (2006) 79, 347-352
© 2006 British Institute of Radiology
doi: 10.1259/bjr/54848398
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Study of scattered radiation for in-air calibration by a multiple-distance method using ionization chambers and an HDR 192Ir brachytherapy source

N P Patel, MSc, DipRP1, B Majumdar, MSc, PhD, PDC2 and V Vijayan, MSc, DipRP, PhD3

1 Department of Medical Physics, Acharya Harihar Regional Cancer Centre, Cuttack-753007, India, 2 Department of Physics, Government College of Science, Raipur-492010, India , 3 Health Physics Unit, Institute of Physics, Bhubaneswar-751005, India


Figure 1
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  		Figure 1. Measurement set-up shows the jig with a 0.1 cm3 chamber and brachytherapy unit.

 

Figure 2
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  		Figure 2. (a) Lateral view of measurement set-up. The chambers 0.6 cm3 and 0.1 cm3 are placed at positions A and B, respectively. The source position c is for the 0.6 cm3 and c' for the 0.1 cm3 chamber that means the heights of the effective points of chambers differed by 1.4 cm. (b) The offsets in positioning of the centre of chamber (c) and source (s). The total offset in the setup distance is given by {Delta} = –c+s.

 

Figure 3
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  		Figure 3. Plan view of the brachytherapy treatment room. Not to scale.

 

Figure 4
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  		Figure 4. The plot of normalizedf(d) values from measured air kerma (dots) and theoretically derived F(d) (lines) vs nominal distance d for the 0.6 cm3 chamber at position B. The measured f(d) curves were fitted by theoretical curves F(d) of corresponding offsets ({Delta}) with varying scattered components (Es). The offsets ({Delta}) in nominal distance were +0.3 cm, +0.1 cm, –0.1 cm, –0.3 cm, and –0.5 cm, respectively. The percentage value in brackets is the scatter component. The mean of scattered components was 0.59±0.06% at the centre of the room for the 0.6 cm3 chamber at position B in the jig.

 

Figure 5
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  		Figure 5. (a) Estimated scattered radiation as a function of distance from the wall in double and single wall scattering by multiple-distance measurement using the 0.6 cm3 chamber at position A inside the jig. (b) Deviation in measured relative ionization from the calculated value as a function of distance from wall for both the chambers (DW, double wall; SW, single wall). The measurement distances were 12 cm, 16 cm, and 18 cm. The calculated relative ionizations were determined from estimated scattered radiations using the 0.6 cm3 chamber.

 




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