flatness & symmetry

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6 Terms

1
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flatness

maximum difference from the CA dose over 80% field dimension

  • defined at depth of 10 cm

  • Max = (Ymax-Y0)/Y0)x100

  • Min= (Ymin-Y0)/(Y0)x100


2
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symmetry

difference in dose rate between any 2 points within 80% of the field dimension

  • points must be equidistant

  • =(Y1-Y2)/Y0×100


3
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flatness & symmetry QA

equipment: tissue equivalent, ion chambers

4
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constancy

average of two middle reading at CA

5
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MLC

primary component, 120 leaves (60 pairs)

  • tongue & groove effect

  • move on x jaws

  • leakage: <2%


6
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MLC QA

  • attenuation properties

  • positional precision

  • MLC setting vs radiation field

  • picket fence: checks for deviation of interlead transmission