Flatness and Symmetry

flatness

•maximum difference from the CA dose over 80% field dimension

•defined at a depth of 10 cm

•Max= (Ymax-Yo)/(Yo)x100%

•Min= (Ymin-Yo)/(Yo)x100%

symmetry

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

•points must be equidistant

=(Y1-Y2)/Yo x 100%

flatness and symmetry qa

•equipment: tissue equivalent, ion chambers

constancy

•average of two middle readings at CA

multileaf collimation

•primary component

•120 leaves typically (60 pairs)

•tongue and groove effect

•move on x jaws

•leakage:

MLC QA

•attenuation properties

•positional precision

•MLC setting vs radiation field

•picket fence: checks for deviation of interleaf transmission