1 45 Degree Wedge Factor Calculation Objective: To obtain the wedge transmission factor for a 45OUT dynamic wedge, for a 10x10 and 20x20 cm 2 field size using a 6 megavotage (MV) beam, and to show the effect of the wedge in a monitor unit calculation. Purpose: Dynamic wedges (EDW) are a commonly used beam modifier in radiation therapy. Wedges essentially attenuate the beam, decreasing intensity from the “toe” end to the “heel” end. 1 The isodose lines are tilted toward the thin “toe” end. Since a wedge is modifying the beam by attenuation, it must be taken into account for a treatment’s monitor unit’s calculation. 1 If a wedge is not taken into account, the prescribed dose will not be accurately met. Methods: The wedge transmission factor (WF) is defined as the ratio of dose with and without the wedge along the central axis at a specified depth. 1 WF= Dose with the wedge along the central axis at a depth Dose without the wedge along the central axis at a depth
14
Embed
zachstauch.weebly.com · Web view45 Degree Wedge Factor Calculation Objective: To obtain the wedge transmission factor for a 45OUT dynamic wedge, for a 10x10 and 20x20 cm2 field size
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
45 Degree Wedge Factor Calculation
Objective: To obtain the wedge transmission factor for a 45OUT dynamic wedge, for a 10x10
and 20x20 cm2 field size using a 6 megavotage (MV) beam, and to show the effect of the wedge
in a monitor unit calculation.
Purpose: Dynamic wedges (EDW) are a commonly used beam modifier in radiation therapy.
Wedges essentially attenuate the beam, decreasing intensity from the “toe” end to the “heel”
end.1 The isodose lines are tilted toward the thin “toe” end. Since a wedge is modifying the beam
by attenuation, it must be taken into account for a treatment’s monitor unit’s calculation.1 If a
wedge is not taken into account, the prescribed dose will not be accurately met.
Methods: The wedge transmission factor (WF) is defined as the ratio of dose with and without
the wedge along the central axis at a specified depth.1
WF= Dose with the wedge along the central axis at a depth
Dose without the wedge along the central axis at a depth
With the aid of a physics resident, the necessary data for the WF was obtained. An open air ion
chamber was placed in a water phantom. The water phantom was placed at 100 SSD and the ion
chamber was at a depth of 10 cm. Also, an energy of 6 MV was used. First, three trials were
conducted using 100 monitor units (MU) for a 10x10 cm2 field size without a wedge. The ion
chamber measured the total charge collected in nanoCoulombs (nC). Next, three trials were
performed for the same field size but with the 45OUT EDW. The same was repeated using a
20x20 cm2 field size, without and with the EDW. The values were recorded and the three trials
for each situation were averaged.
2
Figure 1. Water phantom with ion chamber used to find the WF.
Results:
Table 1. Three measurements for the 10x10 cm2 with and without the 45OUT EDW, and three measurements for the 20x20 cm2 field with and without the 45OUT EDW for a 6 MV beam.
Field Size
(cm)
Measurements without wedge (nC) Measurements with wedge (nC)
10x10 2.500 1.864
2.502 1.860
2.503 1.865
3
20x20 2.673 1.484
2.673 1.485
2.671 1.486
Table 2. Finding the average measurement of both fields with and without the 45OUT EDW for a 6 MV beam.
Field Size
(cm)
Measurements without wedge (nC) Measurements with wedge (nC)