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Influence of monitor chamber calibration on Virtual Wedge™ dosimetrya)
a)Presented in part at the 45th Annual Meeting of the Canadian Organization of Medical Physicists (COMP) in Saskatoon, Saskatchewan, Canada, 31 May–3 June 2006, Med. Phys. 33 (7), 2675–2676 (2006).
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10.1118/1.2710327
/content/aapm/journal/medphys/34/4/10.1118/1.2710327
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/34/4/10.1118/1.2710327

Figures

Image of FIG. 1.
FIG. 1.

Sample of data from our VW QA program for a 60° wedge and a field size at . During this 9 month period there was less than a 1% change in the dose on the central axis in an open (nonwedged) field (A), whereas the central axis dose with the wedge changed by approximately 3% (B). The doses at positions off axis (scaled to isocenter) are also shown. The low dose end of the wedge, labeled “heel” (C) showed a 5% increase in dose while the high dose end or “toe” of the wedge showed a small increase of less than 2%. The off axis positions at the toe and heel points of the wedge profile were inside the geometric field edge.

Image of FIG. 2.
FIG. 2.

Geometry of a VW beam. The moving jaw start position, , is equal to the static jaw position minus for . If , then and the initial gap will be larger than resulting in the profile at the high dose end of the wedge becoming distorted. After delivering a precalculated number of MUs the moving jaw retracts at constant speed, , while the dose rate is modulated as a function of moving jaw position, . Once the moving jaw reaches its final position, , the dose rate returns to its nominal value and the beam remains on until MONITOR 1 reaches its preset value.

Image of FIG. 3.
FIG. 3.

Experimental setup for the validation measurements. The black circles indicate ion chamber positions at central axis (CAX) and at the isocenter plane.

Image of FIG. 4.
FIG. 4.

A comparison of measured and calculated change in dose along the wedged profile as a function of cumulative dose per MU, , on MONITOR 1. The MONITOR 2 calibration was held constant. The data are for a 60° wedge. The high dose (toe) and low dose (heel) measurements were off the central axis. The data points show the average change in dose, with error bars representing . The standard deviation for the CAX and toe points was within the size of the data points. The lines correspond to calculations using Eq. (9).

Image of FIG. 5.
FIG. 5.

A comparison of measured and calculated change in dose along the wedged profile as a function of dose rate calibration, , on MONITOR 2. The MONITOR 1 calibration was held constant. The data are for a 60° wedge. The high dose (toe) and low dose (heel) measurements were off the central axis. The data points show the average change in dose, with error bars representing . The standard deviation for the CAX and toe points was within the size of the data points. The lines correspond to calculations using Eq. (10).

Image of FIG. 6.
FIG. 6.

Calculated fractional change in dose for a 3% reduction in MONITOR 2 calibration vs field size in the wedge direction. The heel point was selected to be inside the geometric field edge. The parameters in the calculation were , , , , .

Tables

Generic image for table
TABLE I.

A comparison of measured and calculated change in dose along the wedged profile when the MONITOR 1 and MONITOR 2 calibrations changed by the same amount, . The data are for a 60° wedge. Measurements reported are the average and standard deviation of five iterations.

Generic image for table
TABLE II.

The energy dependence of the fractional change in VW dose for a 3% reduction in the calibration factor of MONITOR 2. The data are calculated for a 60° wedge with a field size of . The constants used in the calculations were , , . The values of were taken from the Siemens defaults (see Ref. 1).

Generic image for table
TABLE III.

The wedge angle dependence of the fractional change in VW dose for a 3% reduction in the calibration factor of MONITOR 2. The data are calculated for a beam with a field size of . The constants used in the calculations were , , , .

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2007-03-13
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Influence of monitor chamber calibration on Virtual Wedge™ dosimetrya)
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/34/4/10.1118/1.2710327
10.1118/1.2710327
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