Abstract
Some modern treatment‐planning systems (TPSs) provide for input of wedge factor (WF) tables covering the entire range of square and elongated fields available on the LINAC. Depending on the field size increment chosen and the number of available wedge orientations, one may have to take more than 100 measurements per wedge and photon energy to commission the TPS. To expedite TPS commissioning while maintaining high accuracy, we demonstrate a simple method that requires only a few measurements per wedge, from which the remaining wedge factors can be found through linear interpolation based on field area. For the externally mounted wedges of two common LINACs, we have shown that WFs are proportional to field area and are nearly independent of field elongation and wedge orientation. Wedge factors computed from five to seven measurements comprised of square fields and a single, large rectangular field agreed with direct measurements throughout the entire range of achievable field dimensions within 0.6% at 6 MV and within 1% at 15 MV. Making the same set of measurements and using the equivalent square method to find WFs at other field sizes leads to errors up to 2%. Measuring the WF for a 10×10cm2 field and applying the same value to all field sizes can lead to errors of up to 10% at both 6 MV and 15 MV.PACS numbers: 87.53.Bn, 87.53.Mr
Highlights
A common method for finding radiation doses in wedged fields consists of computing the dose at some reference point along the central beam axis and using off-axis factors and tissue maximum ratios or percentage depth doses to determine the radiation intensity at other points in the field
Orientation The effect of orientation was examined by taking the ratio of the I/O wedge factor to the L/R wedge factor at each field size for the data obtained using the 21EX
Perfect linearity would exist if the wedge factor (WF) were proportional to scatter from the wedge, wedge scatter were proportional to field area, and photon scatter from the wedge originated as if from an isotropic point source located on the central axis
Summary
A common method for finding radiation doses in wedged fields consists of computing the dose at some reference point along the central beam axis and using off-axis factors and tissue maximum ratios or percentage depth doses to determine the radiation intensity at other points in the field. In the early years of treatment planning, the dose at the central ray reference point of a wedged field was found by calculating the dose at the same point in an open field and multiplying it by the wedge factor (WF). Later models of treatment-planning systems (TPSs) allowed input of wedge factors for a series of square fields, and accepted separate depth dose or tissue maximum ratio data to take beam hardening by the wedge into account. 1824 6th Avenue South, Birmingham, AL 35233 U.S.A. [205] 934-1762
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