AbstractID: 7124 Title: Determination of Complete Orthogonal Wedge Monitor Factor Tables From Single
Orientation Measurements
The target accuracy of delivered dose is ±5% of planned dose. Accurate dose delivery is important for
biological reasons, as a deviation of delivered dose from planned dose can result in poor outcome, either
failure to eliminate the tumor or normal tissue complications. For wedged fields, the typical method for
calculation of monitor unit setting is to use the open field monitor factor multiplied by the wedge factor.
However, the open field monitor factor can deviate from the wedge monitor factor by up to 5%.
Measurement of complete monitor factor tables can be time consuming, requiring 2-4 hours per wedge.
Furthermore, the time required is doubled by the necessity to measure each wedge in orthogonal
orientations. We present a method for calculation of the wedge monitor factor from the monitor factor for
the wedge in the orthogonal orientation and the open field monitor factor. Because the phantom scatter for
a wedged field of dimension X×Y is the same as that for a field of dimension Y×X with the wedge in the
orthogonal orientation, the difference between the monitor factors in the two geometries is due to
collimator scatter. The ratio of the monitor factors in the two geometries is the ratio of the collimator
scatter for the two jaw settings, which is given by the ratio of the open field monitor factors. Agreement
between calculation and measurement is typically less than 0.5% for a variety of wedge angles and field
sizes This approach reduces the time required to obtain complete monitor factor tables for wedges by a
factor of two.