Measured vs. calculated off-axis wedged dose distributions

Wedge filters are frequently used in the clinic to improve target dose homogeneity. All physical wedges have a uniform thickness along the non-wedged direction. Therefore wedged field profiles show a "sagging effect" in this direction. Most treatment planning systems (TPS) assume an open field profile when calculating wedged dose distributions in the off-axis plane. Although this effect has been discussed in literature, a detailed analysis of calculated and measured dose distributions was not reported. Wedged dose distributions in the non-wedged direction were systematically measured for VARIAN 600CD 6MV photons for all available wedge angles and a range of off-axis distances. Measured results were compared against dose distributions calculated by 3d FOCUS TPS. It was found that, depending on the field size, wedge angle and off-axis distance, the difference between measured and planned calculated doses may exceed 10%. In planes close to the central axis, the TPS overestimated the delivered dose, whereas in the distant off-axis planes the delivered dose was underestimated. This may lead to an appreciable underdosing and/or overdosing in the peripheral regions of the PTV. A three dimensional "wedge compensator" (3DWC) was designed for each wedge angle to be used in conjunction with the wedge filters. This 3DWC was used for dose distribution calculations only. The shape of this "wedge compensator" was determined by analyzing differences between measured and calculated dose distributions. Wedged dose distributions were recalculated using 3DWC. Excellent agreement between calculated and measured dose distributions was achieved.