Dose calculation along the nonwedged direction for externally wedged beams: improvement of dosimetric accuracy with comparatively moderate effort.

Wedge filters ideally modify photon intensities only in one direction. However, in the other, the "nonwedged" direction, the intensity is affected too; it usually decreases with increasing off-axis distance. For external wedges on a particular treatment machine (Varian Clinac 2100C) and 6 MV photons, for example, this decrease is as big as 8%, depending on wedge angle and material, off-axis distance, and phantom depth. We present a way to account for this effect in prescriptions to points off-center in the nonwedged direction. The goal was to minimize the amount of additional data required for this purpose, without unduly compromising the final prescription accuracy. We measure the effective attenuation coefficients in narrow beam geometry for the wedge materials (lead and steel) as a function of thickness and off-axis angle, and the corresponding attenuation in water, again as a function of wedge material thickness and off-axis angle. The data allow us to extract a correction factor for off-axis distances in the nonwedged direction. Neglecting the contribution of scattered radiation and using primary beam data only, shortens data acquisition and simplifies calculations, but still yields surprisingly accurate results. Application of the derived correction reduces the off-axis distance related dose calculation error in wedged fields to < or =1%.