Equivalent square as a predictor of depth of dose maximum for megavoltage therapy beams.

The depths of dose maxima, dmax, and surface doses of 6 MV, 10 MV, and 18 MV photon beams were measured for various square fields and rectangular fields with elongation ratios from 1 to 27. Rectangular fields with elongation ratios below 2 have essentially the same depths of dose maxima and surface doses as their corresponding equivalent square fields. For rectangular fields with elongation ratios above 2, the surface doses increase, and depths of dose maxima decrease with increasing elongation ratio in comparison to their respective values for their corresponding equivalent square fields. The shift of dmax toward the surface is more pronounced when the upper jaws rather than the lower jaws define the long axis of the field. This collimator exchange effect does not influence the surface dose. Even for the largest elongation ratios, the changes in dmax and surface dose from their equivalent square field values were minor and clinically insignificant, suggesting that the equivalent square approach provides a reliable method for predicting the values of dmax and surface dose for rectangular fields from square fields data.