Scatter dose summation for irregular fields: speed and accuracy study.

Overview

Using program IRREG as a standard, we have compared speed and accuracy of several algorithms that calculate the scatter dose in an irregular field. All the algorithms, in some manner, decompose the irregular field into component triangles and obtain the scatter dose as the sum of the contributions from those triangles. Two of the algorithms replace each such component triangle with a sector of a certain "effective radius": in one case the average radius of the triangle, in the other the radius of the sector having the same area as the component triangle. A third algorithm decomposes each triangle further into two right triangles and utilizes the precalculated "equivalent radius" of each, to find the scatter contribution. For points near the center of a square field, all the methods compare favorably in accuracy to program IRREG, with less than a 1% error in total dose and with approximately a factor of 3-5 savings in computation time. Even for extreme rectangular fields (2 cm X 30 cm), the methods using the average radius and the equivalent right triangles agree to within 2% in total dose and approximately a factor of 3-4 savings in computation time.