Scatter dose decrement values for rectangular fields

The scatter dose decrement value (SDV) has been studied by calculational methods for 60Co gamma and 4- and 8-MV x-ray beams of rectangular cross section. In a principal plane and inside the field, the SDV is nearly independent of field size, energy, and depth when the position of the point considered is expressed as the fractional distance to the field edge. From this, it follows that the scatter dose profiles are quite similar in planes parallel to a principal plane. A second corollary is that the SDV for an arbitrary point approximately equals the product of the two SDV values for the projected points in the principal planes. Outside the field, the SDV loses this independence of the various beam conditions, and as a consequence thereof, the "product rule" leads to errors. Mathematical expressions have been derived for the SDV inside and outside small fields. The errors resulting when these small-field formulas are used for large fields have been studied. Expressed as fractions of the total dose on the centerline, these errors are of the order of +/- 1% inside the field and +/- 2% outside.