PARAMETRIZATION OF HEAD-SCATTER FACTORS FOR RECTANGULAR PHOTON FIELDSUSING AN EQUIVALENT SQUARE FORMALISM

Head-scatter factors of symmetric square and rectangular fields (field center on the central beam axis) defined by the upper (Y) and lower ( X) jaws for 6 and 15 MV photon beams from 2300CD and 600C accelerators (Varian Associates Inc., Pale Alto, CA) were measured, as well as tho se for fields shaped by the Y jaws and the multileaf collimator (MLC) of the 2300CD. For rectangular fields, the head-scatter factor for the field (x=a and y=b) was different from that for the field (y=a and x= b). This difference was 2%-3% for fields defined by conventional colli mators when \x-y\ was large, and became 4%-5% when the MLC and Y jaws were used to shape the fields with the X jaws retracted. In-order to c alculate values for head-scatter factors of rectangular fields accurat ely using an equivalent square formalism, the side of the equivalent s quare should be obtained with different weights for lower and upper ja ws, as proposed by Vadash and Bjarngard [Med. Phys. 20, 733-734 (1993) ]. Our measurements demonstrate that the relative weight (C) of upper and lower jaws is strongly dependent on their distances from the x-ray source, while the beam energy has little effect on the value of G. We further show that G can be calculated simply from these distances. An analytical representation for head-scatter factors of square and rect angular fields is also developed in this paper. The quality of this re presentation was judged by the root-mean-square (rms) deviation from m easured head-scatter factors, which ranged from O.11%-0.27%.