ANALYTICAL REPRESENTATION OF HEAD SCATTER FACTORS FOR SHAPED PHOTON BEAMS USING A 2-COMPONENT X-RAY SOURCE MODEL

An analytic representation proposed for the relative intensity distrib ution of the extra-focal source in a two-component x-ray source model serves as the basis for calculation. From this representation, a close d-form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra-focal sourc e plane visible from the measurement point. The resulting expression i s applicable to photon beams from different Varian accelerators and di fferent photon energies, and includes effects arising from beam shapin g with cerrobend blocks or multileaf collimators (MLCs). For 6- and 15 -MV photon beams from Varian 600C and 2300CD accelerators (Varian Onco logy Systems, Pale Alto, CA), 361 measured head scatter factors for sq uare, rectangular, asymmetric, and arbitrarily shaped fields, formed b y either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, we re compared with model calculations. Results show that 93.4% of calcul ated values match corresponding experimental points to within 0.5%, th e average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively desc ribed. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a resul t of the different distances of the X and Y jaws from the focal spot.