EFFECT OF BEAM BLOCKING ON LINAC HEAD SCATTER FACTORS

Purpose: This study aimed to investigate the influence of beam blockin g on head scatter within the context of a two-component x-ray source m odel, Methods and Materials: Head scatter factors were measured for op en rectangular fields defined by X and Y jaws and for fields blocked b y a multileaf collimator (MLC) for a 6-MV photon beam from a Varian 23 00CD accelerator, A simple formula based on consideration of linac hea d geometry and an x-ray source model was derived to determine when the head scatter factor for a shaped field depends only on jaw settings a nd when it is influenced by the blocking (cerrobend blocks or MLC) its elf, Results: Experimental data demonstrate that the assumption that h ead scatter is unaffected by the introduction of beam blocking is not always acceptable, particularly for fields blocked by an MLC. The rati os of open to blocked field dimensions were compared with simple funct ions characterizing the accelerator head geometry to predict changes i n the behavior of head scatter factors observed experimentally, Conclu sion: A simple formula can be used to determine when head scatter fact ors are influenced by beam blocking(cerrobend blocks or MLC). (C) 1997 Elsevier Science Inc.