Verification Of Modeled Photon Beam Attenuation Of Treatment Couch Top In Oncentra Masterplan Treatment Planning System

ABSTRACT During radiotherapy, some accessories used gets into the path of the photon beams. These somehow attenuate the beam to some extent. Therefore it is imperative that every radiotherapy facility quantifies the degree by which these accessories attenuate the beams. This informs the facility operators on the necessary corrections to make to achieve the desired dose to the target area. The purpose of this study was to evaluate the attenuation effects of 6 MV and 15 MV photon beam by an iBeam evo carbon fiber couch top and subsequently correct these effects by using the correction factor method. The couch was first CT scanned to verify it homogeneous construction. Ion chamber output measurements were carried out using polymethylmethacrylate PPMA, phantom. At normal incidence of both photon beams, the 6 MV beam was attenuated by 3.8%, 4.91%, 2.71% and 2.43% and the 15 MV beam by 2.43%, 3.86%, 1.57% and 1.35% at 5, 10, 15 and 20 cm2 field size respectively. An equation for attenuation correction was worked out and correction factors for gantry angles 90° to 180° were obtained. Finally, these correction factors were applied in a typical pelvic case using an anthropomorphic phantom and an AB mimator diode. Photon beam attenuation by iBeam evo couch was well corrected by the correction factors when TPS calculated doses were compared to diode measurements. In conclusion, photon beam attenuation by a carbon fiber couch top was dependent on the gantry angle, field size and photon beam energy. This method using the correction factors was useful in correcting the isocenter dose when attenuation effects cannot be corrected by a treatment planning system. Clinical validation was done with an anthropomorphic phantom and AB Mimator diode dosimetry.