SU-E-P-39: Characterization of Dose Impact On Different Beam Fields Size Attenuation for Elekta IGRT Couch




Quantification of the dosimetric impact of the Elekta IGRT treatment couch in different beam field sizes. Established the relationship of relative dose versus beam angle at different beam field sizes.


Measurements of couch attenuation were performed at gantry angles from 180° to 120°, using a 0.125cc semiflex ionization chamber, isocentrically placed in the center of a homogeneous cylindric sliced RW3 phantom for 6 photon beams. Measurements were performed at six different field sizes (3×3, 5×5, 7×7,10×10, 12×12 and 15×15 cm2). The phantom were positioned at the center of the couche,100 MU were delivered at every gantry angle. The dose difference to the ion chamber was determined.


For oblique fields with 6 MV photons at the same gantry angle the attenuation coefficient value from the lagest to the smallest the order is field size 7 cm2,5 cm2,10 cm2,12 cm2,15 cm2 and 3 cm2. The biggest couch attenuation by up to 4.15% at the gantry angle of 140°for the field size of 7 cm2, while for the field size of 3 cm2 the couch attenuated value only 3.5%. The other field size couch attenuation values are between the couch attenuated value of field size of 7 cm2 and 3 cm2


The recommended treatment couch attenuation measured beam field size is 10×10 or 12×12 cm2. When measured using the beam field size 3×3 cm2 the tested value will be lower, while measuerd using the beam field size 7×7 cm2 the tested value will be higer than the recommended beam field size. This should be noted when modeling the treatment couch in the treatment planning system.