The backscattered radiation from the support arm of Varian a‐Si EPIDs can affect the accuracy of dosimetric measurements using these devices. In this study the effect of insertion of lead sheets between the EPID and the arm has been investigated for the E‐type arms. The optimum lead thickness was determined by comparison of the imager response on and off the arm with increasing lead thicknesses and 2 mm of lead was selected as the optimal thickness considering the reasonable extra weight added to the imager. It reduced the arm backscatter from a maximum of about 6% and 3.5% higher than the off‐arm signal in 6 MV and 18 MV beams to about 2% for both energies. On‐axis EPID response measurements for different field sizes showed a considerable decrease in arm backscatter with the addition of lead. The symmetry improved for the largest field from about 105% and 103% to 101% and 100% using 2 mm lead. Changing the SDD did not affect the backscatter component more than 1%. The addition of lead decreased the contrast‐to‐noise ratio and resolution by 1.3% and 0.8% for 6 MV and by 0.5% and 0.4% for 18 MV beams. The root mean square deviation for the difference in EPID central pixel position with and without lead during a whole gantry rotation was one pixel at maximum. In conclusion a 2 mm thick lead layer seems sufficient for acceptable dosimetry results with no major degradation to the routine performance of the imager.
|Number of pages||1|
|Publication status||Published - 1 Jan 2010|