SU-E-T-226: Image Acquisition and Processing Characteristics of a Siemens EPID: Potential Problems for EPID Dosimetry

Abstract

Purpose: To investigate the behavior of the imagingcontrol software on a Siemens EPID and the resulting impact on EPID based IMRTdosimetry Method and Materials:Four ‘simple’ and four ‘clinical’ IMRT fields were created. Each segment of the ‘simple’ field had a constant field size of 10 × 10 cm2. The ‘simple’ fields had 2, 4, 5, and 10 segments with 10, 5, 4, and 2 MU per segment respectively. The ‘clinical’ fields all consisted of 20 segments with constant MU per segment of 2, 4, 5, and 10 MU. The fields were measured simultaneously with a Siemens EPID and ImRT MatriXX ionisation chamber array (iba) by placing the MatriXX beneath EPID. The integrated EPID dose response was determined from the frame average pixel value multiplied by the reported number of frames. Reproducibility of central axis response, beam profiles, and pixel‐by‐pixel response (2D) was compared from 10 repeat measurements. Results: The percentage standard deviation of the integrated EPID response for the ‘simple’ case was within 1% for the 2 and 10 MU per segment fields but more than 10% for the 4 and 5 MU per segment fields. EPID crossplane profiles also yielded large variations in amplitude and shape for the 4 and 5 MU per segment cases only. No such variations were observed by concurrent MatriXX measurements. A similar but less obvious trend was observed for the ‘clinical’ case. In both cases the number of frames per segment reported for the 2 MU and 10 MU per segment fields remained constant. The number of frames reported for the 4 MU and 5 MU per segment fields varied and appeared unreliable. Conclusions: EPIDimages of IMRT fields displayed large variations in reproducibility depending on the MU per segment. Poor reproducibility correlated with inconsistent reporting of the number of frames. Cancer Institute NSW Research Equipment Grant 10/REG/1‐20