SU‐E‐T‐458: Instability of Electronic Portal Imaging Device Responses for Intensity Modulated Irradiation

• Published in: Medical Physics 2011-06, Vol.38 (6), p.3594-3594
• Main Authors: Yeo, I, Jung, J, Yi, B, Piskulich, F, Choi, D, Nookala, P, Patyal, B
• Format: Article
• Language: English
• Subjects: Field size; Image guided radiation therapy; Multileaf collimators
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.3612412

Purpose: To investigate the response of electronic portal imaging device (EPID) in integration mode for intensity modulated beams including step‐ and‐shoot (SS) and sliding window (SW) deliveries. Methods: We evaluated EPID dose response measurements of open, SS, and SW irradiations. We designed three beams using 6MV x‐rays with 10×10 cm2 field size. For the SS irradiation, two MLC leaves with 0.5 cm opening outside 10 × 10 cm2 were discontinuously moved by 10 cm during each segmental irradiation among ten segments of the entire delivery. For the SW beam, the same MLC leaves were continuously moved by 10 cm ten times during the delivery. The employment of the same open beam area ensures the same dose irradiation by the three. Monitor units were varied. We additionally investigated the dose linearity of EPID response. We used EPID‐ADU1000 operated with IAS3 system for the integrated acquisition. Results: The SS delivery showed a deviation greater than 2% from the open beam, if a segment employed less than 10 MUs. If it employed greater than 10 MUs, then a deviation less than 1% was observed. The SW delivery showed a maximum deviation of 1.4% at the lowest MU. The open beam irradiation showed linearity within 0.8%, the SW irradiation 1.2%, and the SS irradiation 1.7%. Conclusions: The SS delivery is associated with a greater error than the SW delivery due to beam instability at the start of acquisition and reading loss during MLC movement. Using large monitor units greater than 10 MUs/segment can minimize the deviation for the SS delivery from open beam. The linearity of EPID response to dose was better with the SW delivery than the SS delivery. This study will help understand dosimetric response of EPID for the SS delivery. This study was in part supported by Varian Medical Systems, Inc.