title: An EPID response calculation algorithm using spatial beam characteristics of primary, head scattered and MLC transmitted radiation.
abstract: We have developed an independent algorithm for the prediction of electronic portal imaging device (EPID) response. The algorithm uses a set of images [open beam, closed multileaf collimator (MLC), various fence and modified sweeping gap patterns] to separately characterize the primary and head-scatter contributions to EPID response. It also characterizes the relevant dosimetric properties of the MLC: Transmission, dosimetric gap, MLC scatter [P. Zygmansky et al., J. Appl. Clin. Med. Phys. 8(4) (2007)], inter-leaf leakage, and tongue and groove [F. Lorenz et al., Phys. Med. Biol. 52, 5985-5999 (2007)]. The primary radiation is modeled with a single Gaussian distribution defined at the target position, while the head-scatter radiation is modeled with a triple Gaussian distribution defined downstream of the target. The distances between the target and the head-scatter source, jaws, and MLC are model parameters. The scatter associated with the EPID is implicit in the model. Open beam images are predicted to within 1% of the maximum value across the image. Other MLC test patterns and intensity-modulated radiation therapy fluences are predicted to within 1.5% of the maximum value. The presented method was applied to the Varian aS500 EPID but is designed to work with any planar detector with sufficient spatial resolution.
authors.affiliation: North Shore Cancer Center, 17 Centennial Drive, Peabody, Massachusetts 01960, USA. frosca@partners.org
authors.forename: Florin
authors.initials: F
authors.lastname: Rosca
authors.forename: Piotr
authors.initials: P
authors.lastname: Zygmanski
mesh_terms: D000465:Algorithms; D002138:Calibration; D003198:Computer Simulation; D003952:Diagnostic Imaging / Q000379:methods*; D004582:Electronics, Medical / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity