An iterative EPID calibration procedure for dosimetric verification that considers the EPID scattering factor

There has been an increasing interest in the application of electronic port al imaging devices (EPIDs) to dosimetric verification, particularly for int ensity modulated radiotherapy. Although not water equivalent, the phantom s catter factor of an EPID, S-pe, is generally assumed to be that of a full p hantom, S-p, a slab phantom, S-ps or a mini phantom. This assumption may in troduce errors in absolute dosimetry using EPIDs. A calibration procedure t hat iteratively updates S-pe and the calibration curve (pixel value to dose rate) is presented. The EPID (Varian Portal Vision) is irradiated using a 20 x 20 cm(2) field with different beam intensities. The initial guess of d ose rates in the EPID is calculated from ionization chamber measurements in air, multiplied by S-p or S-ps. The calibration curve is obtained by fitti ng EPID readings from pixels near the beam central axis and dose rates in E PID to a quadratic equation. The S-pe is obtained from EPID measurements in 10 x 10 cm(2) and 20 x 20 cm(2) field and from the calibration curve. and is in turn used to adjust the dose rate measurements and hence the calibrat ion curve. The above procedure is repeated until it converges. The final ca libration curve is used to convert portal dose to dose in the slab phantom, using the calibrated S-pe, or assuming S-pe = S-p or S-pe = S-ps. The conv erted doses are then compared with the dose measured using an ionization ch amber. We also apply this procedure to off-axis points and study its depend ence on the energy spectrum. The hypothesis testing results (on the 95% sig nificance level) indicate that systematic errors are introduced when assumi ng S-pe = S-p or S-pe = S-ps, and the dose calculated using S-pe is more co nsistent with ionization chamber measurements, Differences between S-pe and S-ps are as large as 2% for large field sizes. The measured relative dose profile at d(max) using the EPID agrees well with the measured profile at d (max) of the isocentric plane using film in a polystyrene phantom with full buildup and full backup, for open and wedged fields, and for a broad range of field sizes of interest. The dependence of the EPID response on the ene rgy spectrum is removed once the calibration is performed under the same co nditions as the actual measurements. (C) 2001 American Association of Physi cists in Medicine.