Room: Davidson Ballroom A
Purpose: A prototype pEPID is explored for relative/absolute dosimetry and imaging applications in radiotherapy.
Methods: A high resolution dynamic pEPID (0.2mm pixel size) is evaluated using a Varex XRD 0822 detector equipped with plastic scintillators ranging from 2.6 to 41.0mm in thickness. A 6 MV treatment beam (Elekta Infinity Linac, SSD=100cm, FOV=10cmÃ—10cm) is used to establish the dosimetry calibration of the detector signal versus dose in water (based on ion-chamber measurements in solid water). Using a scintillator thickness of 5.2mm, 9.7mm, and 15mm, output factors for field sizes ranging from 3Ã—3 to 20Ã—20cm2 are measured, and pEPID dose response linearity is tested. The 5.2mm pEPID sensitivity is compared to a similar Varex XRD 0822 AP20 Gadox EPID (gEPID). For each scintillator, the median signal of a center 10Ã—10 pixel ROI is computed after offset and gain corrections. The detector signal is modeled using a modified exponential function, and corrected on a pixel-by-pixel basis using output factors and off-axis beam profiles. Imaging performance is studied using a QC-3 phantom. Contrast-to-Noise (CNR) and spatial resolution are compared to published results from standard EPIDs.
Results: The pEPID response shows excellent linearity (R2>0.9997). The 5.2mm pEPID sensitivity is approximately 1% of the gEPID which eliminates the EPID saturation concern. Beam profiles measured by pEPID agree with water scan within 2.5%, and are superior to those measured with SunNuclear Profiler due to smaller detector spacing (1mm). The CNR is 4 times less than with a commercial Varian aS1000 EPID, and similar spatial resolution of 0.76lp/mm is achieved.
Conclusion: The low sensitivity and water-equivalence makes pEPID ideal for dosimetry applications before or during high dose rate treatments. This initial study indicates the feasibility of using pEPID for machine QA and fiducial marker tracking. Further investigations will be performed with clinical IMRT/VMAT/SBRT treatment plans.
Absolute Dosimetry, Electronic Portal Imaging, Quality Assurance