Monte Carlo computation of dosimetric amorphous silicon electronic portal images

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Abstract
This study develops and tests a method to compute dosimetric images for an amorphous siliconflat‐panel detector so that an accurate quantitative comparison between measured and computed portal images may be made. An EGS4‐based Monte Carlo (MC) algorithm is developed to efficiently tally the energy deposition through the use of a virtual detector dose‐scoring methodology. The complete geometry of theimager is utilized in the MC calculation up to the imager rear housing, which is replaced with a uniform thickness material slab. The detector‐mounting hardware is modeled as a uniform backscattering material. The amount of backscatter material required to reproduce the measured backscatter is 0.98 g/cm2of water. A flood‐field irradiation, performed in the measurement imaging session, is used to cross‐calibrate the computed images with the measured images. Calibrated MC‐computed images reproduce measured field‐size dependencies of the electronic portal imaging device (EPID) response to within <1%, without the need for optical glare or other empirical corrections. A 10% dose difference between measured and computed images was observed outside the field edge for afield that was entirely blocked by the multileaf collimator (MLC). However, this error corresponded with less than 0.15% of the open‐field dose. Forfields produced by 5 and 20 mm dynamically sweeping MLC gaps, more than 98% of the points were found to have a gamma less than one with a 2%, 2 mm criteria. For an intensity modulated radiation therapy (IMRT) patient test field, over 99% of the points were found to have a gamma less than one with a 2%, 2 mm criteria. This study demonstrates that MC can be an effective tool for predicting measuredportal images and may be useful for IMRT EPID‐based dosimetry.
Funding Information
  • Varian Oncology Systems and the National Institutes of Health (1R01CA98524)