Monte Carlo computation of dosimetric amorphous silicon electronic portal images.
This study develops and tests a method to compute dosimetric images for an amorphous silicon (a-Si) flat-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 the a-Si imager 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/cm2 of 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 a 10 x 10 cm2 field that was entirely blocked by the multileaf collimator (MLC). However, this error corresponded with less than 0.15% of the open-field dose. For 10 x 10 cm2 fields 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 measured a-Si portal images and may be useful for IMRT EPID-based dosimetry.
Siebers
Jeffrey V
JV
Department of Radiation Oncology, Virginia Commonwealth University Medical Center, Virginia Commonwealth University Richmond, Virginia 23298, USA. jsiebers@vcu.edu
Kim
Jong Oh
JO
Ko
Lung
L
Keall
Paul J
PJ
Mohan
Radhe
R
D000465:Algorithms; D002138:Calibration; D019544:Equipment Failure Analysis / Q000379:methods*; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D009010:Monte Carlo Method; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation* / Q000379:methods; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012825:Silicon / Q000528:radiation effects*
D012825:Silicon