Pretreatment verification of IMSRT using electronic portal imaging and Monte Carlo calculations

The use of an amorphous silicon electronic portal imaging device (EPID) and Monte Carlo calculations were investigated for pretreatment fluence verification in intensity modulated stereotactic radiotherapy (IMSRT). Monte Carlo calculations were performed using BEAM, a general purpose Monte Carlo code to simulate radiation beams from radiotherapy units. The dose distribution to the EPID phosphor was calculated by BEAM and then converted to pixel value using a pixel calibration curve. The calibration correlated calculated pixel dose to the measured pixel value for a range of open fields. Points within the region bounded by the photon jaws were extracted for comparison. Criteria for successful verification were 5% local percent difference in high dose regions, 1 mm distance to agreement in high gradient regions, or 2% of the Monte Carlo calculated central axis pixel value in low dose regions. Software was written to quantitatively compare the measured and calculated EPID images. Successful verification of the modulated field required that >or=95% of compared points fall within the comparison criteria. Dose response of the EPID was found to be linear with Monte Carlo calculated doses over the dose ranges examined in this work Comparison of the measured and calculated EPID dose distributions showed good agreement with 97% of the points passing criteria. The sensitivity of the methodology to detect field shaping errors was tested by introducing positioning errors in segments of the modulated field. These sensitivity tests indicate that the comparison software designed for this work can detect a 1 mm positioning error in a single segment of the composite IMSRT field. It should be noted, however, that the work presented here is a proof of concept and currently not a clinically viable QA tool. It represents a limited evaluation using a single IMSRT field, and verification of additional fields will be required for a comprehensive evaluation of the described methods before broad conclusions can be drawn. Additionally, the results of this work are subject to the comparison criteria that were used. Clinical implementation of the proposed technique should be evaluated for the specific institutional criteria where it will be employed.