Relative profile and dose verification of intensity-modulated radiation therapy

Purpose: To develop a quality assurance (QA) procedure to assess the intens ity profile and dosimetry for intensity-modulated (IM) treatment fields usi ng electronic portal imaging devices (EPIDs), Methods and Materials: A series of rapidly acquired (similar to 1/sec) port al images are summed and converted to dose, For relative intensity QA, the intended profile is subtracted point-by-point from the measured profile for ming a series of error values. The standard deviation, a, of the errors, a measure of the goodness of the match, is minimized by applying a normalizat ion and uniform scatter subtraction from the measured profile. For dose ver ification (dose to isocenter), an empirically determined phantom-correction factor is added to incorporate the effect of patient presence on EPID read ings. Seventy prostate treatment fields were used in a phantom study to ver ify these approaches, Sensitivity was studied by creating artificial mismat ches. Results: The average a for relative profile verification is 3.3% (percentag e of average intended intensity) whereas artificial mismatches resulted in a values from 5% to 27%, The average isocentric dose calculated from EPID r eadings is 1.001 relative to the planned dose with a standard deviation of 0.018. Conclusions: An EPID can be used for profile verification and absolute isoc entric dose measurement for IM fields. (C) 2000 Elsevier Science Inc.