DOSIMETRIC VERIFICATION OF THE DYNAMIC INTENSITY-MODULATED RADIATION-THERAPY OF 92 PATIENTS

Purpose: To verify that optimized dose distributions provided by an in tensity-modulated radiation therapy ((IMRT) system are delivered accur ately to human patients, Methods and Materials: Anthropomorphic phanto ms are used to measure IMRT doses, Four types of verification are deve loped for: I) system commissioning with beams optimized to irradiate s imulated targets in phantoms, II) plans with patient-optimized beams d irected to phantoms simulating the patient, Inj patient-phantom hybrid plans with patient-optimized beams calculated in phantom without furt her optimization, and IV) in vivo measurements, Phantoms containing do simeters are irradiated with patient-optimized beams, Films are scanne d and data were analyzed with software, Percent difference between ver ified and planned maximum target doses is defined as ''dose discrepanc y'' (Delta(vp)). The frequency distribution of type II Delta(vp) from 204 verification films of 92 IR-IRT patients is fit to a Gaussian. Mea surements made in vivo yield discrepancies specified as Delta(ivp), al so fit to a Gaussian, Results and Discussion: Verification methods rev ealed three systematic errors in plans that were corrected prior to tr eatment. Values of \Delta(vp)\ for verification type I are <25%. Type II verification discrepancies are characterized by a Gaussian fit with a peak 0.2% from the centroid, and 158 \Delta(vp)\ <5%. The 36 values of \Delta(vp)\ >5% arise from differences between phantom and patient geometry, and from simulation, calculation, and other errors. Values of \Delta(vp)\ for verification III are less than half of the values o f \Delta(vp)\ for verification II. A Gaussian fit of Delta(ivp) from v erification TV shows more discrepancy than the fit of Delta(vp), attri buted to dose gradients in detectors, and exacerbated by immobilizatio n uncertainty, Conclusions: Dosimetric verification is a critical step in the quality assurance (QA) of IMRT. Hybrid Verification III is sug gested as a preliminary quality standard for IMRT. (C) 1998 Elsevier S cience Inc.