A quality assurance phantom for IMRT dose verification

We have investigated a quality assurance (QA) phantom that was specially designed to verify the accuracy of dose distributions calculated by a commercial inverse planning optimization system (CORVUS, NOMOS Corp., Pittsburgh, PA) and by the Monte Carlo method for intensity-modulated radiotherapy (IMRT). The QA phantom is a PMMA cylinder of 30 cm diameter and 40 cm length with various bone and lung inserts. A procedure was developed to measure the absolute dose at any point inside the QA phantom. Another cylindrical phantom of the same dimensions, but made of water, was used to confirm the results obtained with the PMMA phantom. The PMMA phantom was irradiated by 4, 6 and 15 MV photon beams and the dose was measured using an ionization chamber and compared to the results calculated by CORVUS and by the Monte Carlo method. The results show that the dose distributions calculated by both CORVUS and Monte Carlo agreed well (within 2% of dose maximum) with measured results in the uniform PMMA phantom for both open and intensity- modulated fields. Similar agreement was obtained between Monte Carlo calculation and measured results with the bone and lung heterogeneities inside the PMMA phantom. Following the positive results of this study, our QA phantom has been integrated as a routine QA procedure for patient's IMRT dose verification at Stanford since 1999.