A comparison of measurement techniques for quality assurance of RapidArc treatment plans.

摘要

A new form of intensity modulated radiation therapy (IMRT) using the Varian RapidArcRTM treatment system has the potential to improve cancer treatments by delivering comparable dose distributions as TomoTherapy RTM1 at a rate that is 5 to 15 times faster 2. The goal of this thesis was to compare the sensitivity of an electronic portal imaging device (EPID), film, and the ArcCheck(TM) device as tools for evaluation of IMRT treatments. To accomplish this, we introduced systematic errors in MLC leaf position and perform theoretical and experimental evaluations. The comparisons were made by analyzing changes in the gamma function as systematic MLC gap errors were introduced into the patient treatment plan. To study this, dynamic prostate Rapidarc(TM) plans were developed in Eclipse(TM) using the RANDO anthropomorphic phantom.;After the plans were developed, the files were exported in DICOM-RT (Digital Imaging and Communications in Medicine --- Radiation Therapy) format, and a program was written to modify the MLC leaf position. The files were imported back into Eclipse and recalculated using simulated film, EPID, and ArcCheck phantoms. Before measuring the plans with the devices, each instrument was calibrated and repeatability tests were performed to determine the variation in a single plan. After the delivery of the same plan eight times, film was found to have the largest variation in average gamma of 0.31 +/- 0.13. When the setup was not moved in between measurements, the ArcCheck and EPID had significantly smaller variations in average gamma of 0.10 +/- 0.04 and 0.07 +/- 0.03 respectively. When the setup was moved and realigned in between measurements, the average gamma variation was found to be 0.22 +/- 0.10 and 0.11 +/- 0.05 for the ArcCheck and EPID. As a result of the higher variation, it was decided that testing should be performed without moving the detector during the course of the measurements. Film, unfortunately, had to be changed in between tests, which resulted in a high level of variation. After calibration, the original five plans and their modified MLC gap variants were tested.;A comparison of the Eclipse plans with changes in leaf position to the original plan with 0 mm change in gap shows that the film should have the least sensitivity to changes in average gamma. When looking at the measured results with changes in MLC gap compared to the measured plan with 0 mm change in leaf position, we see that film incorrectly appears to have the highest sensitivity. The cause of this was due to the large variation in average gamma discovered during the repeatability tests. By comparing the repeatability value of 0.31 +/- 0.13 to the sensitivity curve generated by Eclipse, it is estimated that film cannot accurately measure changes in systematic gap less than 2 mm. A similar value was found by Yan et al. 15 when using the 2 % / 2 mm gamma <= 1 pass rate.;The EPID and ArcCheck displayed significantly less variation in average gamma during static tests (0.07 +/- 0.03 and 0.10 +/- 0.04 respectively). Using the Eclipse calculated graph showing changes in average gamma compared to changes in MLC gap, we find that the EPID and ArcCheck should be able to measure systematic MLC errors of greater than 0.3mm. Previous analysis on MLC motion indicates that most positional errors are below 0.5 mm16 , which makes it difficult to conclude if the repeatability of the device is limited by the detector, or by actual errors in MLC leaf position on the treatment machine. For systematic gap width changes in the range of 0 to 1 mm, the EPID and ArcCheck show similar sensitivity.;This thesis has focused on how the average gamma value changes when comparing systematic changes in MLC gap to their original unchanged plan. Although the results are not conclusive, they do indicate that further testing may be warranted. Future testing should involve other treatment sites with steeper dose gradients such as head and neck plans. To ensure clinical relevancy, measurements should also focus on smaller systematic errors based on actual expected MLC gap deviations. If further testing does confirm that sensitivity does change with the geometric shape of the measurement device, it could potentially have an influence on future instrument designs. (Abstract shortened by UMI.).