SU-E-T-16: Statistical Variability and Confidence Intervals for Planar Dose QA Pass Rates.

PURPOSE: The most common metric for comparing measured to calculated dose planes is a pass rate generated using percent difference, distance-to-agreement (DTA), or some combination of the two (e.g. gamma evaluation). The grid of analyzed points often corresponds to a dosimeter array with low areal-density of point detectors. This work examines the statistical uncertainty of planar dose comparison pass rates and proposes methods for establishing confidence intervals for pass rates obtained with low detector-density arrays. METHODS: Absolute dose planes were acquired via EPID for twenty intensity-modulated fields of varying complexity. Matching calculated dose planes were created via treatment planning system. Pass rates for each dose plane pair (centered to CAX) were calculated with various %/DTA composite analysis techniques. Software was designed to selectively sample the high-density EPID matrix to simulate many low-density measured grids, each representing a different alignment with respect to CAX. Simulations were repeated (100 positional iterations per field) using grids of varying detector-densities and both random and orthogonal point-detector orientation. For each simulation, pass rates were calculated with various composite analysis techniques. RESULTS: Repositioning simulated low-density grids leads to a distribution of possible pass rates for each measured/calculated dose plane pair, independent of whether the detector grid is random or uniform. Distributions can be predicted using a binomial distribution by which a confidence interval (function of sampling density and observed pass rate) is approximated for each pass rate. For example, 95% confidence intervals for IMRT pass rates (2%,2mm) average +/-5.3% and +/-3.8% with 1-detector/cm2 and 2-detector/cm2 grids, respectively. CONCLUSIONS: Pass rates for low-density array measurements are not absolute and should be reported with both a full description of calculation method and confidence intervals quantifying their uncertainty. Results extend to 3D detector arrays. The concept of fixed 'action levels' for pass rates must be reexamined for low-density array measurements.

SU-E-T-156: Robust Algorithms for Correction of Predicted Electronic Portal Imager Response.

PURPOSE: Predicted electronic portal imaging device (EPID) response, as calculated by a commercial treatment planning system (TPS), is up to 15% lower than measured EPID response for off-axis IMRT fields. Two original algorithms are presented to correct for EPID prediction errors. The EPID prediction algorithm and a recent image-to-dose conversion algorithm are each tested for ability to identify TPS dose calculation errors. METHODS: By comparing test images to respective predictions, correction factors were calculated to modify the EPID diagonal calibration profile (applied via radial symmetry). Secondly, image/prediction comparisons were used to compute a 2D correction matrix for EPID predictions, to account for radially-asymmetric errors. Over 50 IMRT fields of varying complexity were tested with each correction technique, and with a diode array. Absolute dose and beam-profile errors were separately induced into the TPS and a number of IMRT plans were recalculated and measured with three systems - an EPID prediction system, an EPID image-to-dose conversion system, and a diode array - for comparison to verification plans. RESULTS: With the profile correction, TPS predictions agree much better with EPID measurements, yielding improvement in gamma pass rates (3%,3mm) of over 30% on average for off-axis IMRT fields. Since off-axis prediction errors are not radially-symmetric, the matrix correction further improves pass rates by 5% on average (up to 30%) for fields where the profile correction is limited. The EPID prediction system was unable to catch either induced TPS error, while both the image-to-dose conversion system and the diode array indicated both errors. CONCLUSIONS: Profile correction is effective and efficient though approximate, due to radial symmetry. The matrix correction is comprehensive but requires computational manipulation of DICOM images. Users must be aware that EPID prediction systems may be unable to catch delivered IMRT inaccuracies due to calculation errors downstream from the actual fluence calculation.

Using an EPID for patient-specific VMAT quality assurance.

PURPOSE: A patient-specific quality assurance (QA) method was developed to verify gantry-specific individual multileaf collimator (MLC) apertures (control points) in volumetric modulated arc therapy (VMAT) plans using an electronic portal imaging device (EPID). METHODS: VMAT treatment plans were generated in an Eclipse treatment planning system (TPS). DICOM images from a Varian EPID (aS1000) acquired in continuous acquisition mode were used for pretreatment QA. Each cine image file contains the grayscale image of the MLC aperture related to its specific control point and the corresponding gantry angle information. The TPS MLC file of this RapidArc plan contains the leaf positions for all 177 control points (gantry angles). In-house software was developed that interpolates the measured images based on the gantry angle and overlays them with the MLC pattern for all control points. The 38% isointensity line was used to define the edge of the MLC leaves on the portal images. The software generates graphs and tables that provide analysis for the number of mismatched leaf positions for a chosen distance to agreement at each control point and the frequency in which each particular leaf mismatches for the entire arc. RESULTS: Seven patients plans were analyzed using this method. The leaves with the highest mismatched rate were found to be treatment plan dependent. CONCLUSIONS: This in-house software can be used to automatically verify the MLC leaf positions for all control points of VMAT plans using cine images acquired by an EPID.

Automatic verification of step-and-shoot IMRT field segments using portal imaging.

In step-and-shoot IMRT, many individual beam segments are delivered. These segments are generated by the IMRT treatment planning system and subsequently transmitted electronically through computer hardware and software modules before they are finally delivered. Hence, an independent system that monitors the actual field shape during treatment delivery is an added level of quality assurance in this complicated process. In this paper we describe the development and testing of such a system. The system verifies the field shape by comparing the radiation field detected by the built-in portal imaging system on the linac to the actual field shape planned on the treatment planning system. The comparison is based on a software algorithm that detects the leaf edge positions of the radiation field on the portal image and compares that to the calculated positions. The process is fully automated and requires minimal intervention of the radiation therapists. The system has been tested with actual clinical plan sequences and was able to alert the operator of incorrect settings in real time.