Impact of electronic portal image quality on Elekta AQUA® collimator isocenter.

PURPOSE: The collimator radiation isocenter position determined in AQUA® v3.0 [Elekta AB, Stockholm, Sweden] software test "MLC Leaf and Jaw Position" was independently validated using an in-house MATLAB [Natick, MA: The MathWorks Inc.] script. METHODS: The AQUA test determines radiation isocenter using the mean field center of nine 4 cm × 4 cm electronic portal imager (EPID) exposures at equidistant collimator angles. Impact of EPID image quality on AQUA reported isocenter for thirteen Elekta linear accelerators with Agility MLC heads were evaluated. RESULTS: Of the thirteen, three had visually and quantitatively identifiable artifacts. For the ten good EPID's there was a systematic 0.25 mm offset of the MATLAB calculated mean field center relative to AQUA in the X-axis and Y-axis. This corresponds to one image pixel and was found to be due to differences in software co-ordinate convention. After subtracting this offset there was no significant difference in AQUA and MATLAB calculated isocenter. CONCLUSIONS: For the three machines with poor image quality there was a demonstrated variation in AQUA calculated field center and therefore radiation isocenter relative to MATLAB. Restricting the region of interest (ROI) in AQUA software to only the irradiated section of the EPID brought AQUA and MATLAB result for these three machines into agreement.

Variation in Elekta iView electronic portal imager pixel scale factor with gantry angle, and impact on multi-leaf collimator quality assurance.

For Elekta Agility linear accelerators, the iViewGT electronic portal imaging device (EPID) is positioned at a nominal X-Ray source-to-panel distance of 1600 mm. For display, image registration, and data processing purposes, the image pixels are scaled to spatial units at the treatment isocenter plane. This is achieved by applying a pixel scaling factor (PSF). During this investigation, the dependence of the PSF at cardinal gantry angles was determined along with the resulting effects on the multi-leaf collimator (MLC) quality assurance (QA) results for three linear accelerators (linacs). The PSF was found to vary by 0.0018-0.0022 mm/pixel during gantry rotation, which resulted in variations in the mean MLC reported error of up to 0.8 mm at 100 mm off-axis with the gantry rotated to 180°. Measurement and application of a gantry angle-specific PSF is a simple process that can be implemented to improve the accuracy of EPID-based MLC QA at cardinal gantry angles.

Investigation of pixel scale calibration on the Elekta iView electronic portal imager.

This study investigated the variation in electronic portal imager pixel scale at the isocenter plane for Elekta Agility linear accelerators. An in-house MATLAB script was written to process and calculate the pixel scale based on a metal calibration plate supplied by Elekta. Eight pixel plates were compared and found to have manufacturing tolerances within 0.1 mm of nominal dimensions. The impact of these variations on pixel scale factor was negligible, and plates could be used interchangeably. Uncertainties from other parameters such as source-to-surface distance and user variability summed to a combined uncertainty of 0.0003 mm/pixel, compared to a pixel scale range of 0.003 mm/pixel measured across 10 machines. Most of the inter-machine variation was shown to be attributable to differences in source-to-panel distance. Other factors such as focal spot size and shape, electronic portal imager manufacturing consistency, panel sag, and setup errors may account for the residual variation. Individual characterization of machine and imaging panel pixel scale factors is important to ensure accurate geometric information is derived from electronic portal images, which is critical where the portal imager is used for multi-leaf collimator calibration or other clinical tasks.

Investigation of Elekta AQUA software for kilovoltage to megavoltage radiation isocenter coincidence.

Elekta AQUA v2.02 software (Gantry Runout isocenter test) was investigated as a tool for verification of kilovoltage to megavoltage gantry radiation isocenter coincidence. AQUA reported megavoltage (6 MV) isocenter was independent of field size over the range 5 cm × 5 cm to 20 cm × 20 cm. For the 10 cm × 10 cm field size, standard deviation in AQUA reported 3D megavoltage (6MV) isocenter over ten consecutive deliveries was less than 0.04 mm for any axis. Compared to the full AQUA test delivery (Gantry Runout), the shorter AQUA test version (Gantry Runout short) gave a root mean square MV isocenter (± 1 SD) difference of 0.18 mm ± 0.08. Across 7 machines, root mean square differences between AQUA and PIPS PRO reported MV isocenter (for 6 MV and 6 MV FFF beams) was 0.1 mm ± 0.1 mm, with most of the difference observed in the gun-target (Y-axis). AQUA 6 MV isocentre position was offset to gantry relative to Elekta XVI customer acceptance test (CAT) workflow by 0.25 mm to 0.52 mm. For 6 MV FFF beams, AQUA reported an MV isocenter of between 0.27 and 0.39 mm offset to target relative to Elekta CAT.

Predictive performance of an OVH-based treatment planning quality assurance model for prostate VMAT: Assessing dependence on training cohort size and composition.

Radiotherapy treatment planning quality assurance models are used to assess overall plan quality in terms of dose-volume characteristics, by predicting an optimal dosimetry based on a dataset of prior cases (the training cohort). In this study, a treatment planning quality assurance model for prostate cancer patients treated with volumetric modulated arc therapy was developed using the concept of the overlap volume histogram for geometric comparison to the training cohort. The model was developed on the publically available Erasmus iCycle dataset in order to remove the effect of plan quality/inter-planner variability on the model's predictive capabilities. The model was used to predict anus, rectum, and bladder dose volume histograms. Two versions were developed: the n = 114 case (leave-one-out method) which made predictions using the complete Erasmus dataset, and the similarity index (SI)-based model which used a smaller training cohort allocated in order of geometric similarity determined using an overlap volume histogram-derived SI. The difference in mean dose (predicted-achieved) of the SI model at cohort sizes of 10, 20, 30, 40, 50, 75, and 100 was compared to the leave-one-out method for 5 patients, in an attempt to determine the "optimum" cohort size for the SI-based model in this dataset. Performance of the optimized SI model was compared to the leave-one-out method for all patients using the following metrics: difference in mean and median dose, difference in V65Gy and V75Gy (rectum only), similarity of predicted and achieved mean dose, and mean dose volume histograms residual. The "optimum" cohort size for the SI-based model was determined to be 45. The SI-based model implementing this cohort size yielded slightly better outcomes in all performance metrics for the rectum and anus, but worse for the bladder. SI-based training cohort allocation can lead to better predictive efficacy, but the cohort size should be optimized for each individual organ.

Clinical significance of multi-leaf collimator calibration errors.

This planning study investigates the clinical impact of multi-leaf collimator (MLC) calibration errors on three common treatment sites; head and neck (H&N), prostate and stereotactic body radiotherapy (SBRT) for lung. All plans used using either volumetric modulated adaptive therapy or dynamic MLC techniques. Five patient plans were retrospectively selected from each treatment site, and MLC errors intentionally introduced. MLC errors of 0.7, 0.4 and 0.2 mm were sufficient to cause major violations in the PTV planning criteria for the H&N, prostate and SBRT lung plans. Mean PTV dose followed a linear trend with MLC error, increasing at rates of 3.2-5.9% per millimeter depending on treatment site. The results indicate that an MLC quality assurance program that provides sub-millimeter accuracy is an important component of intensity modulated radiotherapy delivery techniques.