Henry C. Woodruff

Gantry-angle resolved VMAT pretreatment verification using EPID image prediction

PURPOSE Pretreatment verification of volumetric modulated arc therapy (VMAT) dose delivery with electronic portal imaging device (EPID) uses images integrated over the entire delivery or over large subarcs. This work aims to develop a new method for gantry-angle-resolved verification of VMAT dose delivery using EPID. METHODS An EPID dose prediction model was used to calculate EPID images as a function of gantry angle for eight prostate patient deliveries. EPID image frames at 7.5 frames per second were acquired during delivery via a frame-grabber system. The gantry angle for each image was encoded in kV frames which were synchronized to the MV frames. Gamma analysis results as a function of gantry angle were assessed by integrating the frames over 2° subarcs with an angle-to-agreement tolerance of 0.5° about the measured image angle. RESULTS The model agreed with EPID images integrated over the entire delivery with average Gamma pass-rates at 2%, 2 mm of 99.7% (10% threshold). The accuracy of the kV derived gantry angle for each image was found to be 0.1° (1 SD) using a phantom test. For the gantry-resolved analysis all Gamma pass-rates were greater than 90% at 3%, 3 mm criteria (with only two exceptions), and more than 90% had a 95% pass-rate, with an average of 97.3%. The measured gantry angle lagged behind the predicted angle by a mean of 0.3°±0.3°, with a maximum lag of 1.3°. CONCLUSIONS The method provides a comprehensive and highly efficient pretreatment verification of VMAT delivery using EPID. Dose delivery accuracy is assessed as a function of gantry angle to ensure accurate treatment.

The Keck Aperture Masking Experiment: Multiwavelength Observations of Six Mira Variables

The angular diameters of six oxygen-rich Mira-type long-period variables have been measured at various NIR wavelengths using the aperture-masking technique in an extensive observing program from 1997 January to 2004 September. These data sets span many pulsation cycles of the observed objects and represent the largest study of multiwavelength, multiepoch interferometric angular diameter measurements on Mira stars to date. The calibrated visibility data of ο Cet, R Leo, R Cas, W Hya, χ Cyg, and R Hya are fitted using a uniform disk brightness distribution model to facilitate comparison between epochs, wavelengths, and with existing data and theoretical models. The variation of angular diameter as a function of wavelength and time is studied, and cyclic diameter variations are detected for all objects in our sample. These variations are believed to stem from time-dependent changes of density and temperature (and hence varying molecular opacities) in different layers of these stars. The similarities and differences in behavior between these objects are analyzed and discussed in the context of existing theoretical models. Furthermore, we present time-dependent 3.08 μm angular diameter measurements, probing for the first time these zones of probable dust formation, which show unforeseen sizes and are consistently out of phase with other NIR layers shown in this study. The S-type Mira χ Cyg exhibits significantly different behavior compared to the M-type Mira variables in this study, in both its NIR light curves and its diameter pulsation signature. Our data show that the NIR diameters predicted by current models are too small and need to incorporate additional and/or enhanced opacity mechanisms. Also, new tailored models are needed to explain the behavior of the S-type Mira χ Cyg.

Sparse-Aperture Adaptive Optics

Aperture masking interferometry and Adaptive Optics (AO) are two of the competing technologies attempting to recover diffraction-limited performance from ground-based telescopes. However, there are good arguments that these techniques should be viewed as complementary, not competitive. Masking has been shown to deliver superior PSF calibration, rejection of atmospheric noise and robust recovery of phase information through the use of closure phases. However, this comes at the penalty of loss of flux at the mask, restricting the technique to bright targets. Adaptive optics, on the other hand, can reach a fainter class of objects but suffers from the difficulty of calibration of the PSF which can vary with observational parameters such as seeing, airmass and source brightness. Here we present results from a fusion of these two techniques: placing an aperture mask downstream of an AO system. The precision characterization of the PSF enabled by sparse-aperture interferometry can now be applied to deconvolution of AO images, recovering structure from the traditionally-difficult regime within the core of the AO-corrected transfer function. Results of this program from the Palomar and Keck adaptive optical systems are presented.

THE KECK APERTURE MASKING EXPERIMENT: SPECTRO-INTERFEROMETRY OF THREE MIRA VARIABLES FROM 1.1 TO 3.8 μm

We present results from a spectro-interferometric study of the Miras o Cet, R Leo, and W Hya obtained with the Keck Aperture Masking Experiment from 1998 September to 2002 July. The spectrally dispersed visibility data permit fitting with circularly symmetric brightness profiles such as a simple uniform disk (UD). The stellar angular diameter obtained over up to ~ 450 spectral channels spanning the region 1.1-3.8 μm is presented. Use of a simple UD brightness model facilitates comparison between epochs and with existing data and theoretical models. Strong size variations with wavelength were recorded for all stars, probing zones of H2O, CO, OH, and dust formation. Comparison with contemporaneous spectra extracted from our data shows a strong anticorrelation between the observed angular diameter and flux. These variations consolidate the notion of a complex stellar atmosphere consisting of molecular shells with time-dependent densities and temperatures. Our findings are compared with existing data and pulsation models. The models were found to reproduce the functional form of the wavelength versus angular diameter curve well, although some departures are noted in the 2.8-3.5 μm range.

A system for EPID-based real-time treatment delivery verification during dynamic IMRT treatment

PURPOSE To design and develop a real-time electronic portal imaging device (EPID)-based delivery verification system for dynamic intensity modulated radiation therapy (IMRT) which enables detection of gross treatment delivery errors before delivery of substantial radiation to the patient. METHODS The system utilizes a comprehensive physics-based model to generate a series of predicted transit EPID image frames as a reference dataset and compares these to measured EPID frames acquired during treatment. The two datasets are using MLC aperture comparison and cumulative signal checking techniques. The system operation in real-time was simulated offline using previously acquired images for 19 IMRT patient deliveries with both frame-by-frame comparison and cumulative frame comparison. Simulated error case studies were used to demonstrate the system sensitivity and performance. RESULTS The accuracy of the synchronization method was shown to agree within two control points which corresponds to approximately ∼1% of the total MU to be delivered for dynamic IMRT. The system achieved mean real-time gamma results for frame-by-frame analysis of 86.6% and 89.0% for 3%, 3 mm and 4%, 4 mm criteria, respectively, and 97.9% and 98.6% for cumulative gamma analysis. The system can detect a 10% MU error using 3%, 3 mm criteria within approximately 10 s. The EPID-based real-time delivery verification system successfully detected simulated gross errors introduced into patient plan deliveries in near real-time (within 0.1 s). CONCLUSIONS A real-time radiation delivery verification system for dynamic IMRT has been demonstrated that is designed to prevent major mistreatments in modern radiation therapy.

First Experience With Real-Time EPID-Based Delivery Verification During IMRT and VMAT Sessions

PURPOSE Gantry-mounted megavoltage electronic portal imaging devices (EPIDs) have become ubiquitous on linear accelerators. WatchDog is a novel application of EPIDs, in which the image frames acquired during treatment are used to monitor treatment delivery in real time. We report on the preliminary use of WatchDog in a prospective study of cancer patients undergoing intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) and identify the challenges of clinical adoption. METHODS AND MATERIALS At the time of submission, 28 cancer patients (head and neck, pelvis, and prostate) undergoing fractionated external beam radiation therapy (24 IMRT, 4 VMAT) had ≥1 treatment fraction verified in real time (131 fractions or 881 fields). EPID images acquired continuously during treatment were synchronized and compared with model-generated transit EPID images within a frame time (∼0.1 s). A χ comparison was performed to cumulative frames to gauge the overall delivery quality, and the resulting pass rates were reported graphically during treatment delivery. Every frame acquired (500-1500 per fraction) was saved for postprocessing and analysis. RESULTS The system reported the mean ± standard deviation in real time χ 91.1% ± 11.5% (83.6% ± 13.2%) for cumulative frame χ analysis with 4%, 4 mm (3%, 3 mm) criteria, global over the integrated image. CONCLUSIONS A real-time EPID-based radiation delivery verification system for IMRT and VMAT has been demonstrated that aims to prevent major mistreatments in radiation therapy.

The pulsation of χ cygni imaged by optical interferometry: A novel technique to derive distance and mass of mira stars

We present infrared interferometric imaging of the S-type Mira star χ Cygni. The object was observed at four different epochs in 2005-2006 with the Infrared-Optical Telescope Array optical interferometer (H band). Images show up to 40% variation in the stellar diameter, as well as significant changes in the limb darkening and stellar inhomogeneities. Model fitting gave precise time-dependent values of the stellar diameter, and reveals presence and displacement of a warm molecular layer. The star radius, corrected for limb darkening, has a mean value of 12.1 mas and shows a 5.1 mas amplitude pulsation. Minimum diameter was observed at phase 0.94 ± 0.01. Maximum temperature was observed several days later at phase 1.02 ± 0.02. We also show that combining the angular acceleration of the molecular layer with CO (Δ_v = 3) radial velocity measurements yields a 5.9 ± 1.5 mas parallax. The constant acceleration of the CO molecules—during 80% of the pulsation cycle—lead us to argument for a free-falling layer. The acceleration is compatible with a gravitational field produced by a 2.1^(+1.5)_(–0.7) solar mass star. This last value is in agreement with fundamental mode pulsator models. We foresee increased development of techniques consisting in combining radial velocity with interferometric angular measurements, ultimately allowing total mapping of the speed, density, and position of the diverse species in pulsation-driven atmospheres.

3D Dose reconstruction: Banding artefacts in cine mode EPID images during VMAT delivery

Cine (continuous) mode images obtained during VMAT delivery are heavily degraded by banding artefacts. We have developed a method to reconstruct the pulse sequence (and hence dose deposited) from open field images. For clinical VMAT fields we have devised a frame averaging strategy that greatly improves image quality and dosimetric information for three-dimensional dose reconstruction.

Quantifying the reproducibility of lung ventilation images between 4‐Dimensional Cone Beam CT and 4‐Dimensional CT

Purpose Computed tomography ventilation imaging derived from four‐dimensional cone beam CT (CTVI4DCBCT) can complement existing 4DCT‐based methods (CTVI4DCT) to track lung function changes over a course of lung cancer radiation therapy. However, the accuracy of CTVI4DCBCT needs to be assessed since anatomic 4DCBCT has demonstrably poor image quality and small field of view (FOV) compared to treatment planning 4DCT. We perform a direct comparison between short interval CTVI4DCBCT and CTVI4DCT pairs to understand the patient specific image quality factors affecting the intermodality CTVI reproducibility in the clinic. Methods and materials We analysed 51 pairs of 4DCBCT and 4DCT scans acquired within 1 day of each other for nine lung cancer patients. To assess the impact of image quality, CTVIs were derived from 4DCBCT scans reconstructed using both standard Feldkamp‐Davis‐Kress backprojection (Symbol) and an iterative McKinnon‐Bates Simultaneous Algebraic Reconstruction Technique (Symbol). Also, the influence of FOV was assessed by deriving CTVIs from 4DCT scans that were cropped to a similar FOV as the 4DCBCT scans (Symbol), or uncropped (Symbol). All CTVIs were derived by performing deformable image registration (DIR) between the exhale and inhale phases and evaluating the Jacobian determinant of deformation. Reproducibility between corresponding CTVI4DCBCT and CTVI4DCT pairs was quantified using the voxel‐wise Spearman rank correlation and the Dice similarity coefficient (DSC) for ventilation defect regions (identified as the lower quartile of ventilation values). Mann–Whitney U‐tests were applied to determine statistical significance of each reconstruction and cropping condition. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Results The (mean ± SD) Spearman correlation between Symbol and Symbol was 0.60 ± 0.23 (range −0.03–0.88) and the DSC was 0.64 ± 0.12 (0.34–0.83). By comparison, correlations between Symbol and Symbol showed a small but statistically significant improvement with = 0.64 ± 0.20 (range 0.06–0.90, P = 0.03) and DSC = 0.66 ± 0.13 (0.31–0.87, P = 0.02). Intermodal correlations were noted to decrease with an increasing fraction of lung truncation in 4DCBCT relative to 4DCT, albeit not significantly (Pearson correlation R = 0.58, P = 0.002). Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Conclusions This study demonstrates that DIR based CTVIs derived from 4DCBCT can exhibit reasonable to good voxel‐level agreement with CTVIs derived from 4DCT. These correlations outperform previous cross‐modality comparisons between 4DCT‐based ventilation and nuclear medicine. The use of 4DCBCT scans with iterative reconstruction and minimal lung truncation is recommended to ensure better reproducibility between 4DCBCT‐ and 4DCT‐based CTVIs.

MO‐G‐213AB‐03: Simulations of Real‐Time Geometric and Dosimetic Verification System Using EPID

Purpose: To demonstrate a new method for real‐time geometric and dosimetric verification of IMRT and VMAT using synchronization between predicted and measured EPIDimages.Methods: Predicted EPIDimages were calculated using a comprehensive physics‐based model. Each predicted image represents the integrated signal expected from the delivery between control points. The measured images are acquired in cine mode and compared to the set of predicted images in real‐time. The system performs geometric verification prior to dosimetric verification. When the measured image is acquired, the algorithm automatically detects the MLC leaf positions. A comparison between the leaf positions of the measured image and control points in the MLC file is made using the cosine similarity technique. The similarity index(SI) provides geometric MLC verification and synchronization between the measured and predicted images, as a uniform dose‐rate cannot be assumed for IMRT or VMAT deliveries. The SI threshold was based on a series of experiments including 21 dynamic‐IMRT fields defining pass/fail boundary(5 brain, 8 H&N, and 8 prostate cases).If geometric verification is successful, dosimetic verification is performed with the Gamma comparison(3%,3mm).The system reports the verification Result in real‐time. Results: The system was simulated by MATLAB/SIMULINK and detected geometric and dosimetric errors during delivery. Both artificially introduced errors and clinical data were used for testing and analysis of the system performance. For a tested prostate field, the cumulative dose comparisons showed the minimum and maximum number of points with Gamma index<1 as 93.5% and 98.5%, respectively. For individual dose comparisons on the same field, the values were 87% and 97%, respectively. Conclusion: This method includes automatic MLC leaf positioning, synchronization, and dosimetric verification. The pass/fail boundary of geometry was calculated based on the experiments. This system is a useful approach to detect unexpected possible errors occurring in the clinical setting and to prevent patient overdoses during radiotherapy especially in complex deliveries such as arc‐IMRT.