CT reconstruction from portal images acquired during volumetric-modulated arc therapy

Poludniowski, Gavin; Thomas, Michael; Evans, Philip; Webb, S

doi:10.1088/0031-9155/55/19/002

Cited by 17 publications

(19 citation statements)

References 30 publications

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“…It is possible that the images obtained in the absence of errors could be made more consistent by introducing a flex map into the procedure so as to overcome panel sag (28) . The panel sag on the imaging panels used in this study, measured by determining the offset of measured images against their position predicted from the accelerator readout of MLC leaf positions, is in the order of 5 mm peak‐peak (in‐plane) and 3 mm peak‐peak (cross‐plane) (29) . Correcting for this might enable the tolerance level for the method to be lowered, thereby increasing the sensitivity of the method to errors.…”

Section: Discussionmentioning

confidence: 99%

Quality control of VMAT synchronization using portal imaging

Bedford

Chajecka-Szczygielska

Thomas

2015

J Applied Clin Med Phys

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For accurate delivery of volumetric‐modulated arc therapy (VMAT), the gantry position should be synchronized with the multileaf collimator (MLC) leaf positions and the dose rate. This study, therefore, aims to implement quality control (QC) of VMAT synchronization, with as few arcs as possible and with minimal data handling time, using portal imaging. A steel bar of diameter 12 mm is accurately positioned in the G–T direction, 80 mm laterally from the isocenter. An arc prescription irradiates the bar with a 16thinmathspacemm×220thinmathspacemm field during a complete 360° arc, so as to cast a shadow of the bar onto the portal imager. This results in a sinusoidal sweep of the field and shadow across the portal imager and back. The method is evaluated by simulating gantry position errors of 1°–9° at one control point, dose errors of 2 monitor units to 20 monitor units (MU) at one control point (0.3%–3% overall), and MLC leaf position errors of 1 mm ‐ 6 mm at one control point. Inhomogeneity metrics are defined to characterize the synchronization of all leaves and of individual leaves with respect to the complete set. Typical behavior is also investigated for three models of accelerator. In the absence of simulated errors, the integrated images show uniformity, and with simulated delivery errors, irregular patterns appear. The inhomogeneity metrics increase by 67% due to a 4° gantry position error, 33% due to an 8 MU (1.25%) dose error, and 70% due to a 2 mm MLC leaf position error. The method is more sensitive to errors at gantry angle 90°/270° than at 0°/180° due to the geometry of the test. This method provides fast and effective VMAT QC suitable for inclusion in a monthly accelerator QC program. The test is able to detect errors in the delivery of individual control points, with the possibility of using movie images to further investigate suspicious image features.PACS numbers: 87.55.Qr, 87.56.bd, 87.56.Fc

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Section: Discussionmentioning

confidence: 99%

Quality control of VMAT synchronization using portal imaging

Bedford

Chajecka-Szczygielska

Thomas

2015

J Applied Clin Med Phys

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“…In general, it is impossible to make a correct reconstruction with a limited FOV using FBP [ 14 ]. As shown in Figure 1 (b), the area blocked by binary multileaf collimator (MLC) in the sinogram has a lower X-ray intensity than that inside the FOV, so that the conventional reconstruction scheme does not successfully visualize the object.…”

Section: Methodsmentioning

confidence: 99%

“…The sinogram has often been used in the treatment verification [ 12 , 13 ], and one can come up with the visualization of treatment area from the sinogram. For conventional linear accelerators, in fact, CT reconstruction with portal images during rotational treatment such as a volumetric modulated arc therapy (VMAT) has been successfully performed [ 14 , 15 ]. Also the MV CT reconstruction during treatment in helical TomoTherapy® delivery has been first tried in Ref.…”

Section: Introductionmentioning

confidence: 99%

Reconstruction of the treatment area by use of sinogram in helical tomotherapy

et al. 2014

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BackgroundTomoTherapy (Accuray, USA) has an image-guided radiotherapy system with a megavoltage (MV) X-ray source and an on-board imaging device. This system allows one to acquire the delivery sinogram during the actual treatment, which partly includes information from the irradiated object. In this study, we try to develop image reconstruction during treatment with helical tomotherapy.FindingsSinogram data were acquired during helical tomotherapy delivery using an arc-shaped detector array that consists of 576 xenon-gas filled detector cells. In preprocessing, these were normalized with full air-scan data. A software program was developed that reconstructs 3D images during treatment with corrections as; (1) the regions outside the field were masked not to be added in the backprojection (a masking correction), and (2) each voxel of the reconstructed image was divided by the number of the beamlets passing through its voxel (a ray-passing correction).The masking correction produced a reconstructed image, however, it contained streak artifacts. The ray-passing correction reduced this artifact. Although the SNR (the ratio of mean to standard deviation in a homogeneous region) and the contrast of the reconstructed image were slightly improved with the ray-passing correction, use of only the masking correction was sufficient for the visualization purpose.ConclusionsThe visualization of the treatment area was feasible by using the sinogram in helical tomotherapy. This proposed method would be useful in the treatment verification.

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“…It has to be mentioned that not only the here used kV images but also portal images can be provided and CT reconstructions are available from both. 1 By design the LINACs are rotating very slowly with 60 s and more per rotation. For comparison, state-of-the-art clinical CT scanners need about 0.3 s per rotation.…”

Section: Introductionmentioning

confidence: 99%