The utility of megavoltage computed tomography images from a helical tomotherapy system for setup verification purposes
To evaluate the utility of relatively low-dose megavoltage computed tomography (MVCT) images from a clinical helical tomotherapy system for setup verification purposes.
Cross-sectional kilovolt computed tomography (kVCT) images were obtained for treatment planning purposes on a diagnostic third-generation CT scanner, followed by MVCT images from a helical tomotherapy system in 8 pet dogs with spontaneously occurring tumors. The kVCT and MVCT images were aligned for setup verification purposes, allowing repositioning before treatment delivery.
Tumors are readily visualized on the MVCT images. At a dose of 2-3 cGy, the MVCT images are of sufficient quality for verification of treatment setup, but soft-tissue contrast is inferior to that with conventional kVCT. The MV and kVCT images were successfully aligned. When necessary, patients undergoing helical tomotherapy were repositioned before treatment.
Megavoltage CT image quality is sufficient for tumor identification and three-dimensional setup verification in dogs with spontaneous tumors. The MVCT images can be aligned with the planning kVCT to ensure proper patient registration before treatment. Image alignment was successful in these canine patients, despite no skin markings defining patient positioning between the two scans. MVCT images facilitate setup verification, and their tomographic nature offers improvements over conventional portal imaging.
Available from: Arne Gruen
- "Modern linear and helical tomotherapy (HT) accelerators are equipped with an onboard imaging device e.g. kV (kilovoltage; cone-beam) CT scanner or MVCT scanner (megavoltage) to localize the target and are capable of applying intensity modulated radiotherapy (IMRT) or volumetric modulated arc therapy (VMAT) to deliver highly conformal dose distributions . HT uses an MVCT imaging tool  and has been established for the treatment of patients with gynecologic malignancies [3,4]. "
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To analyze setup deviations using daily megavoltage computed tomography (MVCT) and to evaluate three MVCT frequency reducing protocols for gynecologic cancer patients treated with helical tomotherapy.
We recorded the setup errors of 56 patients with gynecological cancer observed throughout their whole course by matching their daily MVCT with the planning CT. Systematic and random errors were calculated on a patient and population basis. We defined three different protocols corresponding to MVCTs from the first five fractions (FFF), the first ten fractions (FTF) or from the first and third weeks (505). We compared theoretical. setup errors calculated using these 5 or 10 early MVCT scans with the actual errors found with the remaining fractions to to analyze the residual deviations.
The total systematic (random) deviations had means of −2.0 (3.8)mm, 0.5 (3.4)mm, 0.5 (6.1)mm and −0.5° (0.9°) in vertical (V), longitudinal (LO), lateral (LA), and roll (R) directions, respectively. The proposed three MVCT protocols resulted in minor residual deviations. In all three protocols, 95% of all calculated residual deviations were less than or equal to 5 mm in all 3 directions. When examining the additional minimal CTV-PTV setup margins that were calculated based on these residual deviations, the 505 protocol would have allowed smaller margins than the FFF and FTF protocol, particularly in the V direction.
For patients with gynecologic cancer, the 505-protocol led to the lowest residual deviations and therefore might offer the best approach in reducing the frequency of pre-treatment MVCTs.
- "Helical tomotherapy's ability to generate megavoltage (MV) computed tomography (CT) images is mainly used to provide proper daily patient positioning. Past studies have shown that although their quality is surpassed by that of kilovoltage (kV) CT images, the MVCT images produced by a helical tomotherapy unit are adequate for image guidance in radiotherapy and can be used reliably for re-contouring. "
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ABSTRACT: Helical tomotherapy's ability to provide daily megavoltage (MV) computed tomography (CT) images for patient set-up verification allows for the creation of adapted plans. As plans become more complex by introducing sharper dose gradients in an effort to spare healthy tissue, inter-fraction changes of organ position with respect to plan become a limiting factor in the correct dose delivery to the target. Tomotherapy's planned adaptive option provides the possibility to evaluate the dose distribution for each fraction and subsequently adapt the original plan to the current anatomy. In this study, 30 adapted plans were created using new contours based on the daily MVCT studies of a bladder cancer patient with considerable anatomical variations. Dose to the rectum and two planning target volumes (PTVs) were compared between the original plan, the dose that was actually delivered to the patient, and the theoretical dose from the 30 adapted plans. The adaptation simulation displayed a lower dose to 35% and 50% of the rectum compared to no adaptation at all, while maintaining an equivalent dose to the PTVs. Although online adaptation is currently too time-consuming, it has the potential to improve the effectiveness of radiotherapy.
Available from: George Li
- "Therefore, automatic registration is susceptible to systematic error and/or can have a higher failure rate. Therefore, it must be validated using a visual-based image fusion method combined with manual adjustment (Forrest et al., 2004; Li et al., 2005). Since all image voxels are utilised in the automatic registration, the result is a compromise between bone structures, which is rigid, and deformable soft tissue. "
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ABSTRACT: As rigid image registration becomes unreliable in the presence of involuntary organ motion, we present a novel approach to register CT images using stable bony landmarks for image-guided patient setup. Using 3D Volumetric Image Registration (3DVIR) technique, bony anatomy is volumetrically-classified as registration landmark, while soft tissues are ignored. Based on 4DCT, it was found that the spine, posterior ribs and clavicles do not move with respiration and remain registered throughout the breathing cycle. However, mutual information based registration produces an error of 1-2 mm due to moving soft tissues. It is suggested that the 3DVIR can improve image-guided setup.
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