Bernd Hesse

Correction of patient positioning errors based on in-line cone beam CTs: clinical implementation and first experiences

BackgroundThe purpose of the study was the clinical implementation of a kV cone beam CT (CBCT) for setup correction in radiotherapy.Patients and methodsFor evaluation of the setup correction workflow, six tumor patients (lung cancer, sacral chordoma, head-and-neck and paraspinal tumor, and two prostate cancer patients) were selected. All patients were treated with fractionated stereotactic radiotherapy, five of them with intensity modulated radiotherapy (IMRT). For patient fixation, a scotch cast body frame or a vacuum pillow, each in combination with a scotch cast head mask, were used. The imaging equipment, consisting of an x-ray tube and a flat panel imager (FPI), was attached to a Siemens linear accelerator according to the in-line approach, i.e. with the imaging beam mounted opposite to the treatment beam sharing the same isocenter. For dose delivery, the treatment beam has to traverse the FPI which is mounted in the accessory tray below the multi-leaf collimator. For each patient, a predefined number of imaging projections over a range of at least 200 degrees were acquired. The fast reconstruction of the 3D-CBCT dataset was done with an implementation of the Feldkamp-David-Kress (FDK) algorithm. For the registration of the treatment planning CT with the acquired CBCT, an automatic mutual information matcher and manual matching was used.Results and discussionBony landmarks were easily detected and the table shifts for correction of setup deviations could be automatically calculated in all cases. The image quality was sufficient for a visual comparison of the desired target point with the isocenter visible on the CBCT. Soft tissue contrast was problematic for the prostate of an obese patient, but good in the lung tumor case. The detected maximum setup deviation was 3 mm for patients fixated with the body frame, and 6 mm for patients positioned in the vacuum pillow. Using an action level of 2 mm translational error, a target point correction was carried out in 4 cases. The additional workload of the described workflow compared to a normal treatment fraction led to an extra time of about 10–12 minutes, which can be further reduced by streamlining the different steps.ConclusionThe cone beam CT attached to a LINAC allows the acquisition of a CT scan of the patient in treatment position directly before treatment. Its image quality is sufficient for determining target point correction vectors. With the presented workflow, a target point correction within a clinically reasonable time frame is possible. This increases the treatment precision, and potentially the complex patient fixation techniques will become dispensable.

Correction of scatter in megavoltage cone-beam CT

The role of scatter in a cone-beam computed tomography system using the therapeutic beam of a medical linear accelerator and a commercial electronic portal imaging device (EPID) is investigated. A scatter correction method is presented which is based on a superposition of Monte Carlo generated scatter kernels. The kernels are adapted to both the spectral response of the EPID and the dimensions of the phantom being scanned. The method is part of a calibration procedure which converts the measured transmission data acquired for each projection angle into water-equivalent thicknesses. Tomographic reconstruction of the projections then yields an estimate of the electron density distribution of the phantom. It is found that scatter produces cupping artefacts in the reconstructed tomograms. Furthermore, reconstructed electron densities deviate greatly (by about 30%) from their expected values. The scatter correction method removes the cupping artefacts and decreases the deviations from 30% down to about 8%.

Tomotherapeutic portal imaging for radiation treatment verification.

In their tomotherapy concept Mackie and co-workers proposed not only a new technique for IMRT but also an appropriate and satisfactory method of treatment verification. This method allows both monitoring of the portal dose distribution and imaging of the patient anatomy during treatment by means of online CT. This would enable the detection of inaccuracies in dose delivery and patient set-up errors. In this paper results are presented showing that a single electronic portal imaging device (EPID) could deliver all data necessary to establish such a complete verification system for tomotherapy and even other IMRT techniques. Consequently it has to be shown that it is able to record both the low-intensity photon fluences encountered in tomographic imaging and the intense photon transmission of each treatment field. The detector under investigation is a video-based EPID, the BIS 710 (manufactured by Wellhöfer Dosimetrie, Schwarzenbruck, Germany). To examine the suitability of the BIS for CT at 6 MV beam quality, different phantoms were scanned and reconstructed. The agreement between a diamond detector and BIS responses is quantitative. Tomographic reconstruction of a complete set of these transmission profiles resulted in images which resolve 3 cm large objects having a (theoretical) contrast to water of less than 9%. Three millimetre objects with a 100% contrast are clearly visible. The BIS signal was shown to measure photon fluence distributions. The reconstructed images possess a spatial and contrast resolution sufficient for accurate imaging of the patient anatomy, needed for treatment verification in many clinical cases.

Scatter rejection methods in megavoltage imaging with an amorphous silicon flat panel array

X-ray scattering in megavoltage portal imaging becomes more of an issue when quantitative results are needed. This is the case in megavoltage computed tomography (MVCT) and transit dosimetry, where the absorbed dose delivered to the patient is to be reconstructed. Although sensor arrays based on amorphous silicon (a-Si) photodiodes show promising results for this application, the scatter problem has so far not been examined. In this paper portal scatter distributions are calculated by means of Monte-Carlo (MC) simulations for typical clinical parameters. The aim of the MC simulations is to design a detector which is able to reject photons and electrons scattered by the phantom. As expected the analysis of the spectrum shows that multiply scattered photons can be differentiated from singly scattered photons by means of their energy. The MC results indicate that by using a detector with a high-Z conversion plate combined with a moderately thick phosphor screen a significant fraction of low energy scattered photons and most electrons can be rejected. However, to reduce the scatter signal further a software correction method based on a dedicated scatter model is still necessary.

Characterization of a flat-panel detector for ion beam spot measurements

Dynamic beam delivery techniques are being increasingly used for cancer therapy. Scanning ion beams require extensive and time-demanding quality assurance procedures and beam tuning. Accordingly, fast measurement techniques improving the efficiency of the procedures and accommodating the safety requirements are highly desirable. Major requirements for a detector used for beam-shape measurements are high spatial resolution in two dimensions, reusability, online readout and easy handling. At the Heidelberg Ion Beam Therapy Facility (Germany), we examined the performance of the RID 256 L flat-panel detector for beam spot measurements. The two-dimensional beam profiles of proton and carbon ion beams measured were compared to measurements with radiographic films at intermediate energies using the index. The difference to the beam width measured with radiographic films of less than 3% demonstrates sufficient accuracy of ion beam width measurements possible with this detector for both proton and carbon ion beams. The beam shapes were also measured at different beam intensities. At both the highest and lowest energies available at the HIT, no beam spot-shape deformation was found with increasing beam intensities, as long as the boundary of the dynamic range was not exceeded. The signal leak along the readout direction was identified as an undesirable effect. However, due to small amplitudes and static beams, this effect is of minor importance for beam spot measurements. Distortion of results due to detector radiation damage was monitored. No detector radiation damage was observed over the experiments. Moreover, the observed short-time detector response stability (within ±0.1%) as well as medium term stability (within 0.5% in 15 months) was excellent. This flat-panel detector is compact and easy to use. Together with its low weight, this helps to speed up measurement procedures substantially. All these properties make this an ideal detector for the fast, high-resolution imaging of static ion beam spots needed for constancy measurements in daily beam quality assurance and for accelerator tuning. For daily use, radiation damage has to be monitored continuously and corrected for if necessary.

3D image guidance in radiotherapy: a feasibility study

Currently, one major research field in radiotheraphy is focused on patient setup verification and on detection of organ motion and deformation. A phantom study is performed to demonstrate the feasibility of image guidance in radiotherapy. Patient setup errors are simulated with a humanoid phantom, which is imaged using a linear accelerator and a therapy simulator to address megavoltage and kilovoltage (kV) computed tomography (CT), respectively. Projections are recorded by a flat panel imager. The various data sets of the humanoid phantom are compared by mutual information matching. The CT investigations show that the spatial resolution is better than 1.6 mm for high contrast objects. The uncertainties remaining after mutual information matching are found to be less than 1 mm for translations and 1 degree(s) for rotations. The phantom study indicates that the detection of patient setup errors as well as organ motion or deformation is possible with a high accuracy, especially if a kV X-ray tube could be attached to the linear accelerator. The presented method allows sophisticated quality assurance of beam delivery in each fraction and may even enable the use of new concepts of adaptive radiotherapy.

Validierung einer Streukorrekturmethode zur IMRT-Verifikation mit Hilfe eines Portal-Imaging-Systems

Zusammenfassung Komplexe Dosisapplikationstechniken, wie sie in der Intensitatsmodulierten Strahlentherapie (IMRT) Anwendung finden, erfordern einen moglichst effizienten Verifikationsprozess. Im Folgenden wird das Problem der Streukorrektur bei der Therapieverifikation mit Hilfe von Transmissionsbildern eines auf amorphem Silicium basierenden Electronic Portal Imaging Devices (EPID) erlautert und eine iterative Methode zur Durchfuhrung einer Streukorrektur von EPID-Aufnahmen vorgestellt, die auf Monte-Carlo-generierten Streukernen basiert. Erste Anwendungen dieser Streukorrekturmethode zur Verifikation der intensitatsmodulierten Strahlentherapie werden anhand einer MVCT- und Dosisrekonstruktion diskutiert. Um die Streukorrektur zu validieren und eine Aussage uber ihre Genauigkeit und Relevanz fur den klinischen Einsatz am Patienten treffen zu konnen, wurden unterschiedliche Experimente sowohl mit homogenen als auch mit anthropomorphen Phantomen durchgefuhrt. Es kann gezeigt werden, dass die hier vorgestellte Methode zur Korrektur der Streustrahlung zu einer deutlichen Verbesserung in der Bild- und Dosisrekonstruktion fuhrt, welche die Grundlage fur eine genaue Online-IMRT-Verifikation bilden.