Biomedical subjects

Eike Rietzel

Publications and source records attributed to Eike Rietzel.

16 recordsLinked to original sources

PubMed2006

Design of 4D treatment planning target volumes.

PURPOSE: When using non-patient-specific treatment planning margins, respiratory motion may lead to geometric miss of the target while unnecessarily irradiating normal tissue. Imaging different respiratory states of a patient allows patient-specific target design. We used four-dimensional computed tomography (4DCT) to characterize tumor motion and create treatment volumes in 10 patients with lung cancer. These were compared with standard treatment volumes. METHODS AND MATERIALS: Four-dimensional CT and free breathing helical CT data of 10 patients were acquired. Gross target volumes (GTV) were delineated on the helical scan as well as on each phase of the 4D data. Composite GTVs were defined on 4DCT. Planning target volumes (PTV) including clinical target volume, internal margin (IM), and setup margin were generated. 4DPTVs with different IMs and standard PTVs were compared by computing centroid positions, volumes, volumetric overlap, and bounding boxes. RESULTS: Four-dimensional PTVs and conventional PTVs differed in volume and centroid positions. Overlap between 4DPTVs generated from two extreme tumor positions only compared with 10 respiratory phases was 93.7%. Comparing PTVs with margins of 15 mm (IM 5 mm) on composite 4D target volumes to PTVs with 20 mm (IM 10 mm) on helical CT data resulted in a decrease in target volume sizes by 23% on average. CONCLUSION: With patient-specific characterization of tumor motion, it should be possible to decrease internal margins. Patient-specific treatment volumes can be generated using extreme tumor positions on 4DCT. To date, more than 150 patients have been treated using 4D target design.

Carcinoma, Non-Small-Cell Lung↗

PubMed2006

Simulations to design an online motion compensation system for scanned particle beams.

Respiration-induced target motion is a major problem in intensity-modulated radiation therapy. Beam segments are delivered serially to form the total dose distribution. In the presence of motion, the spatial relation between dose deposition from different segments will be lost. Usually, this results in over- and underdosage. Besides such interplay effects between target motion and dynamic beam delivery as known from photon therapy, changes in internal density have an impact on delivered dose for intensity-modulated charged particle therapy. In this study, we have analysed interplay effects between raster scanned carbon ion beams and target motion. Furthermore, the potential of an online motion strategy was assessed in several simulations. An extended version of the clinical treatment planning software was used to calculate dose distributions to moving targets with and without motion compensation. For motion compensation, each individual ion pencil beam tracked the planned target position in the lateral as well as longitudinal direction. Target translations and rotations, including changes in internal density, were simulated. Target motion simulating breathing resulted in severe degradation of delivered dose distributions. For example, for motion amplitudes of +/-15 mm, only 47% of the target volume received 80% of the planned dose. Unpredictability of resulting dose distributions was demonstrated by varying motion parameters. On the other hand, motion compensation allowed for dose distributions for moving targets comparable to those for static targets. Even limited compensation precision (standard deviation approximately 2 mm), introduced to simulate possible limitations of real-time target tracking, resulted in less than 3% loss in dose homogeneity.

Carbon↗

PubMed2006

Four-dimensional proton treatment planning for lung tumors.

PURPOSE: In proton radiotherapy, respiration-induced variations in density lead to changes in radiologic path lengths and will possibly result in geometric misses. We compared different treatment planning strategies for lung tumors that compensate for respiratory motion. METHODS AND MATERIALS: Particle-specific treatment planning margins were applied to standard helical computed tomography (CT) scans as well as to "representative" CT scans. Margins were incorporated beam specific laterally by aperture widening and longitudinally by compensator smearing. Furthermore, treatment plans using full time-resolved 4D-computed tomography data were generated. RESULTS: Four-dimensional treatment planning guaranteed target coverage throughout a respiratory cycle. Use of a standard helical CT data set resulted in underdosing the target volume to 36% of the prescribed dose. For CT data representing average target positions, coverage can be expected but not guaranteed. In comparison to this strategy, 4D planning decreased the mean lung dose by up to 16% and the lung volume receiving 20 Gy (prescribed target dose 72 Gy) by up to 15%. CONCLUSION: When the three planning strategies are compared, only 4D proton treatment planning guarantees delivery of the prescribed dose throughout a respiratory cycle. Furthermore, the 4D planning approach results in equal or reduced dose to critical structures; even the ipsilateral lung is spared.

Algorithms↗

PubMed2006

Improving retrospective sorting of 4D computed tomography data.

Respiratory correlated CT is commercially available, and we have implemented its routine clinical use in planning lung tumor patients. Its value is determined by the fidelity of the spatiotemporal data set after processing the acquired reconstructed slices. Retrospective sorting of reconstructed slices is based on respiratory phase. However, the existing commercial software inadequately models respiratory phase for about 30% of the patients, mainly due to irregularities in the respiratory cycle. We have developed software that improves phase determination and consequently leads to an improvement of retrospective data sorting to make 4DCT data acquisition feasible for routine clinical use. Peak inhalation and exhalation respiratory states are selected manually; intermediate phases are interpolated. Residual motion artifacts in the resulting 4DCT volumes are reduced and allow use of the 4D imaging studies for treatment planning.

Algorithms↗

PubMed2006

The susceptibility of IMRT dose distributions to intrafraction organ motion: an investigation into smoothing filters derived from four dimensional computed tomography data.

This study investigated the sensitivity of static planning of intensity-modulated beams (IMBs) to intrafraction deformable organ motion and assessed whether smoothing of the IMBs at the treatment-planning stage can reduce this sensitivity. The study was performed with a 4D computed tomography (CT) data set for an IMRT treatment of a patient with liver cancer. Fluence profiles obtained from inverse-planning calculations on a standard reference CT scan were redelivered on a CT scan from the 4D data set at a different part of the breathing cycle. The use of a nonrigid registration model on the 4D data set additionally enabled detailed analysis of the overall intrafraction motion effects on the IMRT delivery during free breathing. Smoothing filters were then applied to the beam profiles within the optimization process to investigate whether this could reduce the sensitivity of IMBs to intrafraction organ motion. In addition, optimal fluence profiles from calculations on each individual phase of the breathing cycle were averaged to mimic the convolution of a static dose distribution with a motion probability kernel and assess its usefulness. Results from nonrigid registrations of the CT scan data showed a maximum liver motion of 7 mm in superior-inferior direction for this patient. Dose-volume histogram (DVH) comparison indicated a systematic shift when planning treatment on a motion-frozen, standard CT scan but delivering over a full breathing cycle. The ratio of the dose to 50% of the normal liver to 50% of the planning target volume (PTV) changed up to 28% between different phases. Smoothing beam profiles with a median-window filter did not overcome the substantial shift in dose due to a difference in breathing phase between planning and delivery of treatment. Averaging of optimal beam profiles at different phases of the breathing cycle mainly resulted in an increase in dose to the organs at risk (OAR) and did not seem beneficial to compensate for organ motion compared with using a large margin. Additionally, the results emphasized the need for 4D CT scans when aiming to reduce the internal margin (IM). Using only a single planning scan introduces a systematic shift in the dose distribution during delivery. Smoothing beam profiles either based on a single scan or over the different breathing phases was not beneficial for reducing this shift.

Body Burden↗

PubMed2006

Deformable registration of 4D computed tomography data.

Four-dimensional radiotherapy requires deformable registration to track delivered dose across varying anatomical states. Deformable registration based on B-splines was implemented to register 4D computed tomography data to a reference respiratory phase. To assess registration performance, anatomical landmarks were selected across ten respiratory phases in five patients. These point landmarks were transformed according to global registration parameters between different respiratory phases. Registration uncertainties were computed by subtraction of transformed and reference landmark positions. The selection of appropriate registration masks to separate independently moving anatomical subunits is crucial to registration performance. The average registration error for five landmarks for each of five patients was 2.1 mm. This level of accuracy is acceptable for most radiotherapy applications.

Aged↗

PubMed2005

A technique for respiratory-gated radiotherapy treatment verification with an EPID in cine mode.

Respiratory gating based on external surrogates is performed in many clinics. We have developed a new technique for treatment verification using an electronic portal imaging device (EPID) in cine mode for gated 3D conformal therapy. Implanted radiopaque fiducial markers inside or near the target are required for this technique. The markers are contoured on the planning CT set, enabling us to create digitally reconstructed radiographs (DRRs) for each treatment beam. During the treatment, a sequence of EPID images can be acquired without disrupting the treatment. Implanted markers are visualized in the images and their positions in the beam's eye view are calculated off-line and compared to the reference position by matching the field apertures in corresponding EPID and DRR images. The precision of the patient set-up, the placement of the beam-gating window, as well as the residual tumour motion can be assessed for each treatment fraction. This technique has been demonstrated with a case study patient, who had three markers implanted in his liver. For this patient, the intra-fractional variation of all marker positions in the gating window had a 95% range of 4.8 mm in the SI direction (the primary axis of motion). This was about the same (5 mm) as the residual motion considered in the planning process. The inter-fractional variation of the daily mean positions of the markers, which indicates the uncertainty in the set-up procedure, was within +8.3 mm/-4.5 mm (95% range) in the SI direction for this case.

Aged↗

PubMed2005

Temporo-spatial IMRT optimization: concepts, implementation and initial results.

With the recent availability of 4D-CT, the accuracy of information on internal organ motion during respiration has improved significantly. We investigate the utility of organ motion information in IMRT treatment planning, using an in-house prototype optimization system. Four approaches are compared: (1) planning with optimized margins, based on motion information; (2) the 'motion kernel' approach, in which a more accurate description of the dose deposit from a pencil beam to a moving target is achieved either through time-weighted averaging of influence matrices, calculated for different instances of anatomy (subsets of 4D-CT data, corresponding to various phases of motion) or through convolution of the pencil beam kernel with the probability density function describing the target motion; (3) optimal gating, or tracking with beam intensity maps optimized independently for each instance of anatomy; and (4) optimal tracking with beam intensity maps optimized simultaneously for all instances of anatomy. The optimization is based on a gradient technique and can handle both physical (dose-volume) and equivalent uniform dose constraints. Optimization requires voxel mapping from phase to phase in order to score the dose in individual voxels as they move. The results show that, compared to the other approaches, margin expansion has a significant disadvantage by substantially increasing the integral dose to patient. While gating or tracking result in the best dose conformation to the target, the former elongates treatment time, and the latter significantly complicates the delivery procedure. The 'motion kernel' approach does not provide a dosimetric advantage, compared to optimal tracking or gating, but might lead to more efficient delivery. A combination of gating with the 'motion kernel' or margin expansion approach will increase the duty cycle and may provide one with the most efficient solution, in terms of complexity of the delivery procedure and dose conformality to the target.

Algorithms↗

PubMed2005

Four-dimensional image-based treatment planning: Target volume segmentation and dose calculation in the presence of respiratory motion.

PURPOSE: To describe approaches to four-dimensional (4D) treatment planning, including acquisition of 4D-CT scans, target delineation of spatio-temporal image data sets, 4D dose calculations, and their analysis. METHODS AND MATERIALS: The study included patients with thoracic and hepatocellular tumors. Specialized tools were developed to facilitate visualization, segmentation, and analysis of 4D-CT data: maximum intensity volume to define the extent of lung tumor motion, a 4D browser to examine and dynamically assess the 4D data sets, dose calculations, including respiratory motion, and deformable registration to combine the dose distributions at different points. RESULTS: Four-dimensional CT was used to visualize and quantitatively assess respiratory target motion. The gross target volume contours derived from light breathing scans showed significant differences compared with those extracted from 4D-CT. Evaluation of deformable registration using difference images of original and deformed anatomic maps suggested the algorithm is functionally useful. Thus, calculation of effective dose distributions, including respiratory motion, was implemented. CONCLUSION: Tools and methods to use 4D-CT data for treatment planning in the presence of respiratory motion have been developed and applied to several case studies. The process of 4D-CT-based treatment planning has been implemented, and technical barriers for its routine use have been identified.

Algorithms↗

PubMed2005

Four-dimensional computed tomography: image formation and clinical protocol.

Respiratory motion can introduce significant errors in radiotherapy. Conventional CT scans as commonly used for treatment planning can include severe motion artifacts that result from interplay effects between the advancing scan plane and object motion. To explicitly include organ/target motion in treatment planning and delivery, time-resolved CT data acquisition (4D Computed Tomography) is needed. 4DCT can be accomplished by oversampled CT data acquisition at each slice. During several CT tube rotations projection data are collected in axial cine mode for the duration of the patient's respiratory cycle (plus the time needed for a full CT gantry rotation). Multiple images are then reconstructed per slice that are evenly distributed over the acquisition time. Each of these images represents a different anatomical state during a respiratory cycle. After data acquisition at one couch position is completed, x rays are turned off and the couch advances to begin data acquisition again until full coverage of the scan length has been obtained. Concurrent to CT data acquisition the patient's abdominal surface motion is recorded in precise temporal correlation. To obtain CT volumes at different respiratory states, reconstructed images are sorted into different spatio-temporally coherent volumes based on respiratory phase as obtained from the patient's surface motion. During binning, phase tolerances are chosen to obtain complete volumetric information since images at different couch positions are reconstructed at different respiratory phases. We describe 4DCT image formation and associated experiments that characterize the properties of 4DCT. Residual motion artifacts remain due to partial projection effects. Temporal coherence within resorted 4DCT volumes is dominated by the number of reconstructed images per slice. The more images are reconstructed, the smaller phase tolerances can be for retrospective sorting. From phantom studies a precision of about 2.5 mm for quasiregular motion and typical respiratory periods could be concluded. A protocol for 4DCT scanning was evaluated and clinically implemented at the MGH. Patient data are presented to elucidate how additional patient specific parameters can impact 4DCT imaging.

Algorithms↗

PubMed2004

Monte Carlo simulations with time-dependent geometries to investigate effects of organ motion with high temporal resolution.

PURPOSE: To calculate the dose in time-dependent geometry, the results of three-dimensional calculations are usually performed separately and combined. This approach becomes cumbersome when high temporal resolution is required, if the geometry is complex, or if interplay effects between different, independently moving systems are to be studied. The purpose of this project was the implementation of continuous (four-dimensional [4D]) Monte Carlo simulation to study the irradiation of tumors under respiratory motion. METHODS AND MATERIALS: In taking advantage of object-oriented programming, we implemented 4D Monte Carlo dose calculation. Local dose depositions in the patient are calculated while beam configuration and organ positions are changed continuously. Deformable image registration is used to describe the CT voxel displacement over time. RESULTS: The 4D Monte Carlo technique is applied to a lung cancer case planned for proton therapy. We show that the effect of motion on the dose distribution can be simulated effectively based on statistical motion parameterizations acting on the geometry or based on patient-specific 4D CT information. CONCLUSION: We present a novel method able to calculate dose with underlying time-dependent geometry. The technique allows 4D dose calculation in arbitrary time scales in a single simulation even for double-dynamic systems (e.g., time-dependent beam delivery under organ motion).

Feasibility Studies↗

PubMed2004

Online compensation for target motion with scanned particle beams: simulation environment.

Target motion is one of the major limitations of each high precision radiation therapy. Using advanced active beam delivery techniques, such as the magnetic raster scanning system for particle irradiation, the interplay between time-dependent beam and target position heavily distorts the applied dose distribution. This paper presents a simulation environment in which the time-dependent effect of target motion on heavy-ion irradiation can be calculated with dynamically scanned ion beams. In an extension of the existing treatment planning software for ion irradiation of static targets (TRiP) at GSI, the expected dose distribution is calculated as the sum of several sub-distributions for single target motion states. To investigate active compensation for target motion by adapting the position of the therapeutic beam during irradiation, the planned beam positions can be altered during the calculation. Applying realistic parameters to the planned motion-compensation methods at GSI, the effect of target motion on the expected dose uniformity can be simulated for different target configurations and motion conditions. For the dynamic dose calculation, experimentally measured profiles of the beam extraction in time were used. Initial simulations show the feasibility and consistency of an active motion compensation with the magnetic scanning system and reveal some strategies to improve the dose homogeneity inside the moving target. The simulation environment presented here provides an effective means for evaluating the dose distribution for a moving target volume with and without motion compensation. It contributes a substantial basis for the experimental research on the irradiation of moving target volumes with scanned ion beams at GSI which will be presented in upcoming papers.

Computer Simulation↗

PubMed2004

Moving targets: detection and tracking of internal organ motion for treatment planning and patient set-up.

BACKGROUND AND PURPOSE: Clinical target volumes of the thorax and abdomen are typically expanded to account for inter- and intrafractional organ motion. Usually, such expansions are based on clinical experience and planar observations of target motion during simulation. More precise, 4-dimensional motion margins for a specific patient may improve dose coverage of mobile targets and yet limit unnecessarily large field expansions. We are studying approaches to targeting moving tumors throughout the entire treatment process, from treatment planning to beam delivery. MATERIAL AND METHODS: Radio-opaque markers were implanted under CT guidance in the liver at the gross tumor periphery. Organ motion during light respiration was volumetrically imaged by 4D Computed Tomography. Marker motion was also acquired by fluoroscopy and compared with 4DCT data. During treatment, daily diagnostic x-ray images were captured at end-exhale and -inhale for patient setup and target localization. RESULTS: Based on the time-resolved CT data, target volumes can be designed to account for respiratory motion during treatment. Motion of the tumor as derived from 4DCT was consistent with fluoroscopic motion analysis. Radiographs acquired in the treatment room enabled millimeter-level patient set-up and assessment of target position relative to bony anatomy. Daily positional variations between bony anatomy and implanted markers were observed. CONCLUSIONS: Image guided therapy, based on 4DCT imaging, fluoroscopic imaging studies, and daily gated diagonstic energy set-up radiographs is being developed to improve beam delivery precision. Monitoring internal target motion throughout the entire treatment process will ensure adequate dose coverage of the target while sparing the maximum healthy tissue.

Humans↗

PubMed2004

3D online compensation of target motion with scanned particle beam.

Intensity modulated, active beam delivery, like magnetic raster scanning, strongly relies on target immobilisation. If target motion cannot be avoided, interferences between the scanning procedure and the tumour displacement destroy the volume conformity. A prototype setup for 3D online motion compensation (3D-OMC) with a scanned particle beam is described, transferring the full potential of volume conformal irradiation to moving targets.

Humans↗

PubMed2004

Effects of motion on the total dose distribution.

The success of highly target-conformal treatments such as intensity-modulated radiotherapy (IMRT) can be compromised by motion of the inner organs and random patient setup errors. This article gives an overview of different studies that looked at the effect of organ motion and setup errors on radiation therapy dose distributions, both from a qualitative and quantitative point of view. The qualitative findings are generally applicable (ie, case independent). It is found that motion always leads to a blurring of the dose distribution. In addition, there are so-called interplay effects if the treatment delivery involves moving parts, such as multileaf collimators. After a large number of fractions, the interplay effects lead to a normal distribution of the dose value around the average blurred value. Thirdly, organ motion can also cause a spatial deformation of the dose distribution. Quantitatively it has been found that both deformation and interplay effects appear to be small (in the order of 1%-2%) in many typical clinical cases. The dominant effect is the blurring of the dose distribution, which is, in essence, independent of the treatment technique, and is not more pronounced in IMRT than in more conventional treatment techniques. However, because in IMRT there is a tendency to reduce or compromise target margins, the blurring has potentially a bigger effect on the outcome of IMRT, unless precision dose delivery techniques (such as gated or motion-synchronized beams) are used. An alternative to the use of margins is to do the planning based on blurred dose distributions.

Humans↗

PubMed2004

4D-CT imaging of a volume influenced by respiratory motion on multi-slice CT.

We propose a new scanning protocol for generating 4D-CT image data sets influenced by respiratory motion. A cine scanning protocol is used during data acquisition, and two registration methods are used to sort images into temporal phases. A volume is imaged in multiple acquisitions of 1 or 2 cm length along the cranial-caudal direction. In each acquisition, the scans are continuously acquired for a time interval greater than or equal to the average respiratory cycle plus the duration of the data for an image reconstruction. The x ray is turned off during CT table translation and the acquisition is repeated until the prescribed volume is completely scanned. The scanning for 20 cm coverage takes about 1 min with an eight-slice CT or 2 mins with a four-slice CT. After data acquisition, the CT data are registered into respiratory phases based on either an internal anatomical match or an external respiratory signal. The internal approach registers the data according to correlation of anatomy in the CT images between two adjacent locations in consecutive respiratory cycles. We have demonstrated the technique with ROIs placed in the region of diaphragm. The external approach registers the image data according to an externally recorded respiratory signal generated by the Real-Time Position Management (RPM) Respiratory Gating System (Varian Medical Systems, Palo Alto, CA). Compared with previously reported prospective or retrospective imaging of the respiratory motion with a single-slice or multi-slice CT, the 4D-CT method proposed here provides (1) a shorter scan time of three to six times faster than the single-slice CT with prospective gating; (2) a shorter scan time of two to four times improvement over a previously reported multi-slice CT implementation, and (3) images over all phases of a breathing cycle. We have applied the scanning and registration methods on phantom, animal and patients, and initial results suggest the applicability of both the scanning and the registration methods.

Algorithms↗