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F Claus

Publications and source records attributed to F Claus.

12 recordsLinked to original sources
PubMed2002

Dose conformation in IMRT for head and neck tumors: which solution to apply?

At Ghent University Hospital, IMRT for head and neck cancer is routinely performed. The desired dose distribution is defined upfront as a range of acceptable doses assigned to each voxel of volumes of interest. It was found important to specify the range of acceptable doses separately to areas of the PTV either in or outside the buildup zone as well as to areas which do or do not intersect with PTV-dose limiting organs at risk (OAR). To avoid high doses at distance from the PTV, the creation of a "surrounding" OAR which is the whole scanned volume minus the PTV was found efficient, especially if inside this OAR, subvolumes were created at increasing distance from the PTV. By specifying inside these subvolumes maximum dose constraints which decreased with distance from the PTV, conformality is secured. The creation of these additional PTV and OAR subvolumes allows comprehensive and unambiguous definition of the range of acceptable doses and thereby avoids user-interactive assignment of weights to the terms of the objective function during optimization. The efficiency of inverse planning is highly improved. Its outcome is predictable, plan evaluation is objective as the plan either does or does not comply with the predefined range of acceptable doses. Accurate reporting of the planned dose distribution is facilitated by description of the dose range to all volumes. The expense of this procedure is modest and lays mostly 1) in the creation of the subvolumes, which can be done semi-automatically by modern image segmentation tools and 2) in the inclusion of constraints to all subvolumes into the objective function.

Belgium↗
PubMed2001

Leaf position optimization for step-and-shoot IMRT.

PURPOSE: To describe the theoretical basis, the algorithm, and implementation of a tool that optimizes segment shapes and weights for step-and-shoot intensity-modulated radiation therapy delivered by multileaf collimators. METHODS AND MATERIALS: The tool, called SOWAT (Segment Outline and Weight Adapting Tool) is applied to a set of segments, segment weights, and corresponding dose distribution, computed by an external dose computation engine. SOWAT evaluates the effects of changing the position of each collimating leaf of each segment on an objective function, as follows. Changing a leaf position causes a change in the segment-specific dose matrix, which is calculated by a fast dose computation algorithm. A weighted sum of all segment-specific dose matrices provides the dose distribution and allows computation of the value of the objective function. Only leaf position changes that comply with the multileaf collimator constraints are evaluated. Leaf position changes that tend to decrease the value of the objective function are retained. After several possible positions have been evaluated for all collimating leaves of all segments, an external dose engine recomputes the dose distribution, based on the adapted leaf positions and weights. The plan is evaluated. If the plan is accepted, a segment sequencer is used to make the prescription files for the treatment machine. Otherwise, the user can restart SOWAT using the new set of segments, segment weights, and corresponding dose distribution. The implementation was illustrated using two example cases. The first example is a T1N0M0 supraglottic cancer case that was distributed as a multicenter planning exercise by investigators from Rotterdam, The Netherlands. The exercise involved a two-phase plan. Phase 1 involved the delivery of 46 Gy to a concave-shaped planning target volume (PTV) consisting of the primary tumor volume and the elective lymph nodal regions II-IV on both sides of the neck. Phase 2 involved a boost of 24 Gy to the primary tumor region only. SOWAT was applied to the Phase 1 plan. Parotid sparing was a planning goal. The second implementation example is an ethmoid sinus cancer case, planned with the intent of bilateral visus sparing. The median PTV prescription dose was 70 Gy with a maximum dose constraint to the optic pathway structures of 60 Gy. RESULTS: The initial set of segments, segment weights, and corresponding dose distribution were obtained, respectively, by an anatomy-based segmentation tool, a segment weight optimization tool, and a differential scatter-air ratio dose computation algorithm as external dose engine. For the supraglottic case, this resulted in a plan that proved to be comparable to the plans obtained at the other institutes by forward or inverse planning techniques. After using SOWAT, the minimum PTV dose and PTV dose homogeneity increased; the maximum dose to the spinal cord decreased from 38 Gy to 32 Gy. The left parotid mean dose decreased from 22 Gy to 19 Gy and the right parotid mean dose from 20 to 18 Gy. For the ethmoid sinus case, the target homogeneity increased by leaf position optimization, together with a better sparing of the optical tracts. CONCLUSIONS: By using SOWAT, the plans improved with respect to all plan evaluation end points. Compliance with the multileaf collimator constraints is guaranteed. The treatment delivery time remains almost unchanged, because no additional segments are created.

Humans↗
PubMed2001

An anatomy-based beam segmentation tool for intensity-modulated radiation therapy and its application to head-and-neck cancer.

PURPOSE: In segmental intensity-modulated radiation therapy (IMRT), the beam fluences result from superposition of unmodulated beamlets (segments). In the inverse planning approach, segments are a result of ''clipping'' intensity maps. At Ghent University Hospital, segments are created by an anatomy-based segmentation tool (ABST). The objective of this report is to describe ABST. METHODS AND MATERIALS: For each beam direction, ABST generates segments by a multistep procedure. During the initial steps, beam's eye view (BEV) projections of the planning target volumes (PTVs) and organs at risk (OARs) are generated. These projections are used to make a segmentation grid with negative values across the expanded OAR projections and positive values elsewhere inside the expanded PTV projections. Outside these regions, grid values are set to zero. Subsequent steps transform the positive values of the segmentation grid to increase with decreasing distance to the OAR projections and to increase with longer pathlengths measured along rays from their entrance point through the skin contours to their respective grid point. The final steps involve selection of iso-value lines of the segmentation grid as segment outlines which are transformed to leaf and jaw positions of a multileaf collimator (MLC). Segment shape approximations, if imposed by MLC constraints, are done in a way that minimizes overlap between the expanded OAR projections and the segment aperture. RESULTS: The ABST procedure takes about 3 s/segment on a Compaq Alpha XP900 workstation. In IMRT planning problems with little complexity, such as laryngeal (example shown) or thyroid cancer, plans that are in accordance with the clinical protocol can be generated by weighting the segments generated by ABST without further optimization of their shapes. For complex IMRT plans such as paranasal sinus cancer (not shown), ABST generates a start assembly of segments from which the shapes and weights are further optimized. CONCLUSIONS: ABST is a fast procedure to generate a set of segments for IMRT planning. The plan is finalized by assigning weights to the segments or by direct optimization of segment shapes and weights. ABST allows us to avoid the step of translating optimized intensity maps to sequences of segments.

Algorithms↗
PubMed2001

An implementation strategy for IMRT of ethmoid sinus cancer with bilateral sparing of the optic pathways.

PURPOSE: To develop a protocol for the irradiation of ethmoid sinus cancer, with the aim of sparing binocular vision; of developing a strategy of intensity-modulated radiation therapy (IMRT) planning that produces dose distributions that (1) are consistent with the protocol prescriptions and (2) are deliverable by static segmental IMRT techniques within a 15-minute time slot; of fine tuning the implementation strategy to a class solution approach that is sufficiently automated and efficient, allowing routine clinical application; of reporting on the early clinical implementation involving 11 patients between February 1999 and July 2000. patients and methods: Eleven consecutive T1-4N0M0 ethmoid sinus cancer patients were enrolled in the study. For Patients 1-8, a first protocol was implemented, defining a planning target volume prescription dose of 60 to 66 Gy in 30-33 fractions and a maximum dose (Dmax) of 50 Gy to optic pathway structures and spinal cord and limit of 60 Gy to brainstem. For Patients 9-11, an adapted (now considered mature) protocol was implemented, defining a (planning target volume) prescription dose of 70 Gy in 35 fractions and a Dmax to optic pathway structures and brainstem of 60 Gy and to spinal cord of 50 Gy. RESULTS: The class solution-directed strategy developed during this study reduced the protocol translation process from a few days to about 2 hours of planner time. The mature class solution involved the use of 7 beam incidences (20-37 segments), which could be delivered within a 15-minute time slot. Acute side effects were limited and mild. None of the patients developed dry eye syndrome or other visual disturbances. The follow-up period is too short for detection of retinopathy or optic nerve and chiasm toxicity. CONCLUSION: Conventional radiotherapy of ethmoid sinus tumors is associated with serious morbidity, including blindness. We hypothesize that IMRT has the potential to save binocular vision. The dose to the optic pathway structures can be reduced selectively by IMRT. Further enrollment of patients and longer follow-up will show whether the level of reduction tested by the clinical protocol is sufficient to save binocular vision. An adaptive strategy of IMRT planning was too inefficient for routine clinical practice. A class solution-directed strategy improved efficiency by eliminating human trial and error during the IMRT planning process.

Adenocarcinoma↗
PubMed2001

Evaluation of a leaf position optimization tool for intensity modulated radiation therapy of head and neck cancer.

BACKGROUND AND PURPOSE: Since 1996, patients are treated at Ghent University Hospital with a multi-segment technique using MultiLeaf Collimators. The segments were obtained by using the Beam's eye view projections of the planning target volume (PTV) and the organs at risk (OARs), after which the segments weights were optimized. To investigate if optimization of the leaf positions would further improve the intensity modulated radiation therapy (IMRT) plans, a tool optimizing leaf positions and segment weights simultaneously, was developed. This tool is called SOWAT, which is the acronym for segment outline and weight adapting tool. MATERIAL AND METHODS: The tool evaluates the effects of changing the position of each collimating leaf of all segments on the value of the objective function. Only changes that improve the value of the objective function are retained. Between December 1999 and January 2001, 30 head and neck patients were treated with IMRT. Two patient groups were distinguished: pharyngeal and laryngeal tumors (n=17) and sinonasal tumors (n=13). A specific set of physical endpoints was evaluated for each group. Dose statistics of the treatment plans without and with SOWAT were analyzed. RESULTS: When using SOWAT for the pharyngeal and laryngeal cases, the PTV dose homogeneity increased with a median of 11% (range 2-27%), while the maximum dose to the spinal cord was decreased for 14 of the 17 patients. In four plans where parotid function preservation was a goal, the parotid mean dose was lower than 26 Gy in one plan without SOWAT, and in four plans with SOWAT. For the sinonasal tumors, the PTV dose homogeneity increased with a median of 7% (range 1-14%). SOWAT lowered the mean dose to 53 of the 63 optic pathway structures (retina, optic nerve and optic chiasm). SOWAT leaves the number of segments unchanged and has little or no effect on the delivery time. CONCLUSIONS: SOWAT is a powerful tool to perform the final optimization of IMRT plans, without increasing the complexity of the plan or the delivery time.

Algorithms↗
PubMed2001

An isocenter position verification device for electronic portal imaging: physical and dosimetrical characteristics.

The physical and dosimetrical characteristics of a device, designed to visualize the isocenter position on electronic portal images, were examined. The device, to be mounted on the gantry head of the accelerator, containing five spheric lead markers, was designed in order to visualize the isocenter position on portal images. A quality control device was designed to check the reliability of this technique. The disturbance of the dose distribution by the markers was studied with gel dosimetry. The use of markers resulted in a precise and accurate method to visualize the isocenter on portal images. A maximum underdosage of 11%, due to attenuation by the markers, was observed. The use of markers to visualize the isocenter position on portal images, is a fast and reliable method when analyzing patient setup errors with online electronic portal imaging.

Diagnostic Imaging↗
PubMed2001

Improved delivery efficiency for step and shoot intensity modulated radiotherapy using a fast-tuning magnetron.

The delivery efficiency of step and shoot intensity modulated radiotherapy (IMRT) has been improved by the installation of fast-tuning magnetrons into three travelling wave linear accelerators. The IMRT delivery efficiency and the beam start-up performance have been compared before and after installation. Start-up and inter sub-field times were reduced by an average of 3.0 s. A typical start-up time from depression of the start button to beam on is now around 4 s. Delivery efficiency for a variety of clinical and quality control prescriptions was improved by an average of 30.7% (range 7.4-60.9%), depending on a complex combination of the number of sub-fields, distance moved by leaves and dose rate. For the oldest accelerator (7 years old), dosimetric accuracy was significantly improved for low dose sub-fields. The dose output was within 2% for a 1 monitor unit (MU) sub-field and 1% for a 2 MU sub-field. The two newer accelerators displayed similar or better dose characteristics even before fast-tuning magnetron installation. Beam symmetries and flatnesses were acceptable at all energies and dose rates, and showed no obvious degradation in low dose sub-fields. It is recommended that fast-tuning magnetrons are adopted for accelerators of this design performing step and shoot IMRT.

Magnetics↗
PubMed1999

Postoperative radiotherapy of paranasal sinus tumours: a challenge for intensity modulated radiotherapy.

BACKGROUND AND PURPOSE: Intensity modulated radiotherapy (IMRT) is used in our department for treatment of paranasal sinuses. We describe the methodology that was developed together with the clinical implementation, illustrated by a case report. MATERIAL AND METHODS: Patient history, treatment and short follow-up are described. An IMRT, obtained by superposition of static beam segments was implemented. Electronic portal images, compared to digitally reconstructed radiographs (DRR) were used to evaluate and adjust patient positioning. RESULTS, DISCUSSION AND CONCLUSION: IMRT is an appropriate and feasible treatment technique for head and neck cancer in anatomical regions that are difficult to treat. A high tumour dose can be combined with a good sparing of the surrounding organs at risk (OAR's).

Adenocarcinoma↗
PubMed1987

[Diagnostic sensitivity of sonography in varicose and post-thrombotic venous insufficiency].

We report on the application possibilities of sonography in phlebology. Sonographic examination of 131 lower legs showing various clinical forms of chronic venous insufficiency is discussed. Ultrasound offers a timesaving, non-invasive, and reproducible technique to detect insufficiencies concerning orifice or trunk of the great and lower saphenous veins as well as the perforating veins. The method allows varying diagnostic sensitivities. Thrombotic changes in the deep veins may be demonstrated with relative accurateness. As to insufficiencies concerning the epifascial and perforating venous system (in epifascial chronic venous insufficiency, stage I to IV), the question of surgical or sclerotherapeutic treatment can be settled in most cases according to the sonographical findings alone. Subfascial forms of chronic venous insufficiency require additional invasive diagnostic techniques, such as plethysmography, phlebography, and venous pressure measurements. In consideration of the high diagnostic importance of ultrasound, sonographic devices should be available and applied in a large number of phlebologic practices.

Blood Flow Velocity↗
PubMedDate not supplied

[Intensity modulated radiotherapy with dynamic multileaf collimator. Technique and clinical experience].

Since early 1997, dynamic multileaf collimators (DMLCs) have been used in our division for intensity modulated radiotherapy (IMRT). We have used IMRT to: irradiate concave targets (head and neck, paraspinal tumors); combine beams with shallow hinge angles (mediastinum, lung tumors); deliver intentionally inhomogeneous dose distributions (prostate, paranasal sinuses, brain tumors). IMRT is now our standard treatment for locoregional relapse (after high-dose radiotherapy) for head and neck cancer and for radical treatment of localized prostate cancer. For a variety of other tumors, conventional 3D-plans are compared with IMRT-plans, the latter being clinically implemented if superior. We developed a geometry based IMRT planning strategy to create assemblies of static intensity modulated (IM)-beams which consist of uniform (unmodulated) segments. By a translator program, segments are combined in a single prescription which allows delivery under computer control. Cost-containment is further improved by automation of the planning. After manual or semi-automated contouring of PTV and the organs at risk, prostate IMRT plans, based on a class solution, are generated and optimized by a computer. IMRT for pharyngeal relapses and most other tumor sites is planned semi-automatically. IMRT replaces gradually conventional treatments in our division. Interesting dose distributions generated by IMRT allow a better sparing of normal tissues with decreased acute and late toxicity, and offer a window for further dose escalation.

Algorithms↗