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W De Gersem

Publications and source records attributed to W De Gersem.

10 recordsLinked to original sources
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

Combining the advantages of step-and-shoot and dynamic delivery of intensity-modulated radiotherapy by interrupted dynamic sequences.

PURPOSE: A hybrid between step-and-shoot and dynamic operation, called interrupted dynamic sequences, was investigated for prostate intensity-modulated radiotherapy (IMRT) delivered by a multisegment close-in technique. The new delivery mode was compared to the step-and-shoot mode concerning dose distribution. METHODS AND MATERIALS: Segments suitable for dynamic transition were selected using a system of segment classes. Transitions were only allowed between two segments of the same class, keeping intended sharp in-field dose gradients unchanged. Delivery was performed by an Elekta SLiplus (Crawley, UK) linear accelerator equipped with a dynamic multileaf collimator (MLC). Because no modeling of the dose during the transitions is made, accurate dose measurements were performed. Dose profiles were measured using a linear ion chamber array (LA48, PTW-Freiburg). The suitability of this detector for measurements in sharp dose gradients was investigated first. In addition, field flatness was examined for segments with a low monitor unit (MU) count. Uncertainties in dose output were investigated using an ionization chamber (30001, PTW-Freiburg). RESULTS: Because linear array measured penumbrae are only slightly broader (< or = 0.4 mm for MLC collimated field) than those obtained using a diamond detector, the array is a good device for profile measurements. Uncertainties related with the use of low MU beam segments are very small (< 1% for segments of minimum 3 MU), giving no contra-evidence for the step-and-shoot mode. Interrupted dynamic sequences are shown to introduce only small dosimetric differences as compared to the step-and-shoot delivery. CONCLUSION: Both delivery modes, step-and-shoot and interrupted dynamic sequences, result in similar dose distributions for the forward planned prostate class solution.

Humans↗
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↗
PubMed2000

Validation of MR-based polymer gel dosimetry as a preclinical three-dimensional verification tool in conformal radiotherapy.

The aim of this work was to investigate MR-based polymer gel dosimetry as a three-dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water-filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin-spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root-mean-square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized.

Acrylamide↗
PubMed1999

Clinical delivery of intensity modulated conformal radiotherapy for relapsed or second-primary head and neck cancer using a multileaf collimator with dynamic control.

BACKGROUND AND PURPOSE: Concave dose distributions generated by intensity modulated radiotherapy (IMRT) were applied to re-irradiate three patients with pharyngeal cancer. PATIENTS, MATERIALS AND METHODS: Conventional radiotherapy for oropharyngeal (patients 1 and 3) or nasopharyngeal (patient 2) cancers was followed by relapsing or new tumors in the nasopharynx (patients 1 and 2) and hypopharynx (patient 3). Six non-opposed coplanar intensity modulated beams were generated by combining non-modulated beamparts with intensities (weights) obtained by minimizing a biophysical objective function. Beamparts were delivered by a dynamic MLC (Elekta Oncology Systems, Crawley, UK) forced in step and shoot mode. RESULTS AND CONCLUSIONS: Median PTV-doses (and ranges) for the three patients were 73 (65-78), 67 (59-72) and 63 (48-68) Gy. Maximum point doses to brain stem and spinal cord were, respectively, 67 Gy (60% of volume below 30 Gy) and 32 Gy (97% below 10 Gy) for patient 1; 60 Gy (69% below 30 Gy) and 34 Gy (92% below 10 Gy) for patient 2 and 21 Gy (96% below 10 Gy) at spinal cord for patient 3. Maximum point doses to the mandible were 69 Gy for patient 1 and 64 Gy for patient 2 with, respectively, 66 and 92% of the volume below 20 Gy. A treatment session, using the dynamic MLC, was finished within a 15-min time slot.

Adult↗
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↗
PubMed1997

Non-coplanar beam intensity modulation allows large dose escalation in stage III lung cancer.

PURPOSE: To evaluate the feasibility of dose escalation in stage III non-small cell lung cancer, we compared standard coplanar (2D) with non-coplanar beam arrangements, without (3D) and with beam intensity modulation (3D-BIM). MATERIALS AND METHODS: This study was a planning effort performed on a non-selected group of 10 patients. Starting from a serial CT scan, treatment planning was performed using Sherouse's GRATIS 3D planning system. Two target volumes were defined; gross tumor volume (GTV) defined a high-dose target volume that had to receive a dose of at least 80 Gy and GTV plus the lymph node regions with >10% probability of invasion defined an intermediate-dose target volume (GTV + N). It was our intention to irradiate GTV + N up to 56 Gy or more. If the prescribed doses on GTV and GTV + N could not be reached with either the 2D or 3D technique, a 3D-BIM plan was performed. The 3D-BIM plan was a class solution involving identical gantry angles, segment arrangements and relative segment weights for all patients. Dose volume histograms for GTV, GTV + N, lung and spinal cord were calculated. Criteria for tolerance were met if no points inside the spinal cord exceeded 50 Gy and if at least 50% of the lung volume received less than 20 Gy. Under these constraints, maximal achievable doses to GTV and GTV + N were calculated. RESULTS: In all 2D plans, spinal cord was the limiting factor and the prescribed doses for GTV and GTV + N could not be reached in any patient. The non-coplanar 3D plan resulted in a satisfying solution in 4 out of 10 patients under the same constraints. In comparison with 2D, the minimum dose in GTV + N was increased. Six patients had to be planned with the 3D-BIM technique. The theoretical minimum dose to GTV + N ranged between 56 and 98 Gy. The delivery of 80 Gy or more to GTV was possible in all patients. For a minimal dose of 80 Gy to GTV, the maximal dose to any point of the spinal cord varied between 27 and 46 Gy. The lung volume receiving more than 20 Gy ranged from 26 to 46%. CONCLUSION: The potential of 3D-BIM for dose escalation is explained as follows: (i) compared to other planning techniques, a larger amount of lung tissue can be spared by using beam directions that are well-aligned with the mediastinal structures. Such beam directions have narrow angles with the sagittal plane; (ii) dividing all beams into segments with well-specified geometrical restrictions in relation to the spinal cord and well-defined relative weights results in a lower dose to the spinal cord.

Carcinoma, Non-Small-Cell Lung↗