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T Bortfeld

Publications and source records attributed to T Bortfeld.

At least 19 recordsLinked to original sources
PubMed2000

The role of proton therapy in the treatment of large irradiation volumes: a comparative planning study of pancreatic and biliary tumors.

PURPOSE: The purpose of this study was to examine the potential benefit of proton therapy for abdominal tumors. Extensive comparative planning was conducted investigating the most up-to-date photon and proton irradiation technologies. METHODS AND MATERIALS: A number of rival plans were generated for four patients: two inoperable pancreatic tumors, one inoperable and one postoperative biliary duct tumor. The dose prescription goal for these large targets was 50 Gy, followed by a boost dose up to 20 Gy to a smaller planning target volume (PTV). Photon plans were developed using "forward" planning of coplanar and noncoplanar conformal fields and "inverse" planning of intensity-modulated (IM) fields. Proton planning was simulated as administered using the so called spot-scanning technique. Plans were evaluated on the basis of normal tissues' dose-volume constraints (Emami B, Lyman J, Brown A, et al. Tolerance of normal tissue to therapeutic irradiation. Int J Radiat Oncol Biol Phys 1990;21:109-122) and coverage of treatment volumes with prescribed doses. RESULTS: For all cases, none of the forward calculated photon plans was able to deliver 50 Gy to large PTVs at the same time respecting the dose-volume constraints on all critical organs. Nine evenly spaced IM fields achieved or nearly achieved all maximum dose constraints to critical structures for two out of three inoperable patients. IM plans also obtained good results for the postoperative patient, even though the dose to the liver was very close to the maximum allowed. In all cases, photon irradiation of large PTV1s to 50 Gy followed by a 20 Gy boost entailed a risk very close to or higher than 5% for serious complications to the kidneys, liver, or bowel. Simple arrangements of 2, 3, and 4 proton fields obtained better dose conformation to the target, allowing the delivery of planned doses including the boost to all patients, without excessive risk of morbidity. Dose homogeneity inside the targets was also superior with protons. CONCLUSION: For the irradiation of large PTVs located in the abdominal cavity, where multiple, parallel structured organs surround the target volumes, proton therapy, delivered with a sophisticated isocentric technique, has the potential to achieve superior dose distributions compared with state-of-the-art photon irradiation techniques. IM photon plans obtain better results in the postoperative case, because the reduced volume lessens the effect of the unavoidable increase of integral dose to surrounding tissues.

Algorithms↗
PubMed2000

Correlation between CT numbers and tissue parameters needed for Monte Carlo simulations of clinical dose distributions.

We describe a new method to convert CT numbers into mass density and elemental weights of tissues required as input for dose calculations with Monte Carlo codes such as EGS4. As a first step, we calculate the CT numbers for 71 human tissues. To reduce the effort for the necessary fits of the CT numbers to mass density and elemental weights, we establish four sections on the CT number scale, each confined by selected tissues. Within each section, the mass density and elemental weights of the selected tissues are interpolated. For this purpose, functional relationships between the CT number and each of the tissue parameters, valid for media which are composed of only two components in varying proportions, are derived. Compared with conventional data fits, no loss of accuracy is accepted when using the interpolation functions. Assuming plausible values for the deviations of calculated and measured CT numbers, the mass density can be determined with an accuracy better than 0.04 g cm(-3). The weights of phosphorus and calcium can be determined with maximum uncertainties of 1 or 2.3 percentage points (pp) respectively. Similar values can be achieved for hydrogen (0.8 pp) and nitrogen (3 pp). For carbon and oxygen weights, errors up to 14 pp can occur. The influence of the elemental weights on the results of Monte Carlo dose calculations is investigated and discussed.

Female↗
PubMed2000

Direct measurement and analytical modeling of scatter in portal imaging.

In this study a direct measurement of scatter in portal imaging for various air gaps and scatterer thicknesses at a beam energy of 6 MV is presented. The experimental data are compared with results from a Monte Carlo (MC) scatter model. In the regime where the air gap is larger than 9.3 cm the MC and the experiment agree. Based on this MC model an analytical model is developed, which takes all important interaction processes into account. It comprises a rigorous treatment of first order scattering and an estimation of photons scattered more than once within the phantom. This estimation is based on the assumption that higher order scattering can be considered as isotropically distributed around a certain scatter origin located in the midplane of the phantom. It is found that relative deviations between the MC model and the analytical model are of 2% to 3% in regions where scattering is very large.

Biophysical Phenomena↗
PubMed2000

Intensity modulated radiotherapy with charged particle beams: studies of inverse treatment planning for rotation therapy.

The optimization of intensity modulated radiotherapy (IMRT) for charged particle beams is a necessary prerequisite to evaluate the clinical potential of this treatment modality in comparison to IMRT with high energy photons. A theoretical study for IMRT with charged particle beams delivered by rotation therapy is presented. First, the inverse problem for two-dimensional rotation therapy with arbitrary depth dose curves is formulated. Then a numerical strategy is devised to calculate fluence profiles for the simplified case of arbitrary rotationally invariant dose distributions. This mathematical framework is applied to study various aspects of charged particle IMRT. A central topic of the investigation is the evaluation of dose delivery, based on distal edge tracking (DET) and intensity modulation. The potential of DET-IMRT with charged particle beams is studied in comparison to an optimal, conventional dose delivery technique, which employs the concept of a spread-out Bragg peak (SOBP). Moreover, a comparison to photon IMRT is provided for simple geometric dose patterns. The technique of DET-IMRT for the delivery of a homogeneous target dose is only feasible for targets up to a critical radius, depending on the individual shape of the employed Bragg peak. The irradiation of larger targets requires energy modulation in addition to the range modulation for DET. The accurate placement of the Bragg peak with respect to the target edge is found to be of potential importance. Comparing dose delivery via DET-IMRT with the optimal SOBP technique revealed a significant advantage of DET-IMRT, especially a saved integral dose in target-adjacent healthy tissues of up to 30%, and a reduction of the penumbra at the target edge by almost 50%. A saving in integral dose to healthy tissues by a factor of 2-3 was observed for DET-IMRT in comparison to IMRT with high energy photons.

Biophysical Phenomena↗
PubMed1999

Intensity modulation with the "step and shoot" technique using a commercial MLC: a planning study. Multileaf collimator.

PURPOSE/OBJECTIVE: For complex planning situations where organs at risk (OAR) surrounding the target volume place stringent constraints, intensity-modulated treatments with photons provide a promising solution to improve tumor control and/or reduce side effects. One approach for the clinical implementation of intensity-modulated treatments is the use of a multileaf collimator (MLC) in the "step and shoot" mode, in which multiple subfields are superimposed for each beam direction to generate stratified intensity distributions with a discrete number of intensity levels. In this paper, we examine the interrelation between the number of intensity levels per beam for various numbers of beams, the conformity of the resulting dose distribution, and the treatment time on a commercial accelerator (Siemens Mevatron KD2) with built-in MLC. METHODS AND MATERIALS: Two typical, clinically relevant cases of patients with head and neck tumors were selected for this study. Using the inverse planning technique, optimized treatment plans are generated for 3-25 evenly distributed coplanar beams as well as noncoplanar beams. An iterative gradient method is used to optimize a physical treatment objective that is based on the specified target dose and individual dose constraints assigned to each organ at risk (brain stem, eyes, optic nerves) by the radiation oncologist. The intensity distribution of each beam is discretized within the inverse planning program into three to infinitely many intensity levels or strata. These stratified intensity distributions are converted into MLC leaf position sequences, which can be subsequently transferred via computer link to the linac console, and can be delivered without user intervention. The quality of the plan is determined by comparing the values of the objective function, dose-volume histograms (DVHs), and isodose distributions. RESULTS: Highly conformal dose distributions can be achieved with five intensity levels in each of seven beams. The merit of using more intensity levels or more beams is relatively small. Acceptable results are achievable even with three levels only. On average, the number of subfields per beam is about 2-2.5 times the number of intensity levels. The average treatment time per subfield is about 20 s. The total treatment time for the three-level and seven-beam case with a total of 39 subfields is 13 min. CONCLUSION: Optimizing stratified intensity distributions in the inverse planning process allows us to achieve close to optimum results with a surprisingly small number of intensity levels. This finding may help to facilitate and accelerate the delivery of intensity-modulated treatments with the "step and shoot" technique.

Brain Stem↗
PubMed1999

A treatment planning inter-comparison of proton and intensity modulated photon radiotherapy.

PURPOSE: A comparative treatment planning study has been undertaken between standard photon delivery techniques,b intensity modulated photon methods and spot scanned protons in order to investigate the merits and limitations of each of these treatment approaches. METHODS: Plans for each modality were performed using CT scans and planning information for nine patients with varying indications and lesion sites and the results have been analysed using a variety of dose and volume based parameters. RESULTS: Over all cases, it is predicted that the use of protons could lead to a reduction of the total integral dose by a factor three compared to standard photon techniques and a factor two compared to IM photon plans. In addition, in all but one Organ at Risk (OAR) for one case, protons are predicted to reduce both mean OAR dose and the irradiated volume at the 50% mean target dose level compared to both photon methods. However, when considering the volume of an OAR irradiated to 70% or more of the target dose, little difference could be shown between proton and intensity modulated photon plans. On comparing the magnitude of dose hot spots in OARs resulting from the proton and IM photon plans, more variation was observed, and the ranking of the plans was then found to be case and OAR dependent. CONCLUSIONS: The use of protons has been found to reduce the medium to low dose load (below about 70% of the target dose) to OARs and all non-target tissues compared to both standard and inversely planned photons, but that the use of intensity modulated photons can result in similar levels of high dose conformation to that afforded by protons. However, the introduction of inverse planning methods for protons is necessary before general conclusions on the relative efficacy of photons and protons can be drawn.

Algorithms↗
PubMed1999

Optimized planning using physical objectives and constraints.

Intensity-modulated radiation therapy (IMRT) allows one to achieve a better conformation of the high-dose region to the prescribed tumor target volume than uniform beam therapy, especially in complex treatment situations. Still, perfect conformation is impossible. Hence the goal of optimized IMRT planning or inverse planning is to find the beam profiles that yield the optimum among the physically achievable treatment plans. The principal physical advantage of IMRT is best exploited if the optimization is driven by physical criteria. This article presents an overview of such physical, yet clinically relevant, criteria along with optimization algorithms that take these criteria into account. Practical computer implementations are described, which allow one to perform the optimization in an interactive manner within a few minutes. The application of these methods to some complex clinical example cases is presented, and the results are compared with uniform beam treatment plans and with biologically optimized plans.

Algorithms↗
PubMed1999

Inverse planning for x-ray rotation therapy: a general solution of the inverse problem.

Rotation therapy with photons is currently under investigation for the delivery of intensity modulated radiotherapy (IMRT). An analytical approach for inverse treatment planning of this radiotherapy technique is described. The inverse problem for the delivery of arbitrary 2D dose profiles is first formulated and then solved analytically. In contrast to previously applied strategies for solving the inverse problem, it is shown that the most general solution for the fluence profiles consists of two independent solutions of different parity. A first analytical expression for both fluence profiles is derived. The mathematical derivation includes two different strategies, an elementary expansion of fluence and dose into polynomials and a more practical approach in terms of Fourier transforms. The obtained results are discussed in the context of previous work on this problem.

Mathematics↗
PubMed1999

Fast and exact 2D image reconstruction by means of Chebyshev decomposition and backprojection.

A new algorithm for the reconstruction of two-dimensional (2D) images from projections is described. The algorithm is based on the decomposition of the projections into Chebyshev polynomials of the second kind, which are the ideal basis functions for this application. The Chebyshev decomposition is done via the fast discrete sine transform. A discrete reconstruction filter is applied that corresponds to the ramp filter used in standard filtered backprojection (FBP) reconstruction. In contrast to FBP, the filter is applied to the Chebyshev coefficients and not to the Fourier coefficients of the projections. Then the reconstructed image is simply obtained by means of backprojection. Consequently, the method can be considered as a Chebyshev-domain filtered backprojection (CD-FBP). The total calculation time is dominated by the backprojection step only and is comparable to FBP. The merits of CD-FBP as compared with standard FBP are that: (a) The result is exact if the 2D function to be reconstructed can be decomposed into polynomials of finite degree, and if the sampling is adequate. Otherwise a polynomial approximation results. (b) The algorithm is inherently discrete. (c) It is particularly well suited for reconstructions from projections with non-equidistant samples that occur for instance in 2D PET (positron emission tomography) imaging and in a special form of fan beam scanning. Examples of applications comprise reconstructions of the Shepp and Logan head phantom in various sampling geometries, and a real PET test object. In the PET example an increased resolution is observed in comparison with standard FBP.

Algorithms↗
PubMed1998

[Inverse radiotherapy planning for intensity modulated photon fields].

Substantial improvement in conformal radiotherapy is possible using modulated irradiation fields. Such modulated fields may be generated even with conventional accelerators by means of individual metal compensators or with the recently available dynamic multileaf collimators (MLC). For treatment planning a new kind of planning program is required that can calculate the 2 D-intensity matrices for each photon field. At the German Cancer Research Center (Deutsches Krebsforschungszentrum) such a program has been developed under the name of "KonRad" (Conformal Radiotherapy). Although it is an independent application, it is proposed for clinical usage to supplement a planning system that is already present. So-called inverse planning differs from conventional 3 D planning, as the trial-and-error approach for finding good field parameters is nearly completely avoided. Instead, the radio-oncologist is given the chance directly to specify medically oriented criteria like the prescription dose in the target volume, maximal tolerance dose values for each organ at risk and their weighting factors. In addition, the so-called DVH optimization allows aimed, partial overdosage, especially in parallelly structured organs in order to obtain better overall planning results. Because of very fast dose calculation in connection with rapidly converging gradient optimization and an intuitive use interface, the planning is done in a comfortable and interactive manner. Using a workstation or a PC, a typical plan can be created within a few minutes.

Equipment Design↗
PubMed1998

[Stereotactic conforming irradiation of choroid metastases].

UNLABELLED: The aim of our study was to develop noninvasive stereotactic radiotherapy for patients with choroidal metastases. METHODS: The head of the patient was immobilized by an individual cast mask. The target volume and adjacent critical structures were three-dimensionally segmented based on ophthalmological findings and MRI. The beam angles were optimized by a beam's eye view technique with a micro-multileaf collimator. We use a linear accelerator with 6-MeV photons. A patient was treated in a phase I/II trial with a single dose of 22.5 Gy. RESULTS: The accuracy of stereotactic positioning of the eye was 0.8 mm. The dose gradients were 15% mm. It was possible to spare the lens and the lacrimal gland in all cases. The treatment with four to six fields took 35-50 min and was tolerated without acute complications. CONCLUSIONS: We introduce a new method to treat choroid metastases by stereotactic radiotherapy. This method allows to concentrate the dose to the target and to spare adjacent critical structures. One of the advantages of this noninvasive treatment is the short treatment time. The clinical role of this method has to be evaluated based on long-term results of tumor control and visual outcome in these patients.

Adenocarcinoma↗
PubMed1997

Potential role of proton therapy in the treatment of pediatric medulloblastoma/primitive neuroectodermal tumors: reduction of the supratentorial target volume.

PURPOSE: One of the components of radiotherapy (RT) in medulloblastoma/primitive neuroectodermal tumors is the prophylactic irradiation of the whole brain (WBI). With the aim of reducing late neuropsychologic morbidity a CT-scan-based dosimetric study was undertaken in which treatment was confined mainly or exclusively to supratentorial sites considered at high risk for disease recurrence. METHODS AND MATERIALS: A comparative dosimetric study is presented in which a three field (two laterals and one posterior) proton plan (spot scanning method) is compared with a two-field conventional WBI 6 MV x-ray plan, to a 6-field "hand-made" 6 MV x-ray plan, and to a computer-optimized 9-field "inverse" 15 MV x-ray plan. For favorable patients, 30 Gy were delivered to the ventricles and main cisterns, the subfrontal and subtemporal regions, and the posterior fossa. For the unfavorable patients, 10 Gy WBI preceeded a boost to 30 Gy to the same treatment volume chosen for favorable patients. The dose distribution was evaluated with dose-volume histograms to examine the coverage of the targets as well as the dose to the nontarget brain and optical structures. In addition, the risks of radiation-related late neuropsychologic effects after WBI were collected from the literature and used to predict normal tissue complication probabilities (NTCPs) for an intelligence quotient deficit after treatment with photon or proton beams. RESULTS: Proton beams succeeded better in reducing the dose to the brain hemispheres and eye than any of the photon plans. A 25.1% risk of an IQ score <90 was predicted after 30 Gy WBI. Almost a 10% drop in the predicted risk was observed when using proton beams in both favorable and unfavorable patients. However, predicted NTCPs for both optimized photon plans ("hand made" and "inverse") were only slightly higher (0.3-2.5%) than those of proton beams. An age-modifying factor was introduced in the predictive NTCP model to assess for IQ differences in relation with age at irradiation. Children with ages between age 4 to 8 benefitted most from the dose reduction in this exercise (similar NTCP predictions for both proton and "inverse" plans). CONCLUSION: Modulated proton beams may help to significantly reduce the irradiation of normal brain while optimally treating the supratentorial subsites at higher risk for relapse. A decrease in morbidity can be expected from protons and both optimized proton plans compared to WBI.

Brain Neoplasms↗
PubMed1997

Inverse radiotherapy planning for a concave-convex PTV in cervical and upper mediastinal regions. Simulation of radiotherapy using an Alderson-RANDO phantom. Planning target volume.

AIM: Three-dimensional inverse treatment planning with modulated beams was applied for dosimetric optimization of a lengthy (22 cm) and complex (concave-convex) shaped planning target volume (PTV) in the cervical and upper mediastinal regions. MATERIAL AND METHOD: The planning was done for 9 coplanar beams spaced evenly at 40 intervals. Properties of 15 MV photons from a linear accelerator were simulated. The optimization of the fluence modulation profiles for each beam was based on a definition of the desired/permitted relative dose levels in the PTV and organs at risk, and a definition of the strengths of the constraints to achieve these objectives. RESULTS: An adequate dose delivery to the PTV and protection of the spinal cord are completely achievable. The dose delivered to the lungs is clinically acceptable with respect to the risk of radiation-induced pneumonitis. For reasons of physics, no further decrease in the radiation burden on the lungs can be attained with X-rays without compromising the PTV coverage. The radiation burden on some critical part of normal tissues was effectively reduced by application of a dummy organ at risk. CONCLUSION: The inverse planning is an effective method for conformal radiotherapy of large tumors as well. However, the power of the technique is insufficient when the tolerance dose of the neighbouring normal tissue is too low and its volume effect is high. Although requiring further operator interactions, introduction of dummy organs at risk may be of help in reducing the radiation burden on normal tissues.

Body Burden↗
PubMed1997

The effect of stair-step leaf transmission on the 'tongue-and-groove problem' in dynamic radiotherapy with a multileaf collimator.

When intensity-modulated fields are created using a multileaf collimator with dynamic leaf movement the potential problem for underdosage beneath the tongue-and-groove interleaf regions has been identified and a solution based on leaf-movement synchronization has been provided by Van Santvoort and Heijmen. Their first-order analysis ignored the transmission through an exposed stair-step. In this brief Note we provide the extended analysis including this contribution and show the effect on irradiation with synchronization. The result is that the synchronization approach of Van Santvoort and Heijmen solves the tongue-and-groove problem even when the transmission through the stair-step is considered, but partial synchronization is generally sufficient.

Health Physics↗
PubMed1997

A fast algorithm for the dose calculation of irregularly shaped fields in stereotactic convergent beam irradiation.

A fast, irregular field dose calculation algorithm is described for use in stereotactic convergent beam irradiation (CBI) with high-energy photons. The dose distribution of an irregular field is approximated by a convolution of narrow pencil beams. Decomposition of pencil beams and a 'pre-selection' algorithm is used to avoid redundant calculations. A typical dose calculation for CBI with nine couch angles and nine different irregularly shaped fields takes less than 90 s on a DEC VAXstation 3100. Fortran. The difference between the measured and the calculated dose values increases with increasing distance from the isocentre. The error in 10% isodose line is less than 3% of the isocentre dose.

Algorithms↗
PubMed1997

Number and orientations of beams in intensity-modulated radiation treatments.

The fundamental question of how many equispaced coplanar intensity-modulated photon beams are required to obtain an optimum treatment plan is investigated in a dose escalation study for a typical prostate tumor. Furthermore, optimization of beam orientations to improve dose distributions is explored. A dose-based objective function and a fast gradient technique are employed for optimizing the intensity profiles (inverse planning). An exhaustive search and fast simulated annealing techniques (FSA) are used to optimize beam orientations. However, to keep computation times reasonable, the intensity profiles for each beam arrangement are still optimized using inverse planning. A pencil beam convolution algorithm is employed for dose calculation. All calculations are performed in three-dimensional (3D) geometry for 15 MV photons. DVHs, dose displays, TCP, NTCP, and biological score functions are used for evaluation of treatment plans. It is shown that for the prostate case presented here, the minimum required number of equiangular beams depends on the prescription dose level and ranges from three beams for 70 Gy plans to seven to nine beams for 81 Gy plans. For the highest dose level (81 Gy), beam orientations are optimized and compared to equiangular spaced arrangements. It is shown that (1) optimizing beam orientations is most valuable for a small numbers of beams (< or = 5) and the gain diminishes rapidly for higher numbers of beams; (2) if sensitive structures (for example rectum) are partially enclosed by the target volume, beams coming from their direction tend to be preferable, since they allow greater control over dose distributions; (3) while FSA and an exhaustive search lead to the same results, computation times using FSA are reduced by two orders of magnitude to clinically acceptable values. Moreover, characteristics of and demands on biology-based and dose-based objective functions for optimization of intensity-modulated treatments are discussed.

Algorithms↗
PubMed1997

An analytical approximation of the Bragg curve for therapeutic proton beams.

The knowledge of proton depth-dose curves, or "Bragg curves," is a fundamental prerequisite for dose calculations in radiotherapy planning, among other applications. In various cases it is desirable to have an analytical representation of the Bragg curve, rather than using measured or numerically calculated data. This work provides an analytical approximation of the Bragg curve in closed form. The underlying model is valid for proton energies between about 10 and 200 MeV. Its main four constituents are: (i) a power-law relationship describing the range-energy dependency; (ii) a linear model for the fluence reduction due to nonelastic nuclear interactions, assuming local deposition of a fraction of the released energy; (iii) a Gaussian approximation of the range straggling distribution; and (iv) a representation of the energy spectrum of poly-energetic beams by a Gaussian with a linear "tail." Based on these assumptions the Bragg curve can be described in closed form using a simple combination of Gaussians and parabolic cylinder functions. The resulting expression can be fitted to measurements within the measurement error. Very good agreement is also found with numerically calculated Bragg curves.

Calibration↗