Biomedical subjects

Uwe Oelfke

Publications and source records attributed to Uwe Oelfke.

At least 19 recordsLinked to original sources

PubMed2006

Fast multifield optimization of the biological effect in ion therapy.

In this paper, we present a new technique for simultaneous multifield optimization of the biological effect (i.e. relative biological effectiveness times dose) for intensity modulated radiotherapy with ion beams. It offers complete inverse treatment planning by taking into account planning constraints for the target volume as well as for organs at risk. The approach is based on the mixed irradiation formalism of the linear-quadratic model from radiobiology. We employ a novel objective function to directly optimize the biological effect rather than the physical dose. The required biological input data are reduced to a minimum and are completely independent from the optimization itself. They can be derived from any radiobiological model or even from directly measured data. The new optimization method was fully integrated into our inverse treatment planning tool KonRad. Comparisons with the TRiP98 treatment planning code were done for simple spread-out Bragg peaks as well as for three-dimensional treatment plans, where all fields were optimized separately. While the agreement between both planning systems was very good, the calculation time was substantially reduced in KonRad. By enabling the multifield optimization, the quality of the treatment plans and the sparing of healthy tissues can be clearly improved.

Algorithms↗

PubMed2006

Linac-integrated 4D cone beam CT: first experimental results.

A new online imaging approach, linac-integrated cone beam CT (CBCT), has been developed over the past few years. It has the advantage that a patient can be examined in their treatment position directly before or during a radiotherapy treatment. Unfortunately, respiratory organ motion, one of the largest intrafractional organ motions, often leads to artefacts in the reconstructed 3D images. One way to take this into account is to register the breathing phase during image acquisition for a phase-correlated image reconstruction. Therefore, the main focus of this work is to present a system which has the potential to investigate the correlation between internal (movement of the diaphragm) and external (data of a respiratory gating system) information about breathing phase and amplitude using an inline CBCT scanner. This also includes a feasibility study about using the acquired information for a respiratory-correlated 4D CBCT reconstruction. First, a moving lung phantom was used to develop and to specify the required methods which are based on an image reconstruction using only projections belonging to a certain moving phase. For that purpose, the corresponding phase has to be detected for each projection. In the case of the phantom, an electrical signal allows one to track the movement in real time. The number of projections available for the image reconstruction depends on the breathing phase and the size of the position range from which projections should be used for the reconstruction. The narrower this range is, the better the inner structures can be located, but also the noise of the images increases due to the limited number of projections. This correlation has also been analysed. In a second step, the methods were clinically applied using data sets of patients with lung tumours. In this case, the breathing phase was detected by an external gating system (AZ-733V, Anzai Medical Co.) based on a pressure sensor attached to the patient's abdominal region with a fixation belt. The comparison of the reconstructed 4D CBCT images and the corresponding 4D CT images used for the treatment planning provides the required information for the calculation of possible setup errors. So, a repositioning of the patient is feasible even though the patient moves due to respiration. In addition to the external signal, the position of the diaphragm in the cranial-caudal direction could be extracted from each projection. Both independent sources of information show a very good agreement of the phase and even the amplitude of the movement and the external signal respectively. This suggests the usability of such a system for a gated dose delivery approach. However, more studies involving patients with different incidences have to be carried out to confirm these first results.

Algorithms↗

PubMed2006

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

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

Automation↗

PubMed2006

Linac-integrated kV-cone beam CT: technical features and first applications.

One of the most prominent imaging techniques in image-guided radiotherapy (IGRT) is the acquisition of cone beam computed tomographies (CBCTs) at the linac with the patient in treatment position. CBCTs provide accurate 3-dimensional (3D) knowledge about the patient's anatomy for every treatment fraction and are therefore well suited for all adaptive corrections of errors related to interfractional uncertainties of the treatment process. In this paper, we first describe the technical development and implementation of this new imaging technique at our linac, i.e., the hardware components and their operating parameters are discussed in detail for a standard image acquisition of CBCTs. Then, an extension of this approach for the acquisition of complete images for extended field of views--the "shifted detector" technique--is presented followed by a first investigation of how CBCTs can be reliably used for adaptive dose calculations. Finally, a first clinical application, the process of automatic patient positioning based on CBCT images, is discussed. From our investigations, we conclude that the technical development of linac-integrated CBCTs bears an enormous potential for the correction of interfractional treatment errors. However, image quality and reconstruction speed of the images leave room for improvement. The development of clinical strategies for the optimal application of this new image modality in a clinical environment is one the major tasks for the future.

Humans↗

PubMed2006

Assessment of a new multileaf collimator concept using GEANT4 Monte Carlo simulations.

The aim of the work was to investigate in advance the dosimetric properties of a new multileaf collimator (MLC) concept with the help of Monte Carlo (MC) simulations prior to the production of a prototype. The geometrical design of the MLC was implemented in the MC code GEANT4. For the simulation of a 6 MV treatment beam, an experimentally validated phase space and a virtual spatial Gaussian-shaped model placed in the origin were used. For the simulation of the geometry in GEANT4, the jaws and the two leaf packages were implemented with the help of computer-aided design data. First, transmission values for different tungsten alloys were extracted using the simulation codes GEANT4 and BEAMnrc and compared to experimental measurements. In a second step, high-resolution simulations were performed to detect the leakage at depth of maximum dose. The 20%-80% penumbra along the travel direction of the leaves was determined using 10 x 10 cm2 fields shifted along the x- and y-axis. The simulated results were compared with measured data. The simulation of the transmission values for different tungsten alloys showed a good agreement with the experimental measurements (within 2.0%). This enabled an accurate estimation of the attenuation coefficient for the various leaf materials. Simulations with varying width of the spatial Gaussian distribution showed that the leakage and the penumbra depend very much on this parameter: for instance, for widths of 2 and 4 mm, the interleaf leakage is below 0.3% and 0.75%, respectively. The results for the leakage and the penumbra (4.7+/-0.5 mm) are in good agreement with the measurements. This study showed that GEANT4 is appropriate for the investigation of the dosimetric properties of a multileaf collimator. In particular, a quantification of the leakage, the penumbra, and the tongue-and-groove effect and an evaluation of the influence of the beam parameters such as the width of the Gaussian distribution was possible.

Body Burden↗

PubMed2005

Online correction for respiratory motion: evaluation of two different imaging geometries.

One aim of adaptive radiotherapy (ART) is the observation of organ motion followed by a subsequent adaptation of the treatment plan. One way of achieving this goal is a kV x-ray source mounted at a linear accelerator in combination with a flat-panel imager. Two imaging hardware configurations were evaluated for their potential for online tracking and the subsequent correction of organ motion by using fluoroscopic images: x-ray tube positioned with (A) 90 degrees and (B) 180 degrees offset to the MV beam. For one lung case two IMRT plans with five coplanar beams and the table positioned at 0 degrees were optimized for two multileaf collimators (MLCs) with 10 mm and 2.75 mm leaf width. Respiratory motion, modelled by rigid transformation in the lungs, was investigated for different amplitudes. The 3D dose distributions for different cases (no movement, uncorrected movement, correction for the movement perpendicular to the respective kV beam) were evaluated with the help of dose volume histograms (DVHs) and a modified conformity (Baltas et al 1998 Int. J. Radiat. Oncol. Biol. Phys. 40 515-24) and coverage index using the 90% isodose. For the corrected treatment plans the influence of the observed displacement vector caused by organ movement was accounted for by a respective displacement of the target point. For the simulated movement with a small amplitude (3 mm) in the anterior-posterior (AP) direction the dose distributions resulting from the correction of the displacement vector using imaging system A or B showed similar results for both systems and were in good agreement with the dose distribution of the static (not moving) patient. Increasing the amplitude in the AP direction to 6 mm or even 9 mm leads for both amplitudes and both MLCs to almost the same conformity and coverage index as the static dose distribution if imaging system B is used for the online correction. For the dose distribution obtained with correction based on imaging system A the deviation between the optimal and the corrected dose distribution is increasing with increasing amplitude. For the MLC with the smaller leaf width the difference between the optimal and the corrected dose distributions is always significantly larger than for the less conformal dose distributions created by the MLC with the 10 mm leaves. These results can be explained by the fact that system A cannot observe movement in the AP-LR plane perpendicular to the MV beam and therefore cannot correct for these movements whereas system B only fails to observe the motion in the beam direction which for photon irradiation has less impact on the dose distribution.

Body Burden↗

PubMed2005

Compensation for respiratory motion by gated radiotherapy: an experimental study.

Respiratory organ motion is known to be one of the largest intrafractional organ motions. Therefore, it is important to investigate the potential benefit of gated dose delivery approaches which aim to account for the respective dose uncertainties. In this study respiration is simulated by a moving lung phantom; the movement is not restricted to a normal sinusoidal progression and simulates the one of the embedded lung tumour in the cranial-caudal direction. An IMRT plan with a total of 29 beam segments was designed for the treatment of this tumour. It was irradiated in its resting position-which is the position at exhalation-and during movement. Furthermore the irradiation was triggered using different amplitude thresholds, which means that the irradiation only proceeded if the deviation of the tumour's position from its resting position is smaller than the given threshold. We determined the gating-related increase of the treatment time for various gating procedures. We also measured the resulting dose distribution in specific slices of the phantom perpendicular to the direction of the movement using film dosimetry and compared it to the dose distribution of the static case. Since these film measurements cannot be done inside the whole tumour, additionally the movement and gating was simulated using the planning software to calculate the 3D dose distribution inside the tumour and to generate dose volume histograms for different treatment modalities. The total treatment time was observed to increase by 20%-100% depending on the individual gating threshold and can be calculated easily. The analysis of the films showed that irradiation without gating leads to significant underdosages up to 33%, especially at the edge of the tumour. With gating it is possible to considerably reduce this underdosage down to 9% depending on the trigger threshold. The calculation of the dose volume histograms makes it possible to find a reasonable compromise between the improvement of the dose distribution and the increase of the treatment time.

Artifacts↗

PubMed2005

Intensity modulated radiation therapy with multileaf collimators of different leaf widths: a comparison of achievable dose distributions.

PURPOSE: A planning study to analyze the impact of different leaf widths on the achievable dose distributions with intensity modulated radiation therapy (IMRT). METHODS: Five patients (3 intra- and 2 extra-cranial) with projected planning target volume (PTV) sizes smaller than 10 cm by 10 cm were re-planned with four different multileaf collimators (MLC). Two internal collimators with an isocentric leaf width of 4 and 10 mm and two add-on collimators with an isocentric leaf width of 2.75 and were evaluated. The inverse treatment planning system KonRad (Siemens Medical Solutions) was used to create IMRT 'step & shoot' plans. For each patient the same arrangement of beams and the same parameters for the optimization were used for all MLCs. The beamlet size for all treatment plans was chosen to coincide with the leaf width of the respective MLC. To evaluate the treatment plans 3D dose distributions and dose volume histograms were analyzed. As indicators for the quality of the PTV dose distribution the minimum dose, maximum dose and the standard deviation were used. For the organs at risk (OAR) the equivalent uniform dose (EUD) was calculated. To measure the dose coverage of the PTV the volume (V(90)) that received doses higher than 90% of the prescribed dose was calculated where for the conformity the dose conformity index given by Baltas et al. was determined. RESULTS: The MLC with the smallest leaf width yields the best mean value of all five patients for the PTV coverage and for the conformity. For the MLCs with the same leaf width, the add-on MLC leads to superior treatment plans than the internal MLC. This is due to the sharper penumbra of the add-on MLC. The number of IMRT field segments to deliver increased by approximately a factor of two if 2. MLC leafs are used instead of the standard 10 mm leafs. In case of the para-spinal patients the EUD value for the spinal cord is only reduced slightly by using MLCs with leaf widths smaller than 5 mm. For the intra-cranial the EUD value for some organs improved with reduced leaf widths while for some organs the 10 mm MLC leafs give comparable values. CONCLUSION: As expected the MLC with the smallest leaf width always yields the best PTV coverage. Reducing the leaf width from 4 to 2.75 mm results in a slight enhancement of the PTV coverage. With the selected organ parameters no significant improvement for most OAR was found. The disadvantage of the reduction of the leaf width is the increasing number of segments due to the more complex fluence patterns and therefore an increased delivery time.

Equipment Design↗

PubMed2005

Does electron and proton therapy reduce the risk of radiation induced cancer after spinal irradiation for childhood medulloblastoma? A comparative treatment planning study.

The aim of this treatment planning comparison study was to explore different spinal irradiation techniques with respect to the risk of late side-effects, particularly radiation-induced cancer. The radiotherapy techniques compared were conventional photon therapy, intensity modulated x-ray therapy (IMXT), conventional electron therapy, intensity/energy modulated electron therapy (IMET) and proton therapy (IMPT).CT images for radiotherapy use from five children, median age 8 and diagnosed with medulloblastoma, were selected for this study. Target volumes and organs at risk were defined in 3-D. Treatment plans using conventional photon therapy, IMXT, conventional electron therapy, IMET and IMPT were set up. The probability of normal tissue complication (NTCP) and the risk of cancer induction were calculated using models with parameters-sets taken from published data for the general population; dose data were taken from dose volume histograms (DVH). Similar dose distributions in the targets were achieved with all techniques but the absorbed doses in the organs-at-risk varied significantly between the different techniques. The NTCP models based on available data predicted very low probabilities for side-effects in all cases. However, the effective mean doses outside the target volumes, and thus the predicted risk of cancer induction, varied significantly between the techniques. The highest lifetime risk of secondary cancers was estimated for IMXT (30%). The lowest risk was found with IMPT (4%). The risks associated with conventional photon therapy, electron therapy and IMET were 20%, 21% and 15%, respectively. This model study shows that spinal irradiation of young children with photon and electron techniques results in a substantial risk of radiation-induced secondary cancers. Multiple beam IMXT seems to be associated with a particularly high risk of secondary cancer induction. To minimise this risk, IMPT should be the treatment of choice. If proton therapy is not available, advanced electron therapy may provide a better alternative.

Child↗

PubMed2005

Optimization of radiobiological effects in intensity modulated proton therapy.

Today, inverse treatment planning for intensity modulated proton therapy (IMPT) usually employs a constant relative biological effectiveness (RBE). In this paper, the potential clinical relevance of RBE variations for scanning techniques in IMPT is investigated, and a new strategy to include the RBE into the inverse planning process is presented. Three-dimensional RBE distributions are calculated based on a phenomenological model that describes the RBE as a function of dose, linear energy transfer (LET) and tissue type in the framework of the linear-quadratic model. This RBE model is integrated into the optimization loop of inverse planning by using a modified version of the standard quadratic objective function, where the physical dose is replaced by the biological effect. This system for "biological optimization" was implemented into a research version of the inverse planning software KonRad and allows the direct optimization of the product of RBE and physical dose. Several treatment plans for a prostate case are presented, which compare the biological with the conventional physical dose optimization for IMPT scanning techniques, in particular distal edge tracking (DET) and the full three-dimensional (3D) modulation of beam spots. Mainly due to their different LET distributions, the RBE effects for these two techniques are quite different: while the RBE distribution was more or less homogeneous in the planning target volume (PTV) for 3D modulation, considerable RBE variations within the PTV were observed for DET. These unfavorable effects could be compensated for by employing the new biological objective function, which led to a more homogeneous distribution of the product of RBE and physical dose in the PTV. The computation time increased by a factor of 2 compared to the optimization of the physical dose. In conclusion, the proposed method allows the simultaneous multifield optimization of the biological effect in a reasonable time, and is therefore well suited for studying the influence of a variable RBE in IMPT as well as for minimizing potentially adverse effects.

Body Burden↗

PubMed2005

Incorporating organ movements in IMRT treatment planning for prostate cancer: minimizing uncertainties in the inverse planning process.

We investigate an off-line strategy to incorporate inter fraction organ movements in IMRT treatment planning. Nowadays, imaging modalities located in the treatment room allow for several CT scans of a patient during the course of treatment. These multiple CT scans can be used to estimate a probability distribution of possible patient geometries. This probability distribution can subsequently be used to calculate the expectation value of the delivered dose distribution. In order to incorporate organ movements into the treatment planning process, it was suggested that inverse planning could be based on that probability distribution of patient geometries instead of a single snapshot. However, it was shown that a straightforward optimization of the expectation value of the dose may be insufficient since the expected dose distribution is related to several uncertainties: first, this probability distribution has to be estimated from only a few images. And second, the distribution is only sparsely sampled over the treatment course due to a finite number of fractions. In order to obtain a robust treatment plan these uncertainties should be considered and minimized in the inverse planning process. In the current paper, we calculate a 3D variance distribution in addition to the expectation value of the dose distribution which are simultaneously optimized. The variance is used as a surrogate to quantify the associated risks of a treatment plan. The feasibility of this approach is demonstrated for clinical data of prostate patients. Different scenarios of dose expectation values and corresponding variances are discussed.

Artifacts↗

PubMed2004

Intensity modulated radiation therapy with electrons using algorithm based energy/range selection methods.

BACKGROUND AND PURPOSE: In recent years photon intensity modulated radiation therapy (IMRT) has gained attention due to its ability to improve conformity of dose distributions. A potential advantage of electron-IMRT is that the dose fall off in the depth dose curve makes it possible to modulate the dose distribution in the direction of the beam by selecting different electron energies. This paper examines the use of a computer based energy selection in combination with the IMRT technique to optimise the electron dose distribution. MATERIALS AND METHODS: One centimetre square electron beamlets ranging from 2.5 to 50 MeV were pre-calculated in water using Monte Carlo methods. A modified IMRT optimisation tool was then used to find an optimum mix of electron energies and intensities. The main principles used are illustrated in some simple geometries and tested on two clinical cases of post-operated ca. mam. RESULTS: It is clearly illustrated that the energy optimisation procedure lowers the dose to lung and heart and makes the dose in the target more homogeneous. Increasing the energy at steep gradients compensates for lack of target coverage at beam edges and steep gradients. Comparison with a clinically acceptable four segment plan indicates the advantage of the used electron IMRT technique. CONCLUSIONS: Using an intensity optimised mix of computer selected electron energies has the potential to improve electron treatments for mastectomy patients with good target coverage and reduced dose to normal tissue such as lung and heart.

Algorithms↗

PubMed2004

Three-dimensional LET calculations for treatment planning of proton therapy.

The clinical impact of a variable relative biological effectiveness (RBE) in proton therapy is still under investigation. As the RBE depends on the linear energy transfer (LET), we present a fast method for three-dimensional calculations of the dose-averaged LET. The corresponding algorithm, based on an analytical expression for the LET and on Monte Carlo simulations, accounts for tissue inhomogeneities and allows LET calculations for realistic treatment plans and patient geometries given by computed tomography (CT) data sets. LET distributions were calculated for different proton spot scanning techniques, in particular for the full three-dimensional (3D) modulation and for the distal edge tracking technique (DET). In the case of 3D modulation, a more homogeneous distribution of LET in the planning target volume was observed than for the DET. Consequently, the proposed method allows to assess the impact of potential variations in RBE for various dose delivery techniques.

Humans↗

PubMed2004

Inverse planning of intensity modulated proton therapy.

A common requirement of radiation therapy is that treatment planning for different radiation modalities is devised on the basis of the same treatment planning system (TPS). The present study presents a novel multi-modal TPS with separate modules for the dose calculation, the optimization engine and the graphical user interface, which allows to integrate different treatment modalities. For heavy-charged particles, both most promising techniques, the distal edge tracking (DET) and the 3-dimensional scanning (3D) technique can be optimized. As a first application, the quality of optimized intensity-modulated treatment plans for photons (IMXT) and protons (IMPT) was analyzed in one clinical case on the basis of the achieved physical dose distributions. A comparison of the proton plans with the photon plans showed no significant improvement in terms of target volume dose, however there was an improvement in terms of organs at risk as well as a clear reduction of the total integral dose. For the DET technique, it is possible to create a treatment plan with almost the same quality of the 3D technique, however with a clearly reduced number (factor of 5) of beam spots as well as a reduced optimization time. Due to its modular design, the system can be easily expanded to more sophisticated dose-calculation algorithms or to modeling of biological effects.

Humans↗

PubMed2004

[Validation of a scatter correction method for IMRT verification using portal imaging].

Complex dose-delivery techniques, as currently applied in intensity-modulated radiation therapy (IMRT), require a highly efficient treatment-verification process. The present paper deals with the problem of the scatter correction for therapy verification by use of portal images obtained by an electronic portal imaging device (EPID) based on amorphous silicon. It also presents an iterative method for the scatter correction of portal images based on Monte Carlo-generated scatter kernels. First applications of this iterative scatter-correction method for the verification of intensity-modulated treatments are discussed on the basis of MVCT- and dose reconstruction. Several experiments with homogeneous and anthropomorphic phantoms were performed in order to validate the scatter correction method and to investigate the precision and relevance in view of its clinical applicability. It is shown that the devised concept of scatter correction significantly improves the results of MVCT- and dose reconstruction models, which is in turn essential for an exact online IMRT verification.

Humans↗

PubMed2003

CT calibration for two-dimensional scaling of proton pencil beams.

For proton dose calculations in heterogeneous media, it was shown in a previous work that the conventional pencil beam approach based on pathlength scaling does not properly account for scattering effects in nonwater media (Szymanowski and Oelfke 2002 Phys. Med. Biol. 47 3313-30). A two-dimensional scaling method was therefore introduced, which is able to predict with high accuracy the propagation of proton pencil beams both along the depth and the lateral directions in inhomogeneous media. In order to integrate this improved pencil beam algorithm in a CT based treatment planning system, two CT calibration curves are needed. The first one relates the Hounsfield numbers to the relative stopping powers, as for the conventional pencil beam approach. The second curve is to relate the Hounsfield numbers to the material-specific lateral scaling factors. The purpose of this work is to provide the CT calibration curves needed for the integration of the pencil beam algorithm featuring the two-dimensional scaling method. Similarly to as suggested by Schneider et al (1996 Phys. Med. Biol. 41 111-24) for the calibration curve in terms of stopping powers, we follow a stoichiometric procedure to get the calibration curve in terms of material-specific lateral scaling factors. The calibration curves for a CT scanner of the type Siemens Somatom Plus 4 are obtained from the analytical calculation of the CT Hounsfield numbers, relative stopping powers and material-specific lateral scaling factors for human biological tissues.

Algorithms↗

PubMed2003

Development and clinical application of a fast superposition algorithm in radiation therapy.

BACKGROUND AND PURPOSE: Dose calculation algorithms play a central role for the optimization and verification of treatment plans in radiation therapy. Complex treatment techniques like intensity modulated radiotherapy (IMRT) require accurate and fast dose algorithms especially for clinical cases which involve severe tissue inhomogeneities. For these cases the standard dose engine in current treatment planning systems--the convolution of photon pencil beams--usually fails to predict the dose with the required accuracy. The role of more accurate but time consuming dose calculations like superposition algorithms or Monte Carlo simulations in clinical practice is under investigation at several therapy centers. PATIENTS AND METHODS: The paper presents the design, implementation and the first application of a superposition algorithm in a clinical setting at the German Cancer Research Center (DKFZ). It first describes in detail how the superposition algorithm is adapted to the dose delivery system at DKFZ in terms of standard dosimetric data. Then details of the implementation of the algorithm are given with a focus on various methods for the reduction of dose computation times. Next, the algorithm is evaluated in various experiments with dosimetric phantoms. These studies are employed for the development of time efficient sampling strategies of the elemental dose kernels. Finally, the algorithm is applied to dose calculations of clinical cases with tumors adjacent to lung tissue. RESULTS: Severe differences in dose coverage of the tumors and dose burden of the surrounding tissues in comparison to standard pencil beam calculations are observed. A standard 4-7 beam plan in a convenient dose grid (approximately 3 mm in each direction) is calculated in about 30 min on a Pentium 4 (1.9 GHz) applying the superposition algorithm described here.

Algorithms↗

PubMed2003

Analytical linear energy transfer calculations for proton therapy.

As the relative biological effectiveness of protons depends on the linear energy transfer (LET), simple methods for LET calculations are desired for the optimization of proton therapy. This work provides an analytical model for the LET on the central axis of broad proton beams in water, which can also be applied to spread-out Bragg peaks. For realistic treatment situations with polyenergetic beams, the LET is here defined as a local mean of the proton stopping power, weighted by the local energy spectrum. The proposed model considers only Coulomb interactions and neglects nonelastic nuclear interactions. By assuming a Gaussian shape for the energy spectrum and by using a suitable parametrization of the stopping power, analytical expressions for the track averaged and the dose averaged LET are derived, which account for range straggling as well as for the initial width of the energy spectrum. The analytical model was evaluated by Monte Carlo simulations with GEANT 3.21. Local energy spectra were simulated to obtain LET distributions for several cases, using clinical energies between 70 and 250 MeV and varying widths of the initial energy spectrum. Good agreement was found between the analytical model and the Monte Carlo simulations (with maximum deviations of 0.5 keV per micrometer), which justifies the assumptions used in the derivation of the analytical model.

Body Burden↗