Feasibility of TCP-based dose painting by numbers applied to a prostate case with (18)F-choline PET imaging.

INTRODUCTION: A biologically adaptive radiation treatment method to maximize the TCP is shown. Functional imaging is used to acquire a heterogeneous dose prescription in terms of Dose Painting by Numbers and to create a patient-specific IMRT plan. METHOD AND MATERIALS: Adapted from a method for selective dose escalation under the guidance of spatial biology distribution, a model, which translates heterogeneously distributed radiobiological parameters into voxelwise dose prescriptions, was developed. At the example of a prostate case with (18)F-choline PET imaging, different sets of reported values for the parameters were examined concerning their resulting range of dose values. Furthermore, the influence of each parameter of the linear-quadratic model was investigated. A correlation between PET signal and proliferation as well as cell density was assumed. Using our in-house treatment planning software Direct Monte Carlo Optimization (DMCO), a treatment plan based on the obtained dose prescription was generated. Gafchromic EBT films were irradiated for evaluation. RESULTS: When a TCP of 95% was aimed at, the maximal dose in a voxel of the prescription exceeded 100Gy for most considered parameter sets. One of the parameter sets resulted in a dose range of 87.1Gy to 99.3Gy, yielding a TCP of 94.7%, and was investigated more closely. The TCP of the plan decreased to 73.5% after optimization based on that prescription. The dose difference histogram of optimized and prescribed dose revealed a mean of -1.64Gy and a standard deviation of 4.02Gy. Film verification showed a reasonable agreement of planned and delivered dose. CONCLUSION: If the distribution of radiobiological parameters within a tumor is known, this model can be used to create a dose-painting by numbers plan which maximizes the TCP. It could be shown, that such a heterogeneous dose distribution is technically feasible.

Advantage of biological over physical optimization in prostate cancer?

The biological effects of an applied dose can be accounted for by using biological objective functions with IMRT. A commonly used concept is the generalized equivalent uniform dose (gEUD), developed by Niemierko. Unlike the equivalent uniform dose (EUD) which is defined for tumor only, the gEUD can be used for both target volume and organs-at-risk (OAR). In this study, the gEUD has been integrated in our in-house inverse treatment planning system DMCO. DMCO is based on an inverse kernel concept and maintains full Monte-Carlo precision. The system applies direct aperture optimization by means of simulated annealing. Thereby DMCO is per se predestined for the optimization of non-quadratic biological objective functions. In this work, the feasibility of gEUD-based optimization with DMCO is investigated and compared to modified physical optimization. A 'pseudo' Pareto study is performed in order to derive the gEUD-parameters 'a' for the volumes-of-interest of a prostate case. The best biological plan is compared to a physically optimized plan, based on dose-volume objectives (DVO). Furthermore, a hybrid objective function (OF) was developed. It consists of both a biological OF for the OARs and a physical OF for the PTV. The plans are compared to another physically optimized plan, which includes additional zero-DVOs in order to further improve OAR-sparing. As a result of the comparisons it turns out, that the biological OF may improve plan quality with regard to the OARs, but at the price of a degradation of the PTV. This disadvantage can be overcome by a hybrid OF, by which the advantages of both biological and physical OF can be combined. With the application of the physical OF with properly set zero-DVOs, a similar or even superior plan quality may be achieved. The physical OFs do not need the time consuming stochastic optimization, which is mandatory in biological optimization and which is included in DMCO. Furthermore, biological evaluation leaves plan quality rather similar compared to physical optimization, but it cares automatically for the target and the OARs.

Monte Carlo simulations to replace film dosimetry in IMRT verification.

Patient-specific verification of intensity-modulated radiation therapy (IMRT) plans can be done by dosimetric measurements or by independent dose or monitor unit calculations. The aim of this study was the clinical evaluation of IMRT verification based on a fast Monte Carlo (MC) program with regard to possible benefits compared to commonly used film dosimetry. 25 head-and-neck IMRT plans were recalculated by a pencil beam based treatment planning system (TPS) using an appropriate quality assurance (QA) phantom. All plans were verified both by film and diode dosimetry and compared to MC simulations. The irradiated films, the results of diode measurements and the computed dose distributions were evaluated, and the data were compared on the basis of gamma maps and dose-difference histograms. Average deviations in the high-dose region between diode measurements and point dose calculations performed with the TPS and MC program were 0.7 ± 2.7% and 1.2 ± 3.1%, respectively. For film measurements, the mean gamma values with 3% dose difference and 3mm distance-to-agreement were 0.74 ± 0.28 (TPS as reference) with dose deviations up to 10%. Corresponding values were significantly reduced to 0.34 ± 0.09 for MC dose calculation. The total time needed for both verification procedures is comparable, however, by far less labor intensive in the case of MC simulations. The presented study showed that independent dose calculation verification of IMRT plans with a fast MC program has the potential to eclipse film dosimetry more and more in the near future. Thus, the linac-specific QA part will necessarily become more important. In combination with MC simulations and due to the simple set-up, point-dose measurements for dosimetric plausibility checks are recommended at least in the IMRT introduction phase.

Improving the performance of direct Monte Carlo optimization for large tumor volumes.

Direct Monte Carlo Optimization (DMCO) is a powerful method for dose optimization with Monte Carlo accuracy and direct aperture optimization with simulated annealing. Recently, we presented quasi intensity modulated arc therapy (qIMAT), a step-and-shoot technique that simulates a rotational technique by using a high number of beams and reducing the number of segments. In the present work, we applied a combination of both techniques to optimize an anal cancer case. Because of the limited memory of standard computers, two techniques for reducing the size of the inverse kernel (IK) were investigated. The standard deviation degradation technique (SDDT) and the reduced resolution technique (RRT) were applied to a 7-field IMRT plan on the CarPet phantom. Several IKs with an estimated standard deviation (SD) of the MC-calculation of 5%, 10% and 15% and another three IKs with voxel size of 4, 8 and 16mm were calculated. All IKs were optimized with DMCO; after optimization, a final dose calculation with 5% SD and 4mm resolution was carried out. SDDT was a better compromise between plan quality and IK-size reduction than RRT. PTV homogeneity and dose sparing to the OAR was almost identical for SDDT, while for RRT the quality was degraded by low resolution. Therefore, SDDT was applied to the anal cancer case. The IK-file of a quasi-IMAT plan with 30 beams was calculated with XVMC with 15% SD and a voxel size of 4mm. After optimization with DMCO using one segment per beam, a final dose calculation with 2% variance was performed. By comparing the DVHs of qIMAT with a 7-field IMRT (commercial therapy planning system) and with a 7-field IMRT (DMCO), qIMAT showed considerably advantages over IMRT in OARs dose sparing. In this way, the DMCO optimization with qIMAT of complex cases with large treatment volumes, such as anal cancer, are possible. Furthermore, for anal cancer, the comparison of qIMAT with IMRT showed that qIMAT can improve the plan quality.

18F-FET-PET-based dose painting by numbers with protons.

PURPOSE: To investigate the potential of (18)F-fluoroethyltyrosine-positron emission tomography-((18)F-FET-PET-)based dose painting by numbers with protons. MATERIAL AND METHODS: Due to its high specificity to brain tumor cells, FET has a high potential to serve as a target for dose painting by numbers. Biological image-based dose painting might lead to an inhomogeneous dose prescription. For precise treatment planning of such a prescribed dose, an intensity-modulated radiotherapy (IMRT) algorithm including a Monte Carlo dose-calculation algorithm for spot-scanning protons was used. A linear tracer uptake to dose model was used to derive a dose prescription from the (18)F-FET-PET. As a first investigation, a modified modulation transfer function (MTF) of protons was evaluated and compared to the MTF of photons. In a clinically adapted planning study, the feasibility of (18)F-FET-PET-based dose painting with protons was demonstrated using three patients with glioblastome multiforme. The resulting dose distributions were evaluated by means of dose-difference and dose-volume histograms and compared to IMRT data. RESULTS: The MTF for protons was constantly above that for photons. The standard deviations of the dose differences between the prescribed and the optimized dose were smaller in case of protons compared to photons. Furthermore, the escalation study showed that the doses within the subvolumes identified by biological imaging techniques could be escalated remarkably while the dose within the organs at risk was kept at a constant level. CONCLUSION: The presented investigation fortifies the feasibility of (18)F-FET-PET-based dose painting with protons.

Direct machine parameter optimization for intensity modulated radiation therapy (IMRT) of oropharyngeal cancer--a planning study.

The purpose of the study was to investigate the potential of direct machine parameter optimization (DMPO) to achieve parotid sparing without compromising target coverage in IMRT of oropharyngeal cancer as compared to fluence modulation with subsequent leaf sequencing (IM) and forward planned 2-step arc therapy (IMAT). IMRT plans were generated for 10 oropharyngeal cancer patients using DMPO and IM. The resulting dose volume histograms (DVH) were evaluated with regard to compliance with the dose volume objectives (DVO) and plan quality. DMPO met the DVO for the targets better than IM but violated the DVO to the parotids in some cases. DMPO provided better target coverage and dose homogeneity than IM and comparable to IMAT. Dose to the parotids (23Gy) was significantly lower than for IMAT (48Gy), but somewhat higher than for IM (20Gy). Parotid sparing with IM was, however, only achieved at the cost of target coverage and homogeneity. DMPO allows achieving parotid sparing in the treatment of oropharyngeal cancer without compromising target coverage and dose homogeneity in the target as compared to 2-step IMAT. Better overall plan quality can be delivered with less monitor units than with IM.

Analysis of the physical interactions of therapeutic proton beams in water with the use of Geant4 Monte Carlo calculations.

The processes that occur when protons traverse a medium are investigated theoretically for a full therapeutic range of energies [20 MeV, 220 MeV]. The investigation is undertaken using the Geant4 toolkit for water medium. The beam is simulated only inside the phantom, effects of beamline are included in the overall beam properties as lateral width and momentum bandwidth. Every energy deposition is catalogued according to the particle and the process that caused it. The catalogued depositions are analysed statistically. There are only few important processes such as proton ionisation and nuclear scattering (elastic/inelastic) that constitute the main features of the energy distribution. At the same time processes concerning electrons are used very often without obvious effect to the result. Such processes can be therefore approximated in the simulation codes in order to improve the performance of the code. Neutron depositions are most important before the Bragg peak, still they are by an order of magnitude smaller than those of protons. In the region behind the Bragg peak only a small number of neutrons is created in the simulation and their energy contribution through secondary protons is by orders smaller than the effect of proton-produced secondary protons within the Bragg peak. Hence, the effects of neutrons created in the calculation volume can be neglected.

Uncertainty reduction in intensity modulated proton therapy by inverse Monte Carlo treatment planning.

Treatment plans for intensity-modulated proton therapy may be sensitive to some sources of uncertainty. One source is correlated with approximations of the algorithms applied in the treatment planning system and another one depends on how robust the optimization is with regard to intra-fractional tissue movements. The irradiated dose distribution may substantially deteriorate from the planning when systematic errors occur in the dose algorithm. This can influence proton ranges and lead to improper modeling of the Braggpeak degradation in heterogeneous structures or particle scatter or the nuclear interaction part. Additionally, systematic errors influence the optimization process, which leads to the convergence error. Uncertainties with regard to organ movements are related to the robustness of a chosen beam setup to tissue movements on irradiation. We present the inverse Monte Carlo treatment planning system IKO for protons (IKO-P), which tries to minimize the errors described above to a large extent. Additionally, robust planning is introduced by beam angle optimization according to an objective function penalizing paths representing strongly longitudinal and transversal tissue heterogeneities. The same score function is applied to optimize spot planning by the selection of a robust choice of spots. As spots can be positioned on different energy grids or on geometric grids with different space filling factors, a variety of grids were used to investigate the influence on the spot-weight distribution as a result of optimization. A tighter distribution of spot weights was assumed to result in a more robust plan with respect to movements. IKO-P is described in detail and demonstrated on a test case and a lung cancer case as well. Different options of spot planning and grid types are evaluated, yielding a superior plan quality with dose delivery to the spots from all beam directions over optimized beam directions. This option shows a tighter spot-weight distribution and should therefore be less sensitive to movements compared to optimized directions. But accepting a slight loss in plan quality, the latter choice could potentially improve robustness even further by accepting only spots from the most proper direction. The choice of a geometric grid instead of an energy grid for spot positioning has only a minor influence on the plan quality, at least for the investigated lung case.

IMRT of prostate cancer: a comparison of fluence optimization with sequential segmentation and direct step-and-shoot optimization.

BACKGROUND AND PURPOSE: Intensity-modulated radiation therapy (IMRT) has shown its superiority to three-dimensional conformal radiotherapy in the treatment of prostate cancer. Different optimization algorithms are available: algorithms which first optimize the fluence followed by a sequencing (IM), and algorithms which involve the machine parameters directly in the optimization process (DSS). The aim of this treatment-planning study is to compare both of them regarding dose distribution and treatment time. PATIENTS AND METHODS: Ten consecutive patients with localized prostate cancer were enrolled for the planning study. The planning target volume and the rectum volume, urinary bladder and femoral heads as organs at risk were delineated. Average doses, the target dose homogeneity H, D(5), D(95), monitor units per fraction, and the number of segments were evaluated. RESULTS: While there is only a small difference in the mean doses at rectum and bladder, there is a significant advantage for the target dose homogeneity in the DSS-optimized plans compared to the IM-optimized ones. Differences in the monitor units (nearly 10% less for DSS) and the number of segments are also statistically significant and reduce the treatment time. CONCLUSION: Particularly with regard to the tumor control probability, the better homogeneity of the DSS-optimized plans is more profitable. The shorter treatment time is an improvement regarding intrafractional organ motion. The DSS optimizer results in a higher target dose homogeneity and, simultaneously, in a lower number of monitor units. Therefore, it should be preferred for IMRT of prostate cancer.

Fast direct Monte Carlo optimization using the inverse kernel approach.

The loss of treatment plan quality after segmentation following fluence optimization is a problem in IMRT. In a previous publication we showed that re-optimization helps to re-establish part of the plan quality. Recently the so-called direct aperture optimization method has been introduced to successfully overcome that difficulty. The aim of the present paper is to present in detail the integration of the inverse kernel method into direct aperture optimization. It can be shown that this integration leads to a system with high performance with regard to time, while Monte Carlo precision is maintained. The integrated simulated annealing optimization algorithm allows easy adaptation to any multi-leaf collimator and it is open to any complex objective function. Investigations of simulated annealing control parameters are performed to improve the performance. The system denoted by direct Monte Carlo optimization (DMCO) is demonstrated on the Carpet phantom and a clinical prostate case as well. Results are compared to inverse kernel optimizations, showing a remarkable time reduction and simultaneously an improvement in plan quality for the Carpet phantom.

Quasi-IMAT technique and secondary cancer risk in prostate cancer.

BACKGROUND AND PURPOSE: Estimates of secondary cancer risk after radiotherapy are relevant for treatment-planning comparison. Recently, the authors investigated the potential of a step-and-shoot intensity-modulated arc therapy (quasi-IMAT [qIMAT]) to improve the intensity-modulated radiotherapy (IMRT) plan quality. Here, the effect of the primary dose distribution, photon scatter and neutron dose, and the risk of secondary malignancies after qIMAT technique were analyzed and compared to IMRT. METHODS: qIMAT plans with 36 beam directions and IMRT plans with six beam directions were created for 15-MV photons. Both plans were calculated for each of five prostate cancer patients. The obtained differential dose-volume histograms, photon scatter and neutron dose were used to determine the organ-equivalent dose (OED), which is proportional to the secondary cancer risk. Because of the uncertainty of the applicability of biological models to the OED concept both the linear-exponential and the plateau model for the dose-response relationship were applied. RESULTS: Both models gave similar results. The OED in scanned CT volume was lower for the qIMAT technique, but higher in the volume not scanned, compared to IMRT. Using a maximum of 36 segments, the increase of risk resulting from qIMAT was < 1% compared to IMRT for both models. By setting the number of segments to 72, an increase of 8% in secondary cancer risk resulted from qIMAT using the linear-exponential model, compared to IMRT (plateau model: 7%). The primary dose is responsible for 88% of the total OED in IMRT and for 86% in qIMAT. CONCLUSION: Although qIMAT uses a large number of fields and therefore the volume of normal tissue that receives low-dose radiation is larger than for IMRT, the total OED (by considering primary and secondary contributions of radiation) does not increase the risk of developing a secondary cancer compared to a conventional IMRT plan.

Quasi-IMAT study with conventional equipment to show high plan quality with a single gantry arc.

BACKGROUND AND PURPOSE: : Nowadays, intensity-modulated arc therapy (IMAT) for clinical use is mostly based on forward planning. The aim of this work is to investigate the potential of a step-and-shoot quasi-IMAT (qIMAT) technique to improve plan quality. MATERIAL AND METHOD: : qIMAT plans with 18 and 36 beams were generated with a total number of 36 segments. Additionally, the number of segments was increased to 72, in order to investigate if the quality of the plans improves with the number of beams and segments. A conventional six-field intensity-modulated radiation therapy (IMRT) plan was used as a reference. The beam setup was applied to the CarPet phantom and to five prostate cancer patients. RESULTS: : In the phantom case, the dose received by the organ at risk (OAR) decreased considerably by using qIMAT. At the same time, coverage and homogeneity of planning target volume (PTV) remained unaffected. For the prostate cases, a good dose coverage was accomplished inside the PTV. Rectum and bladder were better spared with qIMAT. When increasing the number of segments, only a slight improvement of the plan quality was observed. CONCLUSION: : The study showed that qIMAT improves the sparing of OARs while keeping the uniformity within the PTV, when compared with conventional IMRT. The more concave the PTV, the more noticeable is this behavior. The qIMAT technique has the advantage that it can be realized with a conventional equipment. The plan quality is high even with a single gantry arc and one segment per beam direction.

A biologically adapted dose-escalation approach, demonstrated for 18F-FET-PET in brain tumors.

PURPOSE: To demonstrate the feasibility of a biologically adapted dose-escalation approach to brain tumors. MATERIAL AND METHODS: Due to the specific accumulation of fluoroethyltyrosine (FET) in brain tumors, (18)F-FET-PET imaging is used to derive a voxel-by-voxel dose distribution. Although the kinetics of (18)F-FET are not completely understood, the authors regard regions with high tracer uptake as vital and aggressive tumor and use a linear dose-escalation function between SUV (standard uptake value) 3 and SUV 5. The resulting dose distribution is then planned using the inverse Monte Carlo treatment- planning system IKO. In a theoretical study, the dose range is clinically adapted from 1.8 Gy to 2.68 Gy per fraction (with a total of 30 fractions). In a second study, the maximum dose of the model is increased step by step from 2.5 Gy to 3.4 Gy to investigate whether a significant dose escalation to tracer-accumulating subvolumes is possible without affecting the shell-shaped organ at risk (OAR). For all dose-escalation levels the dose difference Delta D of each voxel inside the target volume is calculated and the mean dose difference Delta D and their standard deviation sigma Delta D are determined. The dose to the OAR is evaluated by the dose values D OAR 50% and D OAR 5%, which are the dose values not exceeded by 50% and 5% of the volume, respectively. RESULTS: The inhomogeneous dose prescription is achieved with high accuracy (Delta D < 0.03 +/- 0.3 Gy/fraction). The maximum dose can be increased remarkably, without increasing the dose to the OAR (standard deviation of D OAR 50% < 0.02 Gy/fraction and of D OAR 5% < 0.05 Gy/fraction). CONCLUSION: Assuming that regions with high tracer uptake can be interpreted as target for radiotherapy, (18)F-FET-PET-based "dose painting by numbers" applied to brain tumors is a feasible approach. The dose, and therefore potentially the chance of tumor control, can be enhanced. The proposed model can easily be transferred to other tracers and tumor entities.

Inverse treatment planning and integration of segmentation procedures.

The inverse treatment planning procedure in IMRT consists of two basic tasks, the fluence optimization and the segmentation into multi leaf collimator (MLC) segments. Both tasks can either be run simultaneously or separated. We focus on the latter case. When using such a separation the fluence given by MLC segments and the optimized one differ. In the same way the quality of the treatment plan decreases. In this paper we discuss two approaches to reduce the negative effect of a separation of the fluence optimization from the segmentation procedure. The fluence smoothing concept is expected to find solutions with, in a mathematical sense, smoother fluences. A mandatory segmentation should then result in less segments and less decrease of plan quality. In the second approach we take the segments and their weights as input to a second optimization cycle which readjusts the weights, so the result is as close to the original demands as possible. We demonstrate the two methods using a 7-field plan applied to the Quasimodo phantom. The results show that the number of segments can be reduced by implementing an additional term to the target function. A penalization of local extrema however does not have the expected effect. In all cases, an additional reoptimization of the segment weights improved the segmented plans significantly.

Investigation of intensity-modulated radiotherapy optimization with gEUD-based objectives by means of simulated annealing.

Inverse treatment planning of intensity-modulated radiation therapy (IMRT) is complicated by several sources of error, which can cause deviations of optimized plans from the true optimal solution. These errors include the systematic and convergence error, the local minima error, and the optimizer convergence error. We minimize these errors by developing an inverse IMRT treatment planning system with a Monte Carlo based dose engine and a simulated annealing search engine as well as a deterministic search engine. In addition, different generalized equivalent uniform dose (gEUD)-based and hybrid objective functions were implemented and investigated with simulated annealing. By means of a head-and-neck IMRT case we have analyzed the properties of these gEUD-based objective functions, including its search space and the existence of local optima errors. We found evidence that the use of a previously published investigation of a gEUD-based objective function results in an uncommon search space with a golf hole structure. This special search space structure leads to trapping in local minima, making it extremely difficult to identify the true global minimum, even when using stochastic search engines. Moreover, for the same IMRT case several local optima have been detected by comparing the solutions of 100 different trials using a gradient optimization algorithm with the global optimum computed by simulated annealing. We have demonstrated that the hybrid objective function, which includes dose-based objectives for the target and gEUD-based objectives for normal tissue, results in equally good sparing of the critical structures as for the pure gEUD objective function and lower target dose maxima.

Comparison of direct machine parameter optimization versus fluence optimization with sequential sequencing in IMRT of hypopharyngeal carcinoma.

BACKGROUND: To evaluate the effects of direct machine parameter optimization in the treatment planning of intensity-modulated radiation therapy (IMRT) for hypopharyngeal cancer as compared to subsequent leaf sequencing in Oncentra Masterplan v1.5. METHODS: For 10 hypopharyngeal cancer patients IMRT plans were generated in Oncentra Masterplan v1.5 (Nucletron BV, Veenendal, the Netherlands) for a Siemens Primus linear accelerator. For optimization the dose volume objectives (DVO) for the planning target volume (PTV) were set to 53 Gy minimum dose and 59 Gy maximum dose, in order to reach a dose of 56 Gy to the average of the PTV. For the parotids a median dose of 22 Gy was allowed and for the spinal cord a maximum dose of 35 Gy. The maximum DVO to the external contour of the patient was set to 59 Gy. The treatment plans were optimized with the direct machine parameter optimization ("Direct Step & Shoot", DSS, Raysearch Laboratories, Sweden) newly implemented in Masterplan v1.5 and the fluence modulation technique ("Intensity Modulation", IM) which was available in previous versions of Masterplan already. The two techniques were compared with regard to compliance to the DVO, plan quality, and number of monitor units (MU) required per fraction dose. RESULTS: The plans optimized with the DSS technique met the DVO for the PTV significantly better than the plans optimized with IM (p = 0.007 for the min DVO and p < 0.0005 for the max DVO). No significant difference could be observed for compliance to the DVO for the organs at risk (OAR) (p > 0.05). Plan quality, target coverage and dose homogeneity inside the PTV were superior for the plans optimized with DSS for similar dose to the spinal cord and lower dose to the normal tissue. The mean dose to the parotids was lower for the plans optimized with IM. Treatment plan efficiency was higher for the DSS plans with (901 +/- 160) MU compared to (1151 +/- 157) MU for IM (p-value < 0.05). Renormalization of the IM plans to the mean of the dose to 95% of the PTV (D95) of the DSS plans, resulted in similar target coverage and dose to the parotids for both strategies, at the cost of a significantly higher dose to the normal tissue and maximum dose to the target. The relative volume of the PTV receiving 107% or more of the prescription dose V107 increased to 35.5% +/- 20.0% for the IM plan as compared to a mean of 0.9% +/- 0.9% for the DSS plan. CONCLUSION: The direct machine parameter optimization is a major improvement compared to the fluence modulation with subsequent leaf sequencing in Oncentra Masterplan v1.5. The resulting dose distribution complies better with the DVO and better plan quality is achieved for identical specification of DVO. An additional asset is the reduced number of MU as compared to IM.

[Certification of an in-house manufactured translational couch unit for total body irradiation in accordance with the medical devices act]. / Zertifizierung einer Translationsliege zur Ganzkörperbestrahlung aus In-Haus-Herstellung gemäss dem Medizinproduktegesetz.

UNLABELLED: A new translational couch unit with extended potentials of dose optimization by variable velocity and a comfortable user interface with integrated patient administration was developed at the university clinic of Regensburg. MATERIALS AND METHODS: The concept and construction were elaborated in legal accordance with the in-house manufacture conditions mentioned in the German Medical Devices Act. In particular we have implemented a concept of functional safety based on a controller unit, an independent monitoring unit and self-testing procedures. Redundant safety relevant hard- and software components are continuously checked against each other. In case of any malfunction the translation movement and the linear accelerator are stopped. Gap-free continuation of the treatment will be possible after elimination of the cause of the interrupt. RESULTS AND CONCLUSION: After the testing of the implemented functional safety features including the risk assessment and management, electrical safety, electromagnetic compatibility by accredited technical experts the translational couch system complies with the requirements of the Medical Devices Act and can be operated in terms of in-house application. The presented certification procedure can motivate other radiation departments to develop own systems for in-house usage.

[Electron fields in clinical application. A comparison of pencil beam and Monte Carlo algorithm]. / Elektronenfelder in der klinischen Anwendung. Ein Vergleich von Pencil-Beam- und Monte-Carlo-Algorithmus.

BACKGROUND AND PURPOSE: For several years three-dimensional treatment-planning systems have used pencil beam algorithms in the calculation of electron fields. Nowadays, exact Monte Carlo methods are commercially available, showing good correspondence to experimental results. Clinical examples are investigated to find differences in the dose distribution of treatment plans, which are calculated with both pencil beam and Monte Carlo algorithm. MATERIAL AND METHODS: Two different clinical applications are regarded: (1) an irradiation of the chest wall, and (2) an electron field to the vertebral column. The dose distributions are calculated by Oncentra MasterPlan on the one hand, using the Monte Carlo code VMC++, and by Helax TMS on the other hand (both Nucletron B.V., Veenendaal, The Netherlands). Profiles and depth dose curves are evaluated by the Verisoft program of PTW (Freiburg, Germany). RESULTS: In the case of chest wall irradiation, the depth dose curves for the three investigated energies, 9, 15 and 21 MeV, agree rather well, also in lung tissue. The mean value for the lung differs only by 4% related to the dose maximum. In the case of vertebral column irradiation, however, the dose difference is more pronounced and, in the prevertebral region, is 56% lower for the VMC++ plan than in the pencil beam calculation. CONCLUSION: For irradiations of the chest wall, dose distribution calculations by means of pencil beam algorithm may be applied. Calculating electron dose distributions in cases of larger bone inhomogeneities, the more exact Monte Carlo algorithm should be preferred.

[Verification and application of the voxel-based Monte Carlo (VMC++) electron dose module of Oncentra MasterPlan]. / Verifikation und Anwendungen des voxelbasierten Monte-Carlo-(VMC++-)Elektronen-Dosismoduls von Oncentra MasterPlan.

UNLABELLED: PURPOSE, MATERIAL AND METHODS: The dose calculation accuracy of the voxel-based Monte Carlo (VMC++) electron dose module of Oncentra MasterPlan (Nucletron B.V., Veenendaal, The Netherlands) was verified by measurements in homogeneous water phantoms. RESULTS: Measured and calculated dose maxima on the central beam axis (calculations with 10,000-20,000 incident electron histories per cm(2)) agree well using standard applicator configurations as well as individually shaped inserts. Profile scans with higher electron energies (>/= 15 MeV) reveal differences up to 5% especially in the penumbra region. Depth dose curves agree best in the vicinity of maximum depths. In the buildup region energy-dependent differences up to 5% in both directions could be observed. In the decay region of depth dose curves calculated doses were up to 10% higher than measured values. CONCLUSION: Good VMC++ accuracy combined with moderate computing times of 1-15 min per beam satisfy all clinical needs. VMC++ allows, for the first time, accurate routine dose evaluations of radiation therapy with electrons. Adequate positioning of the dose reference point is essential. Even small displacements may significantly influence the calculation of monitor units.

Application of the inverse Monte Carlo treatment planning system IKO for an inhomogeneous dose prescription in the sense of dose painting.

Biological imaging (PET SPECTJfMRI, MRS, etc.) is able to provide tri-dimensional biological information, i.e. proliferation, cell density, hypoxia or choline/citrate ratio. The implementation of this information in a treatment plan can be utilised to escalate the dose in target subvolumes. For this purpose, a treatment planning system has to be able to realise an inhomogeneous dose prescription with sufficient spatial resolution. The present study investigated to which extent the inverse Monte Carlo treatment planning system IKO (inverse kernel optimization), developed at our department, can modulate an inhomogeneous dose prescription. As a qualifier to describe this ability, we defined in analogy to imaging a modulation transfer function for treatment planning systems. In addition two clinical cases, a prostate case and a head-and-neck case, were set up with different dose prescriptions in different subtargets. The modulation transfer function revealed that IKO is able to modulate structures larger than 1.3 cm with sharp dose gradients. Also, IKO is able to modulate several subtargets inside a prostate with different escalated doses. The dose-volume histograms of the head-and-neck case showed a good dose coverage of the target volumes, as well as a good protection of the organs at risk according to the dose constraints. As a result, IKO is able to realise a heterogeneous dose prescription in the sense of "dose painting".