@article{HoegeleLoeschelDobleretal., author = {H{\"o}gele, Wolfgang and L{\"o}schel, Rainer and Dobler, Barbara and K{\"o}lbl, Oliver and Zygmanski, Piotr}, title = {Bayesian estimation applied to stochastic localization with constraints due to interfaces and boundaries}, series = {Mathematical Problems in Engineering}, volume = {213}, journal = {Mathematical Problems in Engineering}, publisher = {HIndawi}, issn = {1563-5147}, doi = {10.1155/2013/960421}, pages = {17}, abstract = {Purpose We present a systematic Bayesian formulation of the stochastic localization/triangulation problem close to constraining interfaces. Methods For this purpose, the terminology of Bayesian estimation is summarized suitably for applied researchers including the presentation of Maximum Likelihood (ML), Maximum A Posteriori (MAP), and Minimum Mean Square Error (MMSE) estimation. Explicit estimators for triangulation are presented for the linear 2D parallel beam and the nonlinear 3D cone beam model. The priors in MAP and MMSE optionally incorporate (A) the hard constraints about the interface and (B) knowledge about the probability of the object with respect to the interface. All presented estimators are compared in several simulation studies for live acquisition scenarios with 10,000 samples each. Results First, the presented application shows that MAP and MMSE perform considerably better, leading to lower Root Mean Square Errors (RMSEs) in the simulation studies compared to the ML approach by typically introducing a bias. Second, utilizing priors including (A) and (B) is very beneficial compared to just including (A). Third, typically MMSE leads to better results than MAP, by the cost of significantly higher computational effort. Conclusion Depending on the specific application and prior knowledge, MAP and MMSE estimators strongly increase the estimation accuracy for localization close to interfaces.}, language = {en} } @article{MaerzTreutweinNaboetal., author = {Maerz, Manuel and Treutwein, Marius and Nabo, Jan and Dobler, Barbara}, title = {Three-dimensional printers applied for the production of beam blocks in total body irradiation treatment}, series = {Journal of Applied Clinical Medical Physics}, volume = {23}, journal = {Journal of Applied Clinical Medical Physics}, number = {5}, publisher = {Wiley}, doi = {10.1002/acm2.13592}, pages = {1 -- 8}, abstract = {Purpose: Total body irradiation (TBI) in extended source surface distance (SSD) is a common treatment technique before hematopoietic stem cell transplant. The lungs are organs at risk, which often are treated with a lower dose than the whole body. Methods: This can be achieved by the application of blocks. Three-dimensional (3D) printers are a modern tool to be used in the production process of these blocks. Results: We demonstrate the applicability of a specific printer and printing material, describe the process, and evaluate the accuracy of the product. Conclusion: The blocks and apertures were found to be applicable in clinical routine.}, language = {en} } @article{HoegeleLoeschelDobleretal., author = {Hoegele, W. and L{\"o}schel, Rainer and Dobler, Barbara and Hesser, J. and K{\"o}lbl, Oliver and Zygmanski, Piotr}, title = {Stochastic formulation of patient positioning using linac-mounted cone beam imaging with prior knowledge}, series = {Medical physics}, volume = {38}, journal = {Medical physics}, number = {2}, publisher = {American Association of Physicists in Medicine}, doi = {10.1118/1.3532959}, pages = {668 -- 681}, abstract = {PURPOSE In this work, a novel stochastic framework for patient positioning based on linac-mounted CB projections is introduced. Based on this formulation, the most probable shifts and rotations of the patient are estimated, incorporating interfractional deformations of patient anatomy and other uncertainties associated with patient setup. METHODS The target position is assumed to be defined by and is stochastically determined from positions of various features such as anatomical landmarks or markers in CB projections, i.e., radiographs acquired with a CB-CT system. The patient positioning problem of finding the target location from CB projections is posed as an inverse problem with prior knowledge and is solved using a Bayesian maximum a posteriori (MAP) approach. The prior knowledge is three-fold and includes the accuracy of an initial patient setup (such as in-room laser and skin marks), the plasticity of the body (relative shifts between target and features), and the feature detection error in CB projections (which may vary depending on specific detection algorithm and feature type). For this purpose, MAP estimators are derived and a procedure of using them in clinical practice is outlined. Furthermore, a rule of thumb is theoretically derived, relating basic parameters of the prior knowledge (initial setup accuracy, plasticity of the body, and number of features) and the parameters of CB data acquisition (number of projections and accuracy of feature detection) to the expected estimation accuracy. RESULTS MAP estimation can be applied to arbitrary features and detection algorithms. However, to experimentally demonstrate its applicability and to perform the validation of the algorithm, a water-equivalent, deformable phantom with features represented by six 1 mm chrome balls were utilized. These features were detected in the cone beam projections (XVI, Elekta Synergy) by a local threshold method for demonstration purposes only. The accuracy of estimation (strongly varying for different plasticity parameters of the body) agreed with the rule of thumb formula. Moreover, based on this rule of thumb formula, about 20 projections for 6 detectable features seem to be sufficient for a target estimation accuracy of 0.2 cm, even for relatively large feature detection errors with standard deviation of 0.5 cm and spatial displacements of the features with standard deviation of 0.5 cm. CONCLUSIONS The authors have introduced a general MAP-based patient setup algorithm accounting for different sources of uncertainties, which are utilized as the prior knowledge in a transparent way. This new framework can be further utilized for different clinical sites, as well as theoretical developments in the field of patient positioning for radiotherapy.}, language = {en} } @article{DoblerWalterKnopfetal., author = {Dobler, Barbara and Walter, Cornelia and Knopf, Antje and Fabri, Daniella and L{\"o}schel, Rainer and Polednik, Martin and Schneider, Frank and Wenz, Frederik and Lohr, Frank}, title = {Optimization of extracranial stereotactic radiation therapy of small lung lesions using accurate dose calculation algorithms}, series = {Radiation Oncology}, volume = {1}, journal = {Radiation Oncology}, publisher = {BMC ; Springer}, doi = {10.1186/1748-717X-1-45}, pages = {11}, abstract = {BACKGROUND The aim of this study was to compare and to validate different dose calculation algorithms for the use in radiation therapy of small lung lesions and to optimize the treatment planning using accurate dose calculation algorithms. METHODS A 9-field conformal treatment plan was generated on an inhomogeneous phantom with lung mimics and a soft tissue equivalent insert, mimicking a lung tumor. The dose distribution was calculated with the Pencil Beam and Collapsed Cone algorithms implemented in Masterplan (Nucletron) and the Monte Carlo system XVMC and validated using Gafchromic EBT films. Differences in dose distribution were evaluated. The plans were then optimized by adding segments to the outer shell of the target in order to increase the dose near the interface to the lung. RESULTS The Pencil Beam algorithm overestimated the dose by up to 15\% compared to the measurements. Collapsed Cone and Monte Carlo predicted the dose more accurately with a maximum difference of -8\% and -3\% respectively compared to the film. Plan optimization by adding small segments to the peripheral parts of the target, creating a 2-step fluence modulation, allowed to increase target coverage and homogeneity as compared to the uncorrected 9 field plan. CONCLUSION The use of forward 2-step fluence modulation in radiotherapy of small lung lesions allows the improvement of tumor coverage and dose homogeneity as compared to non-modulated treatment plans and may thus help to increase the local tumor control probability. While the Collapsed Cone algorithm is closer to measurements than the Pencil Beam algorithm, both algorithms are limited at tissue/lung interfaces, leaving Monte-Carlo the most accurate algorithm for dose prediction.}, language = {en} } @article{DoblerStreckKleinetal., author = {Dobler, Barbara and Streck, Natalia and Klein, Elisabeth and L{\"o}schel, Rainer and Haertl, Petra Maria and K{\"o}lbl, Oliver}, title = {Hybrid plan verification for intensity-modulated radiation therapy (IMRT) using the 2D ionization chamber array I'mRT MatriXX--a feasibility study}, series = {Physics in medicine and biology}, volume = {55}, journal = {Physics in medicine and biology}, number = {2}, publisher = {IOP Publishing}, doi = {10.1088/0031-9155/55/2/N02}, pages = {N39 -- N55}, abstract = {The 2D ionization chamber array I'mRT MatriXX (IBA, Schwarzenbruck, Germany) has been developed for absolute 2D dosimetry and verification of intensity-modulated radiation therapy (IMRT) for perpendicular beam incidence. The aim of this study is to evaluate the applicability of I'mRT MatriXX for oblique beam incidence and hybrid plan verification of IMRT with original gantry angles. For the assessment of angular dependence, open fields with gantry angles in steps of 10 degrees were calculated on a CT scan of I'mRT MatriXX. For hybrid plan verification, 17 clinical IMRT plans and one rotational plan were used. Calculations were performed with pencil beam (PB), collapsed cone (CC) and Monte Carlo (MC) methods, which had been previously validated. Measurements were conducted on an Elekta SynergyS linear accelerator. To assess the potential and limitations of the system, gamma evaluation was performed with different dose tolerances and distances to agreement. Hybrid plan verification passed the gamma test with 4\% dose tolerance and 3 mm distance to agreement in all cases, in 82-88\% of the cases for tolerances of 3\%/3 mm, and in 59-76\% of the cases if 3\%/2 mm were used. Separate evaluation of the low dose and high dose regions showed that I'mRT MatriXX can be used for hybrid plan verification of IMRT plans within 3\% dose tolerance and 3 mm distance to agreement with a relaxed dose tolerance of 4\% in the low dose region outside the multileaf collimator (MLC).}, language = {en} }