| 1. |
- Uusijärvi, Helena, 1979, et al.
(författare)
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Dosimetric characterization of radionuclides for systemic tumor therapy: influence of particle range, photon emission, and subcellular distribution.
- 2006
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Ingår i: Medical physics. - : Wiley. - 0094-2405 .- 2473-4209. ; 33:9, s. 3260-9
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Tidskriftsartikel (refereegranskat)abstract
- Various radionuclides have been proposed for systemic tumor therapy. However, in most dosimetric analysis of proposed radionuclides the charged particles are taken into consideration while the potential photons are ignored. The photons will cause undesirable irradiation of normal tissue, and increase the probability of toxicity in, e.g., the bone marrow. The aim of this study was to investigate the dosimetric properties according to particle range, photon emission, and subcellular radionuclide distribution, of a selection of radionuclides used or proposed for radionuclide therapy, and to investigate the possibility of dividing radionuclides into groups according to their dosimetric properties. The absorbed dose rate to the tumors divided by the absorbed dose rate to the normal tissue (TND) was estimated for different tumor sizes in a mathematical model of the human body. The body was simulated as a 70-kg ellipsoid and the tumors as spheres of different sizes (1 ng-100 g). The radionuclides were either assumed to be uniformly distributed throughout the entire tumor and normal tissue, or located in the nucleus or the cytoplasm of the tumor cells and on the cell membrane of the normal cells. Fifty-nine radionuclides were studied together with monoenergetic electrons, positrons, and alpha particles. The tumor and normal tissue were assumed to be of water density. The activity concentration ratio between the tumor and normal tissue was assumed to be 25. The radionuclides emitting low-energy electrons combined with a low photon contribution, and the alpha emitters showed high TND values for most tumor sizes. Electrons with higher energy gave reduced TND values for small tumors, while a higher photon contribution reduced the TND values for large tumors. Radionuclides with high photon contributions showed low TND value for all tumor sizes studied. The radionuclides studied could be divided into four main groups according to their TND values: beta emitters, Auger electron emitters, photon emitters, and alpha emitters. The TND values of the beta emitters were not affected by the subcellular distribution of the radionuclide. The TND values of the Auger electron emitters were affected by the subcellular radionuclide distribution. The photon emitters showed low TND values that were only slightly affected by the subcellular radionuclide distribution. The alpha emitters showed high TND values that were only slightly affected by the subcellular radionuclide distribution. This dosimetric characterization of radionuclides may be valuable in choosing the appropriate radionuclides for specific therapeutic applications.
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| 2. |
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| 3. |
- Ahnesjö, Anders, et al.
(författare)
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Beam modeling and verification of a photon beam multisource model.
- 2005
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 32:6, s. 1722-37
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Tidskriftsartikel (refereegranskat)abstract
- Dose calculations for treatment planning of photon beam radiotherapy require a model of the beam to drive the dose calculation models. The beam shaping process involves scattering and filtering that yield radiation components which vary with collimator settings. The necessity to model these components has motivated the development of multisource beam models. We describe and evaluate clinical photon beam modeling based on multisource models, including lateral beam quality variations. The evaluation is based on user data for a pencil kernel algorithm and a point kernel algorithm (collapsed cone) used in the clinical treatment planning systems Helax-TMS and Nucletron-Oncentra. The pencil kernel implementations treat the beam spectrum as lateral invariant while the collapsed cone involves off axis softening of the spectrum. Both algorithms include modeling of head scatter components. The parameters of the beam model are derived from measured beam data in a semiautomatic process called RDH (radiation data handling) that, in sequential steps, minimizes the deviations in calculated dose versus the measured data. The RDH procedure is reviewed and the results of processing data from a large number of treatment units are analyzed for the two dose calculation algorithms. The results for both algorithms are similar, with slightly better results for the collapsed cone implementations. For open beams, 87% of the machines have maximum errors less than 2.5%. For wedged beams the errors were found to increase with increasing wedge angle. Internal, motorized wedges did yield slightly larger errors than external wedges. These results reflect the increased complexity, both experimentally and computationally, when wedges are used compared to open beams.
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| 4. |
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| 5. |
- Antonovic, Laura, et al.
(författare)
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Evaluation of a lithium formate EPR dosimetry system for dose measurements around Ir-192 brachytherapy sources
- 2009
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Ingår i: MEDICAL PHYSICS. - : Wiley. - 0094-2405. ; 36:6, s. 2236-2247
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Tidskriftsartikel (refereegranskat)abstract
- A dosimetry system using lithium formate monohydrate (HCO2Li center dot H2O) as detector material and electron paramagnetic resonance (EPR) spectroscopy for readout has been used to measure absorbed dose distributions around clinical Ir-192 sources. Cylindrical tablets with diameter of 4.5 mm, height of 4.8 mm, and density of 1.26 g/cm(3) were manufactured. Homogeneity test and calibration of the dosimeters were performed in a 6 MV photon beam. Ir-192 irradiations were performed in a PMMA phantom using two different source models, the GammaMed Plus HDR and the microSelectron PDR-v1 model. Measured absorbed doses to water in the PMMA phantom were converted to the corresponding absorbed doses to water in water phantoms of dimensions used by the treatment planning systems (TPSs) using correction factors explicitly derived for this experiment. Experimentally determined absorbed doses agreed with the absorbed doses to water calculated by the TPS to within +/- 2.9%. Relative standard uncertainties in the experimentally determined absorbed doses were estimated to be within the range of 1.7%-1.3% depending on the radial distance from the source, the type of source (HDR or PDR), and the particular absorbed doses used. This work shows that a lithium formate dosimetry system is well suited for measurements of absorbed dose to water around clinical HDR and PDR Ir-192 sources. Being less energy dependent than the commonly used thermoluminescent lithium fluoride (LiF) dosimeters, lithium formate monohydrate dosimeters are well suited to measure absorbed doses in situations where the energy dependence cannot easily be accounted for such as in multiple-source irradiations to verify treatment plans. Their wide dynamic range and linear dose response over the dose interval of 0.2-1000 Gy make them suitable for measurements on sources of the strengths used in clinical applications. The dosimeter size needs, however, to be reduced for application to single-source dosimetry.
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| 6. |
- Bornefalk, Hans
(författare)
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Implications of unchanged detection criteria with CAD as second reader of mammograms
- 2006
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 33:4, s. 922-929
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we address the use of computer-aided detection (CAD) systems as second readers in mammography. The approach is based on Bayesian decision theory and its implication for the choice of optimal operating points. The choice of a certain operating point along an ROC curve corresponds to a particular tradeoff between false positives and missed cancers. By minimizing a total risk function given this tradeoff, we determine optimal decision thresholds for the radiologist and CAD system when CAD is used as a second reader. We show that under very general circumstances, the performance of the sequential system is improved if the decision threshold of the latent human decision variable is increased compared to what it would have been in the absence of the CAD system. This means that an initial stricter decision criterion should be applied by the radiologist when CAD is used as a second reader than otherwise. First and foremost, the results in this paper should be interpreted qualitatively, but an attempt is made at quantifying the effect by tuning the model to a prospective study evaluating the use of CAD as a second reader. By making some necessary and plausible assumptions, we are able to estimate the effect of the resulting suboptimal operating point. In this study of 12 860 women, we estimate that a 15% reduction in callbacks for masses could have been achieved with only about a 1.5% relative decrease in sensitivity compared to that without using a stricter initial criterion by the radiologist. For microcalcifications the corresponding values are 7% and 0.2%. (c) 2006 American Association of Physicists in Medicine.
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| 7. |
- Bornefalk, Hans, et al.
(författare)
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On the comparison of FROC curves in mammography CAD systems
- 2005
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 32:2, s. 412-417
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Tidskriftsartikel (refereegranskat)abstract
- We present a novel method for assessing the performance of computer-aided detection systems on unseen cases at a given sensitivity level.
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| 8. |
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| 9. |
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| 10. |
- Carlsson, Fredrik
(författare)
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Combining segment generation with direct step-and-shoot optimization in intensity-modulated radiation therapy
- 2008
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 35:9, s. 3828-3838
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Tidskriftsartikel (refereegranskat)abstract
- A method for generating a sequence of intensity-modulated radiation therapy step-and-shoot plans with increasing number of segments is presented. The objectives are to generate high-quality plans with few, large and regular segments, and to make the planning process more intuitive. The proposed method combines segment generation with direct step-and-shoot optimization, where leaf positions and segment weights are optimized simultaneously. The segment generation is based on a column generation approach. The method is evaluated on a test suite consisting of five head-and-neck cases and five prostate cases, planned for delivery with an Elekta SLi accelerator. The adjustment of segment shapes by direct step-and-shoot optimization improves the plan quality compared to using fixed segment shapes. The improvement in plan quality when adding segments is larger for plans with few segments. Eventually, adding more segments contributes very little to the plan quality, but increases the plan complexity. Thus, the method provides a tool for controlling the number of segments and, indirectly, the delivery time. This can support the planner in finding a sound trade-off between plan quality and treatment complexity.
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| 11. |
- Carlsson, Fredrik, et al.
(författare)
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Iterative regularization in intensity-modulated radiation therapy optimization
- 2006
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 33:1, s. 225-234
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Tidskriftsartikel (refereegranskat)abstract
- A common way to solve intensity-modulated radiation therapy (IMRT) optimization problems is to use a beamlet-based approach. The approach is usually employed in a three-step manner: first a beamlet-weight optimization problem is solved, then the fluence profiles are converted into stepand-shoot segments, and finally postoptimization of the segment weights is performed. A drawback of beamlet-based approaches is that beamlet-weight optimization problems are ill-conditioned and have to be regularized in order to produce smooth fluence profiles that are suitable for conversion. The purpose of this paper is twofold: first, to explain the suitability of solving beamlet-based IMRT problems by a BFGS quasi-Newton sequential quadratic programming method with diagonal initial Hessian estimate, and second, to empirically show that beamlet-weight optimization problems should be solved in relatively few iterations when using this optimization method. The explanation of the suitability is based on viewing the optimization method as an iterative regularization method. In iterative regularization, the optimization problem is solved approximately by iterating long enough to obtain a solution close to the optimal one, but terminating before too much noise occurs. Iterative regularization requires an optimization method that initially proceeds in smooth directions and makes rapid initial progress. Solving ten beamlet-based IMRT problems with dose-volume objectives and bounds on the beamlet-weights, we find that the considered optimization method fulfills the requirements for performing iterative regularization. After segment-weight optimization, the treatments obtained using 35 beamlet-weight iterations outperform the treatments obtained using 100 beamlet-weight iterations, both in terms of objective value and of target uniformity. We conclude that iterating too long may in fact deteriorate the quality of the deliverable plan.
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| 12. |
- Carlsson, F, et al.
(författare)
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Iterative regularization of the IMRT optimization problem
- 2005
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Ingår i: Medical physics (Lancaster). - RaySearch Labs AB, Stockholm, Sweden. Royal Inst Technol, Stockholm, Sweden. : AMER ASSOC PHYSICISTS MEDICINE AMER INST PHYSICS. - 0094-2405. ; 32:6, s. 2140-2140
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 13. |
- Carlsson Tedgren, Asa, et al.
(författare)
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Optimization of the computational efficiency of a 3D, collapsed cone dose calculation algorithm for brachytherapy.
- 2008
-
Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 35:4, s. 1611-1618
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Tidskriftsartikel (refereegranskat)abstract
- Brachytherapy dose calculations based on point kernel superposition using the collapsed cone method have been shown to accurately model the influence from finite dimensions of the patient and effects from heterogeneities including those of high atomic numbers. The collapsed cone method is for brachytherapy applications most effectively implemented through a successive-scattering approach, in which the dose from once and higher order of scattered photons is calculated separately and in successive scatter order. The calculation speed achievable is directly proportional to the number of directions used for point kernel discretization and to the number of voxels in the volume. In this work we investigate how to best divide the total number of directions between the two steps of successive-scattering dose calculations. Results show that the largest fraction of the total number of directions should be utilized in calculating the first-scatter dose. Also shown is how the number of directions required for keeping discretization artifacts at acceptably low levels decreases significantly in multiple-source configurations, as a result of the dose gradients being less steep than those around single sources. Investigating the number of kernel directions required to keep artifacts low enough within the high dose region of an implant (i.e., for dose levels above approximately 5%-10% of the mean central target dose) reveals similar figures for brachytherapy as for external beam applications, where collapsed cone superposition is clinically used. Also shown is that approximating point kernels with their isotropic average leads to small dose differences at low and intermediate energies, implying that the collapsed cone calculations can be done in a single operation common to all sources of the implant at these energies. The current findings show that collapsed cone calculations can be achieved for brachytherapy with the same efficiency as for external beams. This, combined with recent results on gains in efficiency through implementing the algorithm on graphical card parallel hardware indicates that dose can be calculated with account for heterogeneities and finite dimensions within a few seconds for large voxel arrays and is therefore of interest for practical application to treatment planning.
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| 14. |
- Carlsson Tedgren, Åsa, et al.
(författare)
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Influence of phantom material and dimensions on experimental Ir-192 dosimetry
- 2009
-
Ingår i: MEDICAL PHYSICS. - : Wiley. - 0094-2405. ; 36:6, s. 2228-2235
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Tidskriftsartikel (refereegranskat)abstract
- In treatment planning of brachytherapy, absorbed dose is calculated by superposing predetermined distributions of absorbed dose to water in water for the single source according to the irradiation pattern [i.e., placement of the source(s) or dwelling position(s)]. Single-source reference water data are derived from Monte Carlo (MC) simulations and/or experiments. For reasons of positional accuracy, experimental brachytherapy dosimetry is most often performed in plastic phantoms. This work investigates the water equivalence of phantoms made from polystyrene, PMMA, and solid water for Ir-192 dosimetry. The EGSnrc MC code is used to simulate radial absorbed dose distributions in cylindrical phantoms of dimensions ranging in size from diameter and height of 20 cm to diameter and height of 40 cm. Water equivalence prevails if the absorbed dose to water in the plastic phantom is the same as the absorbed dose to water in a water phantom at equal distances from the source. It is shown that water equivalence at a specified distance from the source depends not only on the size of the plastic phantom but also on the size of the water phantom used for comparison. Compared to equally sized water phantoms, phantoms of polystyrene are less water equivalent than phantoms of PMMA and solid water but compared to larger water phantoms they are the most water equivalent. Although phantom dimension is the most important single factor influencing the dose distributions around Ir-192 sources, the effect of material properties is non-negligible and becomes increasingly important as phantom dimensions increase. The importance of knowing the size of the water phantom whose data underlies treatment planning systems, when using such data as a reference in, e.g., detector evaluation studies, is discussed. To achieve the highest possible accuracy in experimental dosimetry, phantom-specific correction factors should be used.
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| 15. |
- Carrasco, Pablo, et al.
(författare)
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Comparison of dose calculation algorithms in slab phantoms with cortical bone equivalent heterogeneities
- 2007
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 34:8, s. 3323-3333
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Tidskriftsartikel (refereegranskat)abstract
- To evaluate the dose values predicted by several calculation algorithms in two treatment planning systems, Monte Carlo (MC) simulations and measurements by means of various detectors were performed in heterogeneous layer phantoms with water- and bone-equivalent materials. Percentage depth doses (PDDs) were measured with thermoluminescent dosimeters (TLDs), metal-oxide semiconductor field-effect transistors (MOSFETs), plane parallel and cylindrical ionization chambers, and beam profiles with films. The MC code used for the simulations was the PENELOPE code. Three different field sizes (10 X 10, 5 X 5, and 2 X 2 cm 2) were studied in two phantom configurations and a bone equivalent material. These two phantom configurations contained heterogeneities of 5 and 2 cm of bone, respectively. We analyzed the performance of four correction-based algorithms and one based on convolution superposition. The correction-based algorithms were the Batho, the Modified Batho, the Equivalent TAR implemented in the Cadplan (Varian) treatment planning system (TPS), and the Helax-TMS Pencil Beam from the Helax-TMS (Nucletron) TPS. The convolution-superposition algorithm was the Collapsed Cone implemented in the Helax-TMS. All the correction-based calculation algorithms underestimated the dose inside the bone-equivalent material for 18 MV compared to MC simulations. The maximum underestimation, in terms of root-mean-square (RMS), was about 15% for the Helax-TMS Pencil Beam (Helax-TMS PB) for a 2 X 2 cm2 field inside the bone-equivalent material. In contrast, the Collapsed Cone algorithm yielded values around 3%. A more complex behavior was found for 6 MV where the Collapsed Cone performed less well, overestimating the dose inside the heterogeneity in 3%-5%. The rebuildup in the interface bone-water and the penumbra shrinking in high-density media were not predicted by any of the calculation algorithms except the Collapsed Cone, and only the MC simulations matched the experimental values within the estimated uncertainties. The TLD and MOSFET detectors were suitable for dose measurement inside bone-equivalent materials, while parallel ionization chambers, applying the same calibration and correction factors as in water, systematically underestimated dose by 3%-5%. (c) 2007 American Association of Physicists in Medicine.
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| 16. |
- Danielsson, Mats
(författare)
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MO‐D‐210A‐01 : Photon Counting Detectors for Mammography
- 2009
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Ingår i: Medical physics (Lancaster). - : Elsevier. - 0094-2405. ; 36:6, s. 2699-2699
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Tidskriftsartikel (refereegranskat)abstract
- Mammography is currently one of the most common x‐ray imaging examinations. More than 100 million women worldwide are screened every year and early detection of breast cancer through mammography has proven to be a key to significantly reduced mortality. The requirement on spatial resolution as well as contrast resolution is very high in order to detect and diagnose the cancer. Moreover, because of the large number of women going through this procedure and the fact that more than 99 % are healthy, it also becomes very important to minimize the radiation dose. Photon counting may be one way to meet the demands and mammography is the first modality in x‐ray imaging to implement photon counting detectors. FDA approval is still pending but they are currently in routine clinical use in more than 15 countries. The photon counting enables a discrimination of all electronic noise and a more optimum use of the information in each x‐ray. The absence of electronic noise is particularly important in low dose applications, in for example tomosynthesis a number of exposures from different angles are required and since the dose in each projection is just a fraction of the total dose for a mammogram the sensitivity to electronic noise will increase. Using the spectral information for each x‐ray it is in principle possible to deduce the elemental composition of an object in the breast. This could for example be used to enhance microcalcifications relative to soft tissue and differentiate water from fat in cysts. Recently contrast mammography has attracted significant attention. In this application Iodine is used as a contrast media to visualize the vascular structure. As in breast MRI the cancer stand out because of the leaky vessels resulting from its angiogenesis. A photon counting detector gives a unique opportunity to image the Iodine through spectral imaging by adjusting one of the thresholds to its K‐edge. Challenges for photon counting in mammography are high rates of x‐rays, both to generate the required flux at the source and to handle the rates at the detector without pile‐up. Even more difficult to handle are the charge sharing between detector pixels which, if not corrected for, will compromise the energy information. The current status of photon counting detectors in mammography will be described together with strategies to overcome the pit‐falls. Also future possibilities with spectral imaging in mammography will be investigated and examples from ongoing clinical trials will be given. Learning Objectives: 1. Status of photon counting detectors in mammography 2. Pit‐falls and opportunities with photon counting detectors for mammography 3. Future applications based on spectral detectors for mammography.
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| 17. |
- Das, Indra J., et al.
(författare)
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Accelerator beam data commissioning equipment and procedures : report of the TG-106 of the Therapy Physics Committee of the AAPM
- 2008
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 35:9, s. 4186-4215
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Forskningsöversikt (refereegranskat)abstract
- For commissioning a linear accelerator for clinical use, medical physicists are faced with many challenges including the need for precision, a variety of testing methods, data validation, the lack of standards, and time constraints. Since commissioning beam data are treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data are of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam data commissioning should be performed with appropriate knowledge and proper tools and should be independent of the person collecting the data. To achieve this goal, Task Group 106 (TG-106) of the Therapy Physics Committee of the American Association of Physicists in Medicine was formed to review the practical aspects as well as the physics of linear accelerator commissioning. The report provides guidelines and recommendations on the proper selection of phantoms and detectors, setting up of a phantom for data acquisition (both scanning and no-scanning data), procedures for acquiring specific photon and electron beam parameters and methods to reduce measurement errors (<1%), beam data processing and detector size convolution for accurate profiles. The TG-106 also provides a brief.discussion on the emerging trend in Monte Carlo simulation techniques in photon and electron beam commissioning. The procedures described in this report should assist a qualified medical physicist in either measuring a complete set of beam data, or in verifying a subset of data before initial use or for periodic quality assurance measurements. By combining practical experience with theoretical discussion, this document sets a new standard for beam data commissioning.
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| 18. |
- Das, Indra J., et al.
(författare)
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Small fields : nonequilibrium radiation dosimetry
- 2008
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 35:1, s. 206-215
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Forskningsöversikt (refereegranskat)abstract
- Advances in radiation treatment with beamlet-based intensity modulation, image-guided radiation therapy, and stereotactic radiosurgery (including specialized equipments like CyberKnife, Gamma Knife, tomotherapy, and high-resolution multileaf collimating systems) have resulted in the use of reduced treatment fields to a subcentimeter scale. Compared to the traditional radiotherapy with fields > or =4 x 4 cm2, this can result in significant uncertainty in the accuracy of clinical dosimetry. The dosimetry of small fields is challenging due to nonequilibrium conditions created as a consequence of the secondary electron track lengths and the source size projected through the collimating system that are comparable to the treatment field size. It is further complicated by the prolonged electron tracks in the presence of low-density inhomogeneities. Also, radiation detectors introduced into such fields usually perturb the level of disequilibrium. Hence, the dosimetric accuracy previously achieved for standard radiotherapy applications is at risk for both absolute and relative dose determination. This article summarizes the present knowledge and gives an insight into the future procedures to handle the nonequilibrium radiation dosimetry problems. It is anticipated that new miniature detectors with controlled perturbations and corrections will be available to meet the demand for accurate measurements. It is also expected that the Monte Carlo techniques will increasingly be used in assessing the accuracy, verification, and calculation of dose, and will aid perturbation calculations of detectors used in small and highly conformal radiation beams.
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| 19. |
- Dasu, Alexandru, et al.
(författare)
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Vascular oxygen content and the tissue oxygenation--a theoretical analysis.
- 2008
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Ingår i: Med Phys. - : American Association of Physicists in Medicine (AAPM). - 0094-2405. ; 35:2, s. 539-45
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Several methods exist for evaluating tumor oxygenation as hypoxia is an important prognostic factor for cancer patients. They use different measuring principles that highlight various aspects of oxygenation. The results could be empirically correlated, but it has been suspected that there could be discordances in some cases. This study describes an analysis of the relationship between vascular and tissue oxygenations. Theoretical simulation has been employed to characterize tissue oxygenations for a broad range of distributions of intervessel distances and vascular oxygenations. The results were evaluated with respect to the implications for practical measurements of tissue oxygenations. The findings showed that although the tissue oxygenation is deterministically related to vascular oxygenation, the relationship between them is not unequivocal. Variability also exists between the fractions of values below the sensitivity thresholds of various measurement methods which in turn could be reflected in the power of correlations between results from different methods or in the selection of patients for prognostic studies. The study has also identified potential difficulties that may be encountered at the quantitative evaluation of the results from oxygenation measurements. These could improve the understanding of oxygenation measurements and the interpretation of comparisons between results from various measurement methods.
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| 20. |
- Enger, Shirin A., et al.
(författare)
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Gadolinium neutron capture brachytherapy (GdNCB), a new treatment method for intravascular brachytherapy
- 2006
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 33:1, s. 46-51
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Tidskriftsartikel (refereegranskat)abstract
- Restenosis is a major problem after balloon angioplasty and stent implantation. The aim of this study is to introduce gadolinium neutron capture brachytherapy (GdNCB) as a suitable modality for treatment of stenosis. The utility of GdNCB in intravascular brachytherapy (IVBT) of stent stenosis is investigated by using the GEANT4 and MCNP4B Monte Carlo radiation transport codes. To study capture rate, Kerma, absorbed dose and absorbed dose rate around a Gd-containing stent activated with neutrons, a 30 mm long, 5 mm diameter gadolinium foil is chosen. The input data is a neutron spectrum used for clinical neutron capture therapy in Studsvik, Sweden. Thermal neutron capture in gadolinium yields a spectrum of high-energy gamma photons, which due to the build-up effect gives an almost flat dose delivery pattern to the first 4 mm around the stent. The absorbed dose rate is 1.33 Gy/min, 0.25 mm from the stent surface while the dose to normal tissue is in order of 0.22 Gy/min, i.e., a factor of 6 lower. To spare normal tissue further fractionation of the dose is also possible. The capture rate is relatively high at both ends of the foil. The dose distribution from gamma and charge particle radiation at the edges and inside the stent contributes to a nonuniform dose distribution. This will lead to higher doses to the surrounding tissue and may prevent stent edge and in-stent restenosis. The position of the stent can be verified and corrected by the treatment plan prior to activation. Activation of the stent by an external neutron field can be performed days after catherization when the target cells start to proliferate and can be expected to be more radiation sensitive. Another advantage of the nonradioactive gadolinium stent is the possibility to avoid radiation hazard to personnel.
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| 21. |
- Enger, Shirin A., et al.
(författare)
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Monte Carlo calculations of thermal neutron capture in gadolinium : a comparison of GEANT4 and MCNP with measurements
- 2006
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 33:2, s. 337-341
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Tidskriftsartikel (refereegranskat)abstract
- GEANT4 is a Monte Carlo code originally implemented for high-energy physics applications and is well known for particle transport at high energies. The capacity of GEANT4 to simulate neutron transport in the thermal energy region is not equally well known. The aim of this article is to compare MCNP, a code commonly used in low energy neutron transport calculations and GEANT4 with experimental results and select the suitable code for gadolinium neutron capture applications. To account for the thermal neutron scattering from chemically bound atoms [S(alpha,beta)] in biological materials a comparison of thermal neutron fluence in tissue-like poly(methylmethacrylate) phantom is made with MCNP4B, GEANT4 6.0 patch1, and measurements from the neutron capture therapy (NCT) facility at the Studsvik, Sweden. The fluence measurements agreed with MCNP calculated results considering S(alpha,beta). The location of the thermal neutron peak calculated with MCNP without S(alpha,beta) and GEANT4 is shifted by about 0.5 cm towards a shallower depth and is 25%-30% lower in amplitude. Dose distribution from the gadolinium neutron capture reaction is then simulated by MCNP and compared with measured data. The simulations made by MCNP agree well with experimental results. As long as thermal neutron scattering from chemically bound atoms are not included in GEANT4 it is not suitable for NCT applications.
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| 22. |
- Engström, Emma
(författare)
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Comparison of power spectra for tomosynthesis projections and reconstructed images
- 2009
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 36:5, s. 1753-1758
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Tidskriftsartikel (refereegranskat)abstract
- Burgess [Med. Phys. 28, 419-437 (2001)] showed that the power spectrum of mammographic breast background follows a power law and that lesion detectability is affected by the power-law exponent beta which measures the amount of structure in the background. Following the study of Burgess , the authors measured and compared the power-law exponent of mammographic backgrounds in tomosynthesis projections and reconstructed slices to investigate the effect of tomosynthesis imaging on background structure. Our data set consisted of 55 patient cases. For each case, regions of interest (ROIs) were extracted from both projection images and reconstructed slices. The periodogram of each ROI was computed by taking the squared modulus of the Fourier transform of the ROI. The power-law exponent was determined for each periodogram and averaged across all ROIs extracted from all projections or reconstructed slices for each patient data set. For the projections, the mean beta averaged across the 55 cases was 3.06 (standard deviation of 0.21), while it was 2.87 (0.24) for the corresponding reconstructions. The difference in beta for a given patient between the projection ROIs and the reconstructed ROIs averaged across the 55 cases was 0.194, which was statistically significant (p < 0.001). The 95% CI for the difference between the mean value of beta for the projections and reconstructions was [0.170, 0.218]. The results are consistent with the observation that the amount of breast structure in the tomosynthesis slice is reduced compared to projection mammography and that this may lead to improved lesion detectability.
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| 23. |
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| 24. |
- Fredenberg, Erik, 1979-, et al.
(författare)
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An efficient pre-object collimator based on an x-ray lens
- 2009
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 36:2, s. 626-633
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Tidskriftsartikel (refereegranskat)abstract
- A multiprism lens (MPL) is a refractive x-ray lens with one-dimensional focusing properties. If used as a pre-object collimator in a scanning system for medical x-ray imaging, it reduces the divergence of the radiation and improves on photon economy compared to a slit collimator. Potential advantages include shorter acquisition times, a reduced tube loading, or improved resolution. We present the first images acquired with a MPL in a prototype for a scanning mammography system. The lens showed a gain of flux of 1.32 compared to a slit collimator at equal resolution, or a gain in resolution of 1.31–1.44 at equal flux. We expect the gain of flux in a clinical setup with an optimized MPL and a custom-made absorption filter to reach 1.67, or 1.45–1.54 gain in resolution.
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