Irregular Volume Research Articles

A STOCHASTIC-PROBABILISTIC MODEL FOR SIMULATING PARTICLE DISPERSION IN GENERAL COORDINATES

This article describes an extended work on the development of the stochastic-probabilistic particle dispersion model for turbulent two-phase flows in general coordinates. The extended stochastic-probabilistic modeling of particle dispersion makes it possible to achieve a smooth particle flow field by tracking a small number of particle trajectories. A numerical method was developed for determining the spatial distribution of physical particles in irregular Eulerian control volumes. As a result, the extended particle dispersion model can be used, together with the Eulerian modeling of fluid flow, to predict turbulent two-phase flows with complex geometry. Compared with the conventional stochastic model, the present stochastic-probabilistic model has overcome the deficiency of tracking too large a number of particle trajectories to achieve a smooth particle flow field. Numerical results were reported for a particle-laden turbulent gas flow and liquid flow with available experimental measurements. The performance of the present model was assessed in terms of its agreement with experimental measurements and its computational efficiency as compared to the conventional stochastic particle dispersion model. For the two test cases considered, it was found that computational efficiency has been enhanced by 75%.

Proton radiation therapy for pediatric malignancies: status report.

Proton radiation therapy allows high degrees of conformity of radiation dose around irregular target volumes of variable sizes. Long-term follow-up in adults, and preliminary data for pediatric patients suggest that local control and survival can be improved in histologies requiring high radiation dose, without increased incidence of late toxicities. In the pediatric patient, avoidance of even moderate amounts of irradiation to normal tissues is of paramount importance. Conformal 3-D planned proton irradiation can contribute to this goal. For late effects, one can expect that reduced dose and volume irradiated will reduce radiation effects. However, full expression of late effects may occur in children five to ten years after treatment, or even later. Proton irradiation is therefore also indicated and used at Loma Linda University Medical Center for pediatric solid neoplasms in which conventional dose levels yield satisfactory local control and survival.

Stereotactic radiosurgery for anterior foramen magnum meningiomas

BACKGROUND Total microsurgical resection is the procedure of choice for growing and symptomatic foramen magnum meningiomas. We hypothesized that for patients with advanced age, complicating medical conditions, or residual or recurrent meningiomas at the foramen magnum, stereotactic radiosurgery would be a useful adjunctive ( n = 2) or alternative ( n = 3) treatment. METHODS We report our experience in five elderly patients (73–84 years) who underwent gamma knife radiosurgery. The median tumor volume was 10.5 ml and the tumor margin dose varied from 10 to 16 Gy. Because of the irregular tumor volumes along the inferior clivus, multiple isocenters of irradiation were required (range, 2–8; mean 4.4). RESULTS During the follow-up interval of 1–5 years (median, 3 years), one patient died of an intercurrent illness, and all remaining patients were stable without any further deterioration in their clinical condition. Follow-up imaging studies revealed a reduction in tumor volume in one patient and no further growth in the remaining four. CONCLUSION We believe that stereotactic radiosurgery provides safe and effective management for patients who are poor candidates for resection of their foramen magnum meningiomas.

Potential Gains Using High-Energy Protons for Therapy of Malignant Tumours

High-energy protons have physical properties that virtually always will result in geometrically better dose distributions than can be achieved using photons or electrons. The clinical gains in terms of the probability of higher tumour control and/or the reduced probability of normal tissue complications are, however, not completely known. Comparative model dose planning studies using real patients offer the possibility of estimating the potential gains using a new technique. Several recently completed model studies, including clinically relevant endpoints, indicate that protons may have advantages, even when compared with the conventional treatment that is likely to be introduced at the most advanced hospitals world-wide within the next decade. These advantages can be seen not only in well-demarcated targets close to risk organs, but also when irradiating extended irregular tissue volumes at risk of containing tumour cells.

A technique for manual definition of an irregular volume of interest in single photon emission computed tomography

A technique is described for manually outlining a volume of interest (VOI) in a three-dimensional SPECT dataset. Regions of interest (ROIs) are drawn on three orthogonal maximum intensity projections. Image masks based on these ROIs are backprojected through the image volume and the resultant 3D dataset is segmented to produce the VOI. The technique has been successfully applied in the exclusion of unwanted areas of activity adjacent to the brain when segmenting the organ in SPECT imaging using HMPAO. An example of its use for segmentation in tumour imaging is also presented. The technique is of value for applications involving semi-automatic VOI definition in SPECT.

The Advantages of Three-Dimensional Conformal Radiotherapy for Treatment of Childhood Cancer

Three-dimensional treatment planning with delivery of conformal radiotherapy has been demonstrated to improve tumor targeting and to reduce normal tissue volume exposed in several studies of malignancies in adults. Furthermore, institutional clinical protocols have accomplished escalation of radiation dose with enhanced treatment efficacy and stable to decreased late effects in patients with prostate carcinoma. In the treatment of malignancies in children, there are several aspects of therapy which may be particularly amenable to improvement with three-dimensional treatment planning. First, the occurrence of large, irregular tumor volumes close to critical normal structures is common in solid tumors in children. The acquisition of detailed computerized tomography (CT) images at the time of simulation, along with the use of three-dimensional reconstruction and beam's-eye-view capabilities, should greatly improve tumor targeting and allow more selective blocking of normal structures. In addition, the enhanced sensitivity of developing organs and tissues to radiation in combination with high expectations for overall survival with current treatment programs has resulted in greater awareness of the late effects of treatment. Today, the goal of pediatric radiotherapy is to provide high rates of local tumor control without significant late effects, such as impairment of growth and development and injury to organ function. The use of conformal radiotherapy is likely to be an important advance in realizing this goal. The ability to use novel beam arrangements and to evaluate radiotherapy treatment plans with quantitative tools, such as dose-volume histogram analysis or probabilities of normal tissue complications, should enable the use of treatment programs with a lessened risk of late effects. However, the rarity of pediatric tumors is unlikely to permit a controlled trial of conformal compared to conventional therapy. In this article, we use three case illustrations to demonstrate and discuss the potential advantages of conformal radiotherapy in the treatment of childhood cancer.

Seed specification for displaying a streamline in an irregular volume

Two techniques are presented for specifying seed points, which are the starting points for streamlines, in an irregular volume. One is an interactive seed specification technique, in which efficient data probing is achieved by traversing the adjacency graph of the volume. The other is an automatic seed specification technique. A seed point is placed near a critical point, where the velocity vector is zero. Efficient algorithms are given for finding and classifying the critical point by using a tetrahedral cell. A successful application of these techniques to a problem in a clean room is also described.

Avoiding errors in progressive tetrahedralizations

This paper describes some fundamental issues for robust implementations of progressively refined tetrahedralizations generated through sequences of edge collapses. We address the definition of appropriate cost functions and explain on various tests which are necessary to preserve the consistency of the mesh when collapsing edges. Although being considered a special case of progressive simplicial complexes [4], the results of our method are of high practical importance and can be used in many different applications, such as finite element meshing, scattered data interpolation or rendering of irregular volume data. CR Categories: I.3.5 [Computer Graphics]: Computational Geometry and Object Modeling – surfaces and object representations.

Comparison of radiosurgery treatment modalities based on physical dose distributions

As a means of selecting the optimal stereotactic radiosurgery (SRS) treatment modality, a comparison of physical dose distributions to defined targets and nontarget brain tissue has been made for a group of test cases selected to represent a range of treatment-planning situations from small, nearly spherical volumes to large irregular volumes. Plans were developed for each case using photon beams from the Leksell Gamma Unit (LGU), multiarc bremsstrahlung photon beams from a linear accelerator (linac) and proton beams, with the objective of encompassing the target as closely as possible with the prescription isodose line, and minimizing dosage to normal tissue within the bounds of standard clinical practice. Dose-volume histograms (DVHs) were calculated for target and for nontarget brain tissue and compared for the various modalities. In general, protons delivered less dosage to normal brain than other modalities for large and peripheral lesions and LGU plans were more successful at conforming to highly irregular shapes than conventional linac plans. Differences were observed to depend on treatment modality, target characteristics (shape, size and location), and the amount of effort expended on treatment planning and the time allotted for treatment implementation.

Comparison of radiosurgery treatment modalities based on complication and control probabilities

The relative efficacy of Gamma Knife, Linac, and Proton treatment modalities for stereotactic radiosurgery (SRS) was investigated on the basis of normal tissue complication probability (NTCP) and tumor control probability (TCP), calculated for representative test cases. Five radiosurgery patient cases were selected to cover a range of treatment-planning situations from small spherical volumes to large irregular volumes. A target volume consisting of contours drawn on CT transverse slices was prepared for each case. Plans were developed using the three treatment modalities for each case, with the objective of encompassing the target as closely as possible with a prescription isodose line and minimizing dose to normal tissue, within the constraints of current clinical practice. Dose-volume histograms (DVH) were calculated for the target and for normal tissue, and these histograms were used to calculate NTCP and TCP values for each plan. Differences in NTCP and TCP values were found to depend on treatment modality, size, shape, and location of the target, the amount of effort devoted to treatment planning, and the complexity of the plan.

Dose-response relationship for late functional changes in the rat brain after radiosurgery evaluated by magnetic resonance imaging

Only few quantitative data are available on late effects in the healthy brain after radiosurgery. An animal model can contribute to systematically investigate such late effects. Therefore, a model applying radiosurgery at the rat brain was established. A long-term (19 months) follow up study with 66 animals after radiosurgery was carried out. In 60 animals, an area in the frontal lobe of the brain was irradiated stereotactically with a 15 MV linac. Different doses of 20, 30, 40, 50, and 100 Gy with two field sizes (3.9 and 5.9 mm collimator) were selected, using the integrated logistic formula with input parameters from human brain. The induced alteration of the blood-brain barrier permeability was investigated by means of contrast enhanced magnetic resonance imaging. A first intracranial signal enhancement was observed in one animal 160 days after irradiation with 100 Gy. Beginning at 5 months all animals in the two 100 Gy groups homogeneously showed contrast enhancement, but none of the other groups. This remained until 13 months after irradiation. The volume of contrast enhancement as well as the increase of signal intensity were different between the two 100 Gy groups. After 19 months, the animals irradiated with lower doses also showed contrast enhancements, although not uniformly throughout one group. A maximum likelihood fit of the logistic formula P(D) = 1/[1 + (D50/D)k] to the incidence of late effects for the 5.9 mm collimator at 19 months after irradiation results in the parameters D50 = 37.4(-5.2,+6.1) Gy and k = 4.7 +/- 2.4. An animal model was established to study late normal brain tissue response. The observed late effects appeared very similar to the estimation of the integrated logistic formula for human brain. Based on these radiosurgery techniques, future experiments will focus on modifications in the irradiation modalities, i.e., irregular volumes, radiation quality or fractionation.

EFFICIENT PARTICLE TRACKING ALGORITHM FOR TWO-PHASE FLOWS IN GEOMETRIES USING CURVILINEAR COORDINATES

Particle dispersion in turbulent carrier flows is described using a Eulerian-Lagrangian hybrid model The body-fitted coordinates are used for the finite-volume solution to the Eulerian equations, whereas the mesh-free Lagrangian formulation is used for the solution to the Lagrangian equations. A novel numerical approach is developed to locate whether a particle stays in an arbitrary Eulerian control volume and to handle particle-wall interactions. The developed approach is characterized by its robustness and high computational efficiency. Details of the approach are given for a two-dimensional case. However, it is straightforwardly extendible to three-dimensional two-phase flows. As a demonstration, numerical results are presented for a three-dimensional, particle-laden turbulent gas flow in an industrial gas meter. It is found that the developed approach offers much higher computational efficiency than the existing circular-search approach. As a result, the particle-locating procedure with an optimal search path for irregular Eulerian control volumes becomes as efficient as that of the conventional tabular expression in rectangular Eulerian control volumes.

An efficient lagrangian trajectory model for predicting complex turbulent two-phase flows using the curvilinear coordinates

In this article, a numerical procedure is developed to extend the SPEED (Stochastic-Probabilistic Efficiency Enhanced Dispersion) model to the non-orthogonal curvilinear coordinates for the efficient prediction of dispersed turbulent two-phase flows with complex geometries. The partial-transformation approach using the Cartesian velocity vectors is employed to compute the fluid and dispersed flow fields. The Lagrangian trajectory model accounts for both the stochastic and probabilistic effects of discrete particle dispersion induced by fluid turbulence. A robust numerical algorithm is developed to determine the probability distribution in irregular Eulerian control volumes. The extended SPEED model is validated against a 90 deg turbulent bend flow laden with solid particles. It is found that the computational efficiency is greatly enhanced by using a very fewer number of particle trajectories than the conventional stochastic model while achieving the noise-free computational results.

Accelerating techniques in volume rendering of irregular data

Volume rendering of irregular data has become a challenging research topic in the area of scientific visualization in recent years. In comparison with regular data rendering, irregular data volume rendering faces many new problems. Because cells in an irregular volume have various shape and size, volume rendering by the raycasting method has more difficulty in calculating the intersection between rays and cells, and the projection method has even greater trouble in queuing up cells, in particular concave cells. To accelerate volume rendering of irregular data, this paper presents a hybrid method which possesses the advantages of both raycasting and projection methods. The main strategy in the method is using a polygon scan conversion algorithm in 3-D space to set up a ray path with high efficiency, and by establishing intersection links to allow the operation of color composition to be locally conducted on-the-fly. As a result, it is possible to produce high quality images without the requirement of the heavy task for cell queuing up and complex ray intersection calculation.

2057 A phase [formula omitted] study of fractionated intensity modulated radiation therapy in the treatment of patients with primary or recurrent high grade gliomas

‘Department of Radiation Oncology,and lDepartment of Neurosurgery, New England Medical Center, Tufts University School of Medicine, Boston, Massachusetts. ‘Baystate Medical Center, Springfield, Massachusetts. Pumose To determine the safety and effectiveness of fractionated intensity modulated radiation therapy (IMRT) in the treatment of primary and recurrent high grade gliomas Materials and Methods: Beginning in 1995, two prospective trials were undertaken at our institution to 1.) assess the potential role of dose escalation using hypofractionated IMRT for primary high grade gliomas, and 2.) re-irradiation using IMRT for recurrent high grade gliomas. Dosimetry and radiobiologic fractionation schemes for IMRT were designed for implant and stereotactic radiosurgery (SRS) ineligible patients to emulate V temporary interstitial implant (50 Gy @ .60 Gylhr) or single fraction radiosurgery of 15-20 Gy. Sixteen patients ( 14 Glioblastoma Multiforme (GBMI, 1 Anaplastic Astrocytoma (AA), 1 High grade Oligoastrocytoma) with a median age of 52 years (range 9-70 years1 have been treated on these protocols. Eight patients were treated for primary brain lesions, while the remaining 8 had recurrent disease after previous high dose conventional radiation therapy (median dose 5940 cGy). All patients had a Karnofsky Performance Status (KPS) of > 60. Patients were seen in follow-up two weeks after completion of therapy, then every 4 to 8 weeks. CT or MRI scans were performed at 6 to 8 weeks, then every 2-3 months. Cases were analyzed with regard to survival, acute toxicities, and corticosteroid use. Results: The median follow-up of the 16 patients was 5 months (range 2-8 months). Patients were treated with a median prescribed dose of 3325 cGy (range 1600-3500) using 400-500 cGy per fraction. The median tumor volume was 27.93 cc (range 10-l 19 cc). At last follow-up, 9 (56%) patients were alive. Of the patients who received a primary boost with IMRT, one patient at 7 months has no evidence of disease by CT/MRI while the remaining patients have persistent abnormalities. All patients who have been retreated for recurrence have persistent radiographic abnormalities on CT/MRI which could correspond to either radionecrosis or residual tumor. There were no acute toxicities or worsening of symptoms associated with the treatment. Of 10 evaluable patients,4 (40%) were steroid dependent at last follow-up. Conclusion; Hypofractionated IMRT is a feasible alternative with no appreciable short term toxicity to SRSlinterstitial brachytherapy ineligible patients for boost es part of primary treatment, or in the treatment of recurrent high grade gliomas. High dose fractionated treatment with IMRT may be most suitable for patients with highly irregular or large target volumes and/or are located in critically sensitive areas.

The alphabet soup revisited: the chronic interstitial pneumonias in the 1990s.

Liebow classified the idiopathic interstitial pneumonias as usual (UIP), desquamative (DIP), bronchiolitis obliterans (BIP), lymphoid (LIP), and giant cell (GIP) interstitial pneumonias. This classification was modified to exclude LIP and GIP. UIP, the most common type, is characterized by synchronous foci of inflammation, collagen deposition, and fibrosis with interspersed normal lung. It usually affects men 40-60 years old and manifests radiologically with bilateral, basilar irregular opacities and volume loss. In most cases, a confident diagnosis can be made at high-resolution computed tomography because of characteristic subpleural irregular linear opacities, ground-glass opacities, honeycombing, and traction bronchiectasis. DIP affects younger patients and is characterized by diffuse intraalveolar macrophage aggregation. Typical radiologic features include bilateral, basilar ground-glass opacities and preserved lung volumes. BIP, renamed bronchiolitis obliterans with organizing pneumonia, affects middle-aged patients and manifests with multifocal plugs of immature fibroblasts in the air spaces. Typical radiologic features include bilateral consolidations and normal lung volumes. Recently described entities include acute (AIP) and nonspecific (NIP) interstitial pneumonias and respiratory bronchiolitis with interstitial lung disease (RB-ILD). AIP is a rapidly progressive, often fatal, illness characterized by diffuse alveolar damage and manifests with clinical and radiologic features of adult respiratory distress syndrome. NIP is a heterogeneous group of fibrosing disorders that cannot be otherwise classified. RB-ILD is a disease of smokers with a good prognosis.

A natural extension of the conventional finite volume method into polygonal unstructured meshes for CFD application

A new general cell-centered solution procedure based upon the conventional control or finite volume (CV or FV) approach has been developed for numerical heat transfer and fluid flow which encompasses both structured and unstructured meshes for any kind of mixed polygon cell. Unlike conventional FV methods for structured and block structured meshes and both FV and FE methods for unstructured meshes, the irregular control volume (ICV) method does not require the shape of the element or cell to be predefined because it simply exploits the concept of fluxes across cell faces. That is, the ICV method enables meshes employing mixtures of triangular, quadrilateral, and any other higher order polygonal cells to be exploited using a single solution procedure. The ICV approach otherwise preserves all the desirable features of conventional FV procedures for a structured mesh; in the current implementation, collocation of variables at cell centers is used with a Rhie and Chow interpolation (to suppress pressure oscillation in the flow field) in the context of the SIMPLE pressure correction solution procedure. In fact all other FV structured mesh-based methods may be perceived as a subset of the ICV formulation. The new ICV formulation is benchmarked using two standard computational fluid dynamics (CFD) problems i.e., the moving lid cavity and the natural convection driven cavity. Both cases were solved with a variety of structured and unstructured meshes, the latter exploiting mixed polygonal cell meshes. The polygonal mesh experiments show a higher degree of accuracy for equivalent meshes (in nodal density terms) using triangular or quadrilateral cells; these results may be interpreted in a manner similar to the CUPID scheme used in structured meshes for reducing numerical diffusion for flows with changing direction.

Conformal therapy using maximum entropy optimization

AbstractConformal therapy offers considerable advantages over conventional radiotherapy treatment, because it has the potential for matching almost exactly the delivered dose distribution to the prescribed dose distribution. Associated (inverse) treatment planning methods address a constrained linear optimization problem. In this article, a method based on maximizing the total entropy of the beam profiles is developed. Maximum entropy optimization constrains the computed dose to be within well‐defined tolerances of the prescribed dose, and has advantages of robustness, fast convergence, and high accuracy. For the work reported here, it is assessed using clinically prescribed irregular target dose volumes based on magnetic resonance imaging and computed tomography images. Results are shown for a two‐dimensional, homogeneous absorption, primary dose computation model, to illustrate the feasibility of the approach; however, the method may be extended to accommodate a more general three‐dimensional model, including inhomogeneities and scatter dose contributions. Optimization of beam offset for a regular angular displacement of beams is also considered, with particular regard to implications on total beam energy, entropy, and computation time.

Fast generation of spherical slicing surfaces for irregular volume rendering

An efficient algorithm for generating a set of concentric spherical slicing surfaces for volume rendering of irregular volume datasets is presented. Our original algorithm, which approximates volume rendering by accumulating concentric spherical slicing surfaces from back to front, generates these surfaces by means of a conventional isosurface generation algorithm. However, this causes a performance bottleneck. To solve the problem, we propose a proliferous generation of slicing surfaces from seed cells, which are automatically determined according to the extremum points of the values of distances from a viewing point. A benchmark test shows that this approach can improve the performance significantly. In addition, we compare this algorithm with a raycasting algorithm that we proposed previously, and discuss a criterion for selecting which one to use for maximizing the performance

Optimization of interstitial volume implants

For interstitial applications of high dose rate (HDR) afterloading brachytherapy, generally a single stepping iridium-192 source is used, enabling optimization of the dose distribution by optimization of the relative time (dwell time) that the source remains at a certain position (dwell position). We analysed the effects of geometric optimization in a regular volume implant, with strictly parallel catheters, and in an irregular volume implant, such as an implant for tumours of the base of the tongue characterized by a non-parallel geometry and varying catheter separations. In both examples the reference dose is specified at 85% of the mean central dose (as is done in the Paris system for dose specification) in the non-optimized as well as the optimized plan. The irradiated volume, the dose uniformity, and the choice of the reference dose of optimized and non-optimized dose distributions were compared. This was done by isodose plots for representative planes, volume dose histograms (distributed, contiguous, and natural), and dose non-uniformity ratios (DNRs). For the regular implant, optimization results in a 28% increase in the treated volume with a similar increase in the overdosed volumes. In order to keep the treated volume comparable with the non-optimized dose distribution, 90–95% of the mean central dose should be chosen as a reference dose or the range of active dwell positions should be shortened in case of optimization. In the case of the irregular volume implant at the base of the tongue, the method for dose specification should be kept unchanged after geometric optimization as the volume enclosed by the reference isodose does not increase. It is clear from the volume-dose histograms that there is a reduction of the overdosed volume due to optimization. This is accompanied by an increase in the uniformity index and a decrease of the DNR. In conclusion, geometric optimization appears to be an effective tool to improve the dose distribution of interstitial volume implants. Contiguous and natural volume dose histograms appear, apart from planar dose plots, valuable methods for evaluating the dose distribution of an implant.