Asymmetric Margins Research Articles

Uterus motion analysis for radiotherapy planning optimization. The innovative contribution of on-board hybrid MR imaging

IntroductionOrgan motion (OM) and volumetric changes pose challenges in radiotherapy (RT) for locally advanced cervical cancer (LACC). Magnetic Resonance-guided Radiotherapy (MRgRT) combines improved MRI contrast with adaptive RT plans for daily anatomical changes. Our goal was to analyze cervico-uterine structure (CUS) changes during RT to develop strategies for managing OM. Materials and methodsLACC patients received chemoradiation by MRIdian system with a simultaneous integrated boost (SIB) protocol. Prescription doses of 55–50.6 Gy at PTV1 and 45–39.6 Gy at PTV2 were given in 22 and 25 fractions. Daily MRI scans were co-registered with planning scans and CUS changes were assessed.Six PTVs were created by adding 0.5, 0.7, 1, 1.3, 1.5, and 2 cm margins to the CUS, based on the simulation MRI. Adequate margins were determined to include 95 % of the CUSs throughout the entire treatment in 95 % of patients. ResultsAnalysis of 15 LACC patients and 372 MR scans showed a 31 % median CUS volume decrease. Asymmetric margins of 2 cm cranially, 0.5 cm caudally, 1.5 cm posteriorly, 2 cm anteriorly, and 1.5 cm on both sides were optimal for PTV, adapting to CUS variations. Post-14th fraction, smaller margins of 0.7 cm cranially, 0.5 cm caudally, 1.3 cm posteriorly, 1.3 cm anteriorly, and 1.3 cm on both sides sufficed. ConclusionCUS mobility varies during RT, suggesting reduced PTV margins after the third week. MRgRT with adaptive strategies optimizes dose delivery, emphasizing the importance of streamlined IGRT with reduced PTV margins using a tailored MRgRT workflow with hybrid MRI-guided systems.

Abstract PO1-22-02: Optimal intra-fraction PTV margins for patients receiving stereotactic partial breast radiation (SPBI) using cone beam adaptive radiation therapy

Abstract Objectives To quantify the intra-fraction tumor bed motion using image guidance and fiducial markers to determine optimal PTV margins in early-stage breast cancer patients receiving adaptive radiation therapy SPBI after partial mastectomy. Methods A retrospective cohort of five patients with pT1a-pT2N0, grade 1-2, ER+/PR+ early-stage invasive ductal carcinoma of either the right breast or left breast underwent partial mastectomy and adjuvant five-fraction SPBI using cone beam based online adaptation. A total of 62 CBCTs and 22 total fractions were analyzed after post-processing with an average time span of 39.1±8.9 minutes. Two methods were utilized to determine optimal CTV to PTV margins: one contouring-based and another fiducial-based. Our contouring-based method involved independently contouring on each fraction’s CBCTs including pre-treatment CBCT, intra-treatment CBCT, and post-treatment CBCT. After image registration with the original simulation CT image, these contours were propagated to and combined to create a new union CTV. Using the original CTV on the simulation CT, optimal PTV margin was determined to achieve 95% overlap of the union CTV created from fractionated CBCTs. In addition to the contouring-based method, our fiducial-based method utilized an in-house script to calculate the centroid of all fiducials per CBCT. The average and maximum anterior-posterior, medial-lateral, superior-inferior shifts of fiducials between intra-fraction CBCTs were recorded. Results Using our contouring-based method, we calculated and determined that a uniform 3.5 mm expansion around the original CT simulation CTV is needed to achieve 95% PTV overlap encompassing the pre-treatment, intra-treatment, and post-treatment union CTV. Furthermore, with our fiducial-based method, we calculated average centroid anterior-posterior, medial-lateral and superior-inferior shifts to be the following: 1.51±0.79mm, 1.04±0.55 mm, 1.78±0.94 mm respectively. Average maximum shifts were determined to be 2.13±1.18mm anterior-posterior, 1.45±0.76mm medial-lateral and 2.47±1.40mm superior-inferior shifts. Conclusion With our contouring-based analysis, this preliminarily study suggests CTV to PTV margin of 3.5mm is adequate. On the other hand, our fiducial-based method suggests decreased movement and smaller asymmetric margin. Further CBCT adaptation images will be analyzed. In conjunction with these two methods, we currently recommend utilizing a 3.5mm uniform margin for intrafraction motion and to assess adequate coverage with image guidance during daily adaptation for patients with early-stage breast cancer receiving SPBI. Citation Format: Meng-Lun Hsieh, Justin Visak, Sean Domal, Gabrielle Gard, Steven Montalvo, Liyuan Chen, Xinran Zhong, Zohaib Iqbal, Tingliang Zhuang, Bin Cai, Heejung Kim, Mona Arbab, Prasanna Alluri, Nathan Kim, Mu-Han Lin, David Parsons, Asal Rahimi. Optimal intra-fraction PTV margins for patients receiving stereotactic partial breast radiation (SPBI) using cone beam adaptive radiation therapy [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-22-02.

Turning kinematics of the scyphomedusa Aurelia aurita

Scyphomedusae are widespread in the oceans and their swimming has provided valuable insights into the hydrodynamics of animal propulsion. Most of this research has focused on symmetrical, linear swimming. However, in nature, medusae typically swim circuitous, nonlinear paths involving frequent turns. Here we describe swimming turns by the scyphomedusa Aurelia aurita during which asymmetric bell margin motions produce rotation around a linearly translating body center. These jellyfish ‘skid’ through turns and the degree of asynchrony between opposite bell margins is an approximate predictor of turn magnitude during a pulsation cycle. The underlying neuromechanical organization of bell contraction contributes substantially to asynchronous bell motions and inserts a stochastic rotational component into the directionality of scyphomedusan swimming. These mechanics are important for natural populations because asynchronous bell contraction patterns are common in situ and result in frequent turns by naturally swimming medusae.

Assessing systemic risk and connectedness among dirty and clean energy markets from the quantile and expectile perspectives

In response to environmental and climate change issues in recent decades, clean energy resources have been promoted as substitutes for fossil fuel-based dirty energy resources. While receiving great attention from environmentally conscious investors and policy-makers, the markets for the former energies could be affected by financial instability in the markets for the latter, which could lead to systemic risk and directional connectedness. In this paper, we aimed to assess potential systemic risk in such markets by employing conditional quantile- and conditional expectile-based Value-at-Risk (henceforth CoVaR and CoEVaR, respectively). Due to their definition through expected conditional asymmetric loss functions, we adopted Kabaila’s efficient method for the computation of conditional expectations and proposed a backtesting technique based on Diebold and Mariano’s test. Using Delta-CoVaR and Delta-CoEVaR, we constructed conditional tail risk networks to describe the directional tail risk connectedness. We revealed that the CoVaR and CoEVaR for almost all pairs of energy markets had the best forecast accuracy when computed using a copula exhibiting symmetric tail dependence with asymmetric and heavy-tailed margins. Compared to CoVaR, CoEVaR possessed a relatively better conditional coverage performance in several cases, complementing its advantage of being more sensitive to (extreme) loss magnitude. In addition, we found crude oil, heating oil, and sectoral clean energy indices to be highly interconnected, but the connectedness weakened following the 2015 Paris Climate Agreement. In particular, crude oil and the renewable energy sector played a consistent role as the most systemically important net receiver and net transmitter, respectively. These results provided important implications for policy-makers and investors aware of systemic risk and connectedness among dirty and clean energy markets.

Unsupervised Domain Adaptation with Asymmetrical Margin Disparity loss and Outlier Sample Extraction

Unsupervised domain adaptation (UDA) trains models using labeled data from a specific source domain and then transferring the knowledge to certain target domains that have few or no labels. Many prior measurement-based works achieve lots of progress, but their feature distinguishing abilities to classify target samples with similar features are not enough; they do not adequately consider the confusing samples in the target domain that are similar to the source domain; and they don't consider negative transfer of the outlier sample in source domain. We address these issues in our work and propose an UDA method with asymmetrical margin disparity loss and outlier sample extraction, called AMD-Net with OSE. We propose an Asymmetrical Margin Disparity Discrepancy (AMD) method and a training strategy based on sample selection mechanism to make the network have better feature extraction ability and the network gets rid of local optimal. Firstly, in the AMD method, we design a multi-label entropy metric to evaluate the marginal disparity loss of the confusing samples in the target domain. This asymmetric marginal disparity loss designment uses the different entropy measurement algorithms of the two domains to excavate the differences of the two domains as much as possible, so as to find the common features of the two domains. Secondly, A sample selection mechanism is designed to evaluate which part of the sample in target domain is confusable. We define the certainty of the sample in the target domain, adopt a progressive learning scheme, and adopt one-hot marginal disparity loss for most of the samples in the target domain with low uncertainty and easy to distinguish. The multi-label marginal calculation method is used only for the uncertainty samples in the target domain whose certainty is less than the threshold value, so that the network can get rid of the local optimal as much as possible. At last, we further propose an outlier sample extraction algorithm (OSE) based on weighted cosine similarity distance for source domain to reduce the negative migration effect caused by outlier samples in the source domain. Extensive experiments on four datasets Office-31, Office-Home, VisDA-2017 and DomainNet demonstrate that our method works well in various UDA settings and outperforms the state-of-the-art methods.

Asymmetry and evolution of craton-influenced rifted margins

Abstract Rifting can result in asymmetric conjugate margins. In numerical models with laterally homogeneous lithosphere, the polarity of the asymmetry is random and results from spontaneous strain localization on a dominant fault/shear zone. However, along the central South Atlantic, margin width is well correlated to the proximity of the rift to a craton during rifting. To understand the evolution of rifting close to a craton, we used numerical experiments that included a thick craton lithosphere adjacent to a thinner, mobile-belt lithosphere. When rifting starts close to the craton, i.e., ≤ 100 km, deformation focuses along a fault/shear zone dipping toward the craton, leading to cratonward asymmetric asthenospheric uplift and rift migration. This results in a narrow margin on the craton side and a wide margin on the mobile-belt side. Craton-related asymmetry results in the wide margin subsiding more than the narrow one, inducing the formation of large synrift sag basins in the wide conjugate, as in the South Atlantic. This differential subsidence is preserved for ~100 m.y. after breakup and may partly explain the present-day higher dynamic topography close to cratons in the central South Atlantic. Mantle flow during asymmetric rifting leads to weakening of the craton lithosphere, partial intermixing, and recycling into the convective mantle, as interpreted for this sector of the South Atlantic ridge. Thus, rifting adjacent to a craton may have wide-ranging tectonic, stratigraphic, and compositional consequences that affect not only margin architecture, but also the development of subsequent dynamic topography and the spreading system.

Population-based asymmetric margins for moving targets in real-time tumor tracking.

Both geometric and dosimetric components are commonly considered when determining the margin for planning target volume (PTV). As dose distribution is shaped by controlling beam aperture in peripheral dose prescription and dose-escalated simultaneously integrated boost techniques, adjusting the margin by incorporating the variable dosimetric component into the PTV margin is inappropriate; therefore, geometric components should be accurately estimated for margin calculations. We introduced an asymmetric margin-calculation theory using the guide to the expression of uncertainty in measurement (GUM) and intra-fractional motion. The margins in fiducial marker-based real-time tumor tracking (RTTT) for lung, liver, and pancreatic cancers were calculated and were then evaluated using Monte Carlo (MC) simulations. A total of 74705, 73235, and 164968 sets of intra- and inter-fractional positional data were analyzed for 48 lung, 48 liver, and 25 pancreatic cancer patients, respectively, in RTTT clinical trials. The 2.5th and 97.5th percentiles of the positional error were considered representative values of each fraction of the disease site. The population-based statistics of the probability distributions of these representative positional errors (PD-RPEs) were calculated in six directions. A margin covering 95% of the population was calculated using the proposed formula. The content rate in which the clinical target volume (CTV) was included in the PTV was calculated through MC simulations using the PD-RPEs. The margins required for RTTT were at most 6.2, 4.6, and 3.9mm for lung, liver, and pancreatic cancer, respectively. MC simulations revealed that the median content rates using the proposed margins satisfied 95% for lung and liver cancers and 93% for pancreatic cancer, closer to the expected rates than the margins according to van Herk's formula. Our proposed formula based on the GUM and motion probability distributions (MPD) accurately calculated the practical margin size for fiducial marker-based RTTT. This was verified through MC simulations.

Effects of Tectonic Quiescence Between Orogeny and Rifting

AbstractTectonic quiescence is a period of nearly null relative plate motion. During the Atlantic Ocean opening, rifting cut across continental segments of supercontinent Pangea that had been through different periods of quiescence, which varied from tens to hundreds of Myr. Using numerical models, we explored the effects of quiescence between orogeny and rifting on the pre‐rift lithosphere and subsequent conjugate rifted margin configuration. Our model results showed that quiescence of ∼30–60 Myr caused wide orogenic wedges (>300 km) to be heated beneath their centers, which led to the breakup of the lithospheric mantle before the breakup of the continental crust during rifting, resulting in hyperextended conjugate rifted margins. In these scenarios, continental lithosphere breakup away from the suture contributed to the preservation of previously subducted lower crust along the subduction zone. Long periods of tectonic quiescence (∼100 Myr) experienced by a wide orogen permitted efficient orogenic collapse and lateral spreading, with pronounced lithospheric thermal weakening. Consequently, post‐quiescence rifting reactivated the suture and effectively drove plate eduction, exhuming most of the subducted continental lower crust and mantle lithosphere. Ultimately, ultra‐wide domains developed into asymmetric conjugate margins. Contrastingly, in narrow orogenic wedges (<200 km), different quiescence durations had minor effects on the margin architecture. In these cases, a longer quiescence time contributed to a larger preservation of subducted crust in the fossil subduction zone after rifting. Our models agree with conjugate rifted margins in the North and South Atlantic, which probably developed after the collapse of wide orogens and subsequent periods of tectonic quiescence.

Mesorectal motion evaluation in rectal cancer MR-guided radiotherapy: an exploratory study to quantify treatment margins

BackgroundMesorectal motion (MM) is a source of uncertainty during neoadjuvant chemoradiotherapy (nCRT) delivery for locally advanced rectal cancer (LARC). Previously published experiences using cone-beam computed tomography imaging have already described significant movement. Aim of this analysis is to assess inter-fraction MM using the higher tissue contrast provided by hybrid magnetic resonance imaging (MRI) in LARC patients (pts) treated with MRI guided radiation therapy (MRgRT).MethodsThe total mesorectum, its superior (Msup), middle (Mmid) and lower (Mlow) regions were contoured on the positioning MRIs acquired on simulation day and on each treatment day. Six PTVs were obtained adding 0.5, 0.7, 1, 1.3, 1.5 and 2 cm margin to the whole mesorectum, starting from the simulation MRI. Margins including 95% of the mesorectal structures during whole treatment in 95% of patients (pts) were considered adequate.ResultsA total number of 312 fractions of 12 consecutive pts was retrospectively analyzed. The different mesorectum regions show specific motion variability. In particular, Msup shows larger variability in left, right and anterior directions, while the Mlow in caudal and posterior ones. The anterior margin is significantly larger in the Msup than in the other regions.ConclusionDifferent mesorectal regions move differently throughout the radiotherapy treatment, with the largest MM in the Msup anterior direction. Asymmetrical margins are recommended.

Quantification of Organ Motion in Male and Female Patients Undergoing Long-Course Radiation Therapy for Rectal Cancer in the Supine Position

PurposePrevious studies have reported data on the internal rectal motion of patients with rectal cancer treated in the prone position. With the introduction of intensity modulated techniques, more patients are treated in the more reproducible supine position. Data informing specific margins for this treatment position are sparse, as are data comparing rectal motion characteristics and factors in male and female patients. The purpose of this retrospective study was to quantify and compare the interfractional rectal movement characteristics of male and female patients with rectal cancer treated with long-course chemoradiation therapy in the supine position. The data will aid the generation of internal target volume margins accounting for this organ's internal physiological movements. Methods and MaterialsCone beam computed tomography (CBCT) images were acquired from 19 male and 16 female patients with rectal cancer on the first 3 days of treatment and weekly thereafter. The rectum, bladder, and femoral heads were delineated on the planning CT (PCT) and 6 CBCT for each patient. Overall, 245 images were analyzed. All patients were treated with a full bladder. The rectum was divided into three 5-cm segments (upper, mid, and lower). The motion of the rectum was quantified by documenting the anteroposterior and lateral distances as measured using fixed anatomic landmarks, namely from the anterior aspect of the sacrum and mid-left femoral head, respectively. These measurements were taken at 1-cm intervals from the inferior border of L5 vertebrae. The sigmoid was excluded from these measurements. Estimations of systematic and random physiological movement error were determined and margins were calculated. ResultsTwo hundred forty-five image sets (19 PCT + 114 CBCT for male, 16 PCT + 96 CBCT for female) on patients who had undergone long-course radiation therapy were analyzed. Rectal tumor location was 31% in the inferior rectum, 46% in the mid rectum, and 23% in the superior rectum. Random rectal motion (mean of the per-patient standard deviation [σ]) was largest for the upper and mid rectum in the anterior direction. There were statistically significant differences in σ between male and female patients in the left lateral motion of the mid and inferior rectum as well as the anterior, posterior, and right motion of the inferior rectum (mid left: P < .0005; lower left: P < .0005; lower posterior: P = .001; lower anterior: P = .032; lower right: P = .001). Suggested internal target volume margin guidelines are therefore nonisotropic and vary per segment of rectum and sex. ConclusionsIn our present study, interfractional rectal motion is shown to be significantly different between male and female patients. Our data suggest that the use of asymmetrical sex-specific margins in patients with rectal cancer treated in the supine position should be considered.

Minimum non-isotropic and asymmetric margins for taking into account intrafraction prostate motion during moderately hypofractionated radiotherapy

PurposeTo investigate the impact on dose distribution of intrafraction motion during moderate hypofractionated prostate cancer treatments and to estimate minimum non-isotropic and asymmetric (NI-AS) treatment margins taking motion into account. MethodsProstate intrafraction 3D displacements were recorded with a transperineal ultrasound probe and were evaluated in 46 prostate cancer patients (876 fractions) treated by moderate hypofractionated radiation therapy (60 Gy in 20 fractions). For 18 patients (346 fractions), treatment plans were recomputed increasing CTV-to-PTV margins from 0 to 6 mm with an auto-planning optimization algorithm. Dose distribution was estimated using the voxel shifting method by displacing CTV structure according to the retrieved movements. Time-dependent margins were finally calculated using both van Herk’s formula and the voxel shifting method. ResultsMean intrafraction prostate displacements observed were −0.02 ± 0.52 mm, 0.27 ± 0.78 mm and −0.43 ± 1.06 mm in left–right, supero-inferior and antero-posterior directions, respectively. The CTV dosimetric coverage increased with increased CTV-to-PTV margins but it decreased with time. Hence using van Herk’s formula, after 7 min of treatment, a margin of 0.4 and 0.5 mm was needed in left and right, 1.5 and 0.7 mm in inferior and superior and 1.1 and 3.2 mm in anterior and posterior directions, respectively. Conversely, using the voxel shifting method, a margin of 0 mm was needed in left–right, 2 mm in superior, 3 mm in inferior and anterior and 5 mm in posterior directions, respectively. With this latter NI-AS margin strategy, the dosimetric target coverage was equivalent to the one obtained with a 5 mm homogeneous margin. ConclusionsNI-AS margins would be required to optimally take into account intrafraction motion.

Effects of total ionizing dose on transient ionizing radiation upset sensitivity of 40–180 nm SRAMs

Effects of total ionizing dose (TID) on the transient radiation upset sensitivity of commercial static random access memories (SRAMs) were investigated. Commercial SRAMs of three technology nodes were irradiated with Co-60 gamma rays for total dose effects and with pulsed x rays generated using a “Chen-Guang” accelerator for doserate effects. It is shown that TID irradiation and the memory pattern written to the memory array during TID irradiation can substantially affect the transient radiation upset sensitivity of all the SRAMs. First, the memory pattern written to the memory array during TID irradiation is more sensitive to pulsed x rays for all SRAMs. Second, the variation in transient radiation upset sensitivity with total dose has different tendencies for different SRAMs, but the static power current increased with the increase in total dose for all SRAMs. The increase in static power current with total dose and the increasing sensitivity of the memory pattern during TID irradiation are caused by the increasing TID induced leakage current, which results in an asymmetric noise margin of individual memory cells.

Multiparametric MRI-guided Hypofractionated Intensity-modulated Radiation Therapy With Simultaneous Integrated Boost for Glioblastoma

This study investigated the feasibility and efficacy of multiparametric magnetic resonance imaging (MRI)-guided dose-escalated hypofractionated intensity-modulated radiation therapy with simultaneous integrated boost (IMRT-SIB) for glioblastoma. Eighteen patients underwent postoperative IMRT-SIB for glioblastoma using three MRI sequences: double inversion recovery (DIR), diffusion tensor imaging (DTI), and post-gadolinium T1-weighted imaging. Prescribed doses were 60 Gy and 40 Gy in 15 fractions for residual enhancing lesions and surrounding tumor-infiltrating areas, respectively. For surrounding tumor-infiltrating areas, asymmetric margins were set with reference to DTI imaging. The 1-year overall survival rate was 58.0%, and the 1-year local control rate for the residual enhancing lesions was 76.2%, while that for surrounding tumor-infiltrating areas was 39.4%. One patient (6%) developed grade 2 cerebral radiation necrosis 10 months after IMRT-SIB, but there was no grade 3 or higher adverse event. Multiparametric MRI-guided dose-escalated IMRT-SIB with DIR and DTI imaging has the potential to improve local control rates without increasing adverse events.

Buoyancy versus shear forces in building orogenic wedges

Abstract. The dynamics of growing collisional orogens are mainly controlled by buoyancy and shear forces. However, the relative importance of these forces, their temporal evolution and their impact on the tectonic style of orogenic wedges remain elusive. Here, we quantify buoyancy and shear forces during collisional orogeny and investigate their impact on orogenic wedge formation and exhumation of crustal rocks. We leverage two-dimensional petrological–thermomechanical numerical simulations of a long-term (ca. 170 Myr) lithosphere deformation cycle involving subsequent hyperextension, cooling, convergence, subduction and collision. Hyperextension generates a basin with exhumed continental mantle bounded by asymmetric passive margins. Before convergence, we replace the top few kilometres of the exhumed mantle with serpentinite to investigate its role during subduction and collision. We study the impact of three parameters: (1) shear resistance, or strength, of serpentinites, controlling the strength of the evolving subduction interface; (2) strength of the continental upper crust; and (3) density structure of the subducted material. Densities are determined by linearized equations of state or by petrological-phase equilibria calculations. The three parameters control the evolution of the ratio of upward-directed buoyancy force to horizontal driving force, FB/FD=ArF, which controls the mode of orogenic wedge formation: ArF≈0.5 causes thrust-sheet-dominated wedges, ArF≈0.75 causes minor wedge formation due to relamination of subducted crust below the upper plate, and ArF≈1 causes buoyancy-flow- or diapir-dominated wedges involving exhumation of crustal material from great depth (&gt;80 km). Furthermore, employing phase equilibria density models reduces the average topography of wedges by several kilometres. We suggest that during the formation of the Pyrenees ArF⪅0.5 due to the absence of high-grade metamorphic rocks, whereas for the Alps ArF≈1 during exhumation of high-grade rocks and ArF⪅0.5 during the post-collisional stage. In the models, FD increases during wedge growth and subduction and eventually reaches magnitudes (≈18 TN m−1) which are required to initiate subduction. Such an increase in the horizontal force, required to continue driving subduction, might have “choked” the subduction of the European plate below the Adriatic one between 35 and 25 Ma and could have caused the reorganization of plate motion and subduction initiation of the Adriatic plate.

A comprehensive active learning method for multiclass imbalanced data streams with concept drift

A challenge to many real-world applications is multiclass imbalance with concept drift. In this paper, we propose a comprehensive active learning method for multiclass imbalanced streaming data with concept drift (CALMID). First, we design a comprehensive online active learning framework that includes an ensemble classifier, a drift detector, a label sliding window, sample sliding windows and an initialization training sample sequence. Next, a variable threshold uncertainty strategy based on an asymmetric margin threshold matrix is designed to comprehensively address the problem that a given class can simultaneously be a majority to a given subset of classes while also being a minority to others. Last but not least, we design a novel sample weight formula that comprehensively considers the class imbalance ratio of the sample’s category and the prediction difficulty. On 10 multiclass synthetic streams with different imbalance ratios and concept drifts, and on 5 real-world imbalanced streams with 7 to 55 classes and unknown drifts, the experimental results demonstrate that the proposed CALMID is more effective and efficient than several state-of-the-art learning algorithms.

Lithospheric Strength and Rift Migration Controls on Synrift Stratigraphy and Breakup Unconformities at Rifted Margins: Examples From Numerical Models, the Atlantic and South China Sea Margins

AbstractSynrift stratigraphy and the distribution of breakup‐related erosional unconformities vary vastly between passive margins and cannot be explained by classical rifting models. Here we use numerical modeling to predict their spatiotemporal distribution. We show that synrift stratigraphy mimics rift architecture, which is controlled by lithospheric strength. Basinward rift migration during extension produces (1) oceanward younging, syntectonic and posttectonic sequences, (2) rift migration unconformities, RMUs, predating breakup, and (3) a breakup unconformity, BU, that only extends over the outermost margins, since breakup is not linked with a sudden stress drop. With small synrift sedimentation, the RMUs and BU laterally merge to form a margin‐wide unconformity. In symmetric, wide conjugate margins, which arise for weak lithospheres such as the South China Sea, a long phase of distributed deformation with little subsidence results in early synrift sediment over most of the margins. RMUs merge into a single event that marks the subsequent focusing of deformation into a narrow breakup area, which experiences short‐lived intense thinning and subsidence. In asymmetric conjugate margins, lateral rift migration transports shallowly deposited, early synrift sediments from the narrow to the wide, hyperextended margin, leading to a condensed syntectonic sequence and a single BU in the narrow margin and a series of RMUs in the wide one. For very weak lower crusts, lateral rift migration generates large synrift sag basins in the wide margin, as in Angola and Congo margins. Our models resemble the observed margins tectonic diversity and may be used as templates for interpreting their distal, unexplored areas.

Methods to optimize conformal dynamic arcs in radiation therapy treatment planning

Conformal Dynamic Arcs (CDA) can provide a useful alternative in certain clinical situations which require a higher level of 3-dimensional (3D) conformation than shaped static fields but for which modulated fields (fixed or arc) are inappropriate. Due to lack of modulation, the quality of the dose distribution produced by a CDA is highly dependent on the specific patient geometry. The ideal geometry for a CDA, in terms of achievable conformation and uniformity, is a spherical target perfectly centered in a cylindrical medium or body and utilizing a full 360 degree of rotation. This manuscript will provide or review several methods a treatment planner may employ to improve dose distributions produced by CDA as the patient and/or target geometry or degrees of arc rotation vary from the ideal. These include:1. Weighting arc segments to improve homogeneity.2. Defining asymmetric margins for fitting leaves.3. Hybrid static/CDA compensating for CDA with limited degrees of rotations.4. Improving conformation in irregular target via use of pseudo-PTV.5. Creating concave features via the use of avoidance structures.6. Editing CDA multi-leaf collimator (MLC) to remove irregular hot spots.

Study of normal tissue dosimetric benefit using asymmetric margin-based biological fuzzy decision making: volumetric modulated arc therapy of prostate cancer

AbstractAim:Radiation therapy has historically used margins for target volume to ensure dosimetric planning criteria. The size of margin for a given treatment site is still uncertain particularly for moving targets along with set-up variations leading to a fuzziness of target volume. In this study, we have estimated the dosimetric benefit of normal structures using biological-based optimal margins. The treatment margins are derived by knowledge-based fuzzy logic technique which is considering the radiotherapy uncertainties in treatment planning.Materials and methods:All treatment plans were performed using stepped increments of asymmetric margins to estimate prostate radiobiological indices such as tumour control probability (TCP) and normal tissue complication probability (NTCP). An absolute NTCP of 5% was considered to be the maximum acceptable value while TCP of 85% was considered to be the minimal acceptable limit for each volumetric modulated arc therapy (VMAT) plan of localised prostate cancer radiotherapy. Results were used to formulate rules and membership functions for Mamdani-type fuzzy inference system (FIS). In implementing the rules for the fuzzy system for ΔNTCP values above 10%, the PTV margin was not permitted to exceed 5 mm to avoid rectal complications due to margin selection. The new margins were applied in VMAT planning of prostate cancer for standard displacement errors. The dosimetric results of normal tissue predictors were estimated such as organ mean doses, rectum V60 (volume receiving 60 Gy), bladder V65 (volume receiving 65 Gy) and other clinically significant dose–volume indicators and compared with VMAT plans using current margin formulations.Results:Dosimetric results compared well to the results obtained by current techniques. Good agreement was obtained between proposed fuzzy model margins and currently used margins in lower error magnitude, but significant results were observed at higher error magnitude when organ toxicity concerned without compromising the target volumes.Findings:The new margins may be helpful to estimate possible outcomes of normal tissue complications and thus may improve complication free survival particularly when organ motion errors are inevitable, case by case.

Quantification of Rectal Motion in Male and Female Patients Undergoing Long Course Radiotherapy for Rectal Cancer in the Supine Position

Previous studies have reported data on internal rectal motion of rectal cancer patients treated in the prone position. With the introduction of intensity modulated techniques, more patients are treated in the supine position. Data informing specific margins for this treatment position is sparse as is data comparing rectal motion characteristics and factors in male and female patients. The purpose of this retrospective study is to quantify and compare the inter-fractional rectal movement characteristics of male and female rectal cancer patients treated with long course chemo radiotherapy in the supine position. As well as to generate Internal Target Volume (ITV) margins accounting for this organ’s internal physiological movements. Cone Beam Computer Tomography (CBCT) images were acquired from 20 male and 17 female rectal cancer patients on the first 3 days of treatment and weekly thereafter. The rectum, bladder and femoral heads were delineated on the planning CT (PCT) and six CBCT for each patient. Overall, 249 images were analyzed. The site of disease varied along the rectum. All patients were treated with a full bladder. The rectum was divided into three 5cm segments (inf, med, and sup) The motion of the rectum was quantified by documenting the anteroposterior and lateral distances as measured using fixed anatomical landmarks namely from the anterior aspect of the sacrum and mid-left femoral head, respectively. These measurements were taken at 1-cm intervals from the inferior border of L5. Sigmoid was excluded from these measurements. Estimate of systemic and random physiological movement error was determined, and margins were calculated using the Van Herk formula. 249 CBCT data sets (120 male, 102 female) on patients who had undergone long course chemotherapy were analyzed. The mean of the per-patient standard deviation (σ) was largest for the mid rectum in all directions (Anterior, Left and Right) except for the posterior motion of the lower rectal wall. There were statistically significant differences in σ between males and females in the lateral directions in all three rectal segments. Suggested ITV margin guidelines are therefore non-isotropic and vary per segment of rectum and sex. Table 1 shows the ITV margins calculated based on this study’s patient population. In our present study, rectal motion is shown to be significantly different between males and females. Our data suggests using asymmetrical sex specific margins in all patients should be considered.Abstract 3377; TableFEMALE ITV MARGINSUP [cm]MID [cm]INF [cm]POST1.940.931.39ANT1.751.671.56LEFT1.691.471.03RIGHT1.941.61.22MALE ITV MARGINSUP [cm]MID [cm]INF [cm]POST1.260.990.89ANT1.411.451.19LEFT0.961.060.85RIGHT1.291.161.06 Open table in a new tab

Gondwanan fragments in the southern Appalachians

Abstract Upper-plate and lower-plate asymmetric passive margin fragments are preserved within Carolinia, one of several terranes that rifted from Gondwana in the Furongian (late Cambrian) to form the Rheic Ocean. In the upper plate, 1–2 km of preserved rocks are middle Cambrian (Drumian, Ptychagnostus atavus zone) trilobite-bearing mudstones that lie above an angular unconformity and are the youngest stratified rocks in Carolinia. In the lower plate, 4–5 km of stratigraphy preserved in the Kings Mountain terrane are particularly interesting, because a 4 km thick Cambrian Series 2 clastic sedimentary section increasingly dominated by western Amazonian detritus lies above a Carolinian volcanic arc basement. Here, we describe for the first time the origin and setting of the youngest rocks in the Appalachians of wholly Gondwanan origin.