Asymmetric Margins Research Articles

Effects of lateral strength contrasts and inherited heterogeneities on necking and rifting of continents

Besides the intrinsic rheological layering of the lithosphere and its thermal structure, inherited heterogeneities may play an important role in strain localization during continental extension. This is similar to the role that defects play in the failure and necking of other materials. Here, we consider both inherited small-scale weak zones and the effects of lateral juxtaposition of two lithospheres with differing properties as mechanisms to localize deformation and initiate necking instabilities. Using 2D finite-element models that contain lateral lithospheric boundaries, in combination with smaller scale heterogeneities, we illustrate that two controls determine how necking instabilities grow and thus lead to varying styles of rifting: Control 1, the stiff/pliable nature of the lithosphere and Control 2, the background strain rate in the lithosphere. Control 1 depends on the lithospheric rheology, such that necking instabilities grow faster in materials with high power-law creeping flow exponents (stiff, brittle lithosphere) than in those with low power-law creeping flow exponents (pliable, viscous lithosphere). Control 2 prevails in lithosphere where background strain rates are highest. This happens because necking amplifies the background strain rate in power-law materials, leading to faster necking where strain rates are highest. The model results show that Control 2 determines the location of localization, unless the background strain rate is equal or near equal in both lithospheres, in which case Control 1 wins. These results explain why rifting does not localize in cratons even though they contain heterogeneities. The results also provide a mechanism for the formation of asymmetric rifted margins.

Comparison of planning target volumes based on three-dimensional and four-dimensional CT imaging of thoracic esophageal cancer.

Background and purposeTo investigate the definition of planning target volumes (PTVs) based on four-dimensional computed tomography (4DCT) compared with conventional PTV definition and PTV definition using asymmetrical margins for thoracic primary esophageal cancer.Materials and methodsForty-three patients with esophageal cancer underwent 3DCT and 4DCT simulation scans during free breathing. The motions of primary tumors located in the proximal (group A), middle (group B), and distal (group C) thoracic esophagus were obtained from the 4DCT scans. PTV3D was defined on 3DCT using the tumor motion measured based on 4DCT, PTV conventional (PTVconv) was defined on 3DCT by adding a 1.0 cm margin to the clinical target volume, and PTV4D was defined as the union of the target volumes contoured on the ten phases of the 4DCT images. The centroid positions, volumetric differences, and dice similarity coefficients were evaluated for all PTVs.ResultsThe median centroid shifts between PTV3D and PTV4D and between PTVconv and PTV4D in all three dimensions were <0.3 cm for the three groups. The median size ratios of PTV4D to PTV3D were 0.80, 0.88, and 0.71, and PTV4D to PTVconv were 0.67, 0.73, and 0.76 (χ2=−3.18, −2.98, and −3.06; P=0.001, 0.003, and 0.002) for groups A, B, and C, respectively. The dice similarity coefficients were 0.87, 0.90, and 0.81 between PTV4D and PTV3D and 0.80, 0.84, and 0.83 between PTV4D and PTVconv (χ2 =−3.18, −2.98, and −3.06; P=0.001, 0.003, and 0.002) for groups A, B, and C, respectively. The difference between the degree of inclusion of PTV4D in PTV3D and that of PTV4D in PTVconv was <2% for all groups. Compared with PTVconv, the amount of irradiated normal tissue for PTV3D was decreased by 11.81% and 11.86% in groups A and B, respectively, but was increased by 2.93% in group C.ConclusionFor proximal and middle esophageal cancer, 3DCT-based PTV using asymmetrical margins provides good coverage of PTV4D; however, for distal esophageal cancer, 3DCT-based PTV using conventional margins provides ideal conformity with PTV4D.

Asymmetric Forecast Densities for U.S. Macroeconomic Variables from a Gaussian Copula Model of Cross-Sectional and Serial Dependence

Most existing reduced-form macroeconomic multivariate time series models employ elliptical disturbances, so that the forecast densities produced are symmetric. In this article, we use a copula model with asymmetric margins to produce forecast densities with the scope for severe departures from symmetry. Empirical and skew t distributions are employed for the margins, and a high-dimensional Gaussian copula is used to jointly capture cross-sectional and (multivariate) serial dependence. The copula parameter matrix is given by the correlation matrix of a latent stationary and Markov vector autoregression (VAR). We show that the likelihood can be evaluated efficiently using the unique partial correlations, and estimate the copula using Bayesian methods. We examine the forecasting performance of the model for four U.S. macroeconomic variables between 1975:Q1 and 2011:Q2 using quarterly real-time data. We find that the point and density forecasts from the copula model are competitive with those from a Bayesian VAR. During the recent recession the forecast densities exhibit substantial asymmetry, avoiding some of the pitfalls of the symmetric forecast densities from the Bayesian VAR. We show that the asymmetries in the predictive distributions of GDP growth and inflation are similar to those found in the probabilistic forecasts from the Survey of Professional Forecasters. Last, we find that unlike the linear VAR model, our fitted Gaussian copula models exhibit nonlinear dependencies between some macroeconomic variables. This article has online supplementary material.

Correlation of primary middle and distal esophageal cancers motion with surrounding tissues using four-dimensional computed tomography.

PurposeTo investigate the correlation of gross tumor volume (GTV) motion with the structure of interest (SOI) motion and volume variation for middle and distal esophageal cancers using four-dimensional computed tomography (4DCT).Patients and methodsThirty-three patients with middle or distal esophageal carcinoma underwent 4DCT simulation scan during free breathing. All image sets were registered with 0% phase, and the GTV, apex of diaphragm, lung, and heart were delineated on each phase of the 4DCT data. The position of GTV and SOI was identified in all 4DCT phases, and the volume of lung and heart was also achieved. The phase relationship between the GTV and SOI was estimated through Pearson’s correlation test.ResultsThe mean peak-to-peak displacement of all primary tumors in the lateral (LR), anteroposterior (AP), and superoinferior (SI) directions was 0.13 cm, 0.20 cm, and 0.30 cm, respectively. The SI peak-to-peak motion of the GTV was defined as the greatest magnitude of motion. The displacement of GTV correlated well with heart in three dimensions and significantly associated with bilateral lung in LR and SI directions. A significant correlation was found between the GTV and apex of the diaphragm in SI direction (rleft=0.918 and rright=0.928). A significant inverse correlation was found between GTV motion and varying lung volume, but the correlation was not significant with heart (rLR=−0.530, rAP=−0.531, and rSI=−0.588) during respiratory cycle.ConclusionFor middle and distal esophageal cancers, GTV should expand asymmetric internal margins. The primary tumor motion has quite good correlation with diaphragm, heart, and lung.

Esthetic Rehabilitation through Crown Lengthening Surgery and Conservative CAD/CAM Veneers: A Multidisciplinary Case Report.

This case report describes a successful multidisciplinary approach used to improve the smile esthetics of a patient presenting with excessive gingival display, asymmetric gingival margins, and small upper anterior teeth and lower anterior teeth. The treatment combined esthetic crown lengthening, dental bleaching, and restorative dentistry using CAD/CAM veneer. The 6-month follow-up examination confirmed the stability of the modification and absence of adverse effects.

Asymmetric margin setting at the cranial and caudal sides in respiratory gated and non-gated stereotactic body radiotherapy for lung cancer.

To evaluate total errors, including setup errors, and tumour motion changes with a electronic portal imaging device (EPID) cine at the cranial and caudal sides in respiratory gated and non-gated radiotherapy. Co-ordinates of the tumour centres (TCs) in the craniocaudal direction were obtained by using four-dimensional CT (4DCT) for each bin and EPID cine frame. During the 100% duty cycle (DC100), 50% duty cycle (DC50) and 30% duty cycle (DC30), both centred on the 50 phase, the co-ordinates of the TCs were compared at the most cranial and caudal positions on both 4DCT and EPID cine. During DC100, total errors were -0.2 ± 2.1 and 1.1 ± 2.6 mm at the cranial and caudal sides, respectively. During DC50, the corresponding values were -0.2 ± 2.1 and 1.7 ± 3.2 mm, respectively; during DC30, they were -0.1 ± 2.1 and 1.8 ± 2.9 mm, respectively. The tumour motion changes at the caudal side were strongly correlated with tumour motion observed on 4DCT during DC100 (R(2) = 0.59). Total errors and tumour motion changes on the caudal side were larger than on the cranial side because of the patients' breathing levels. Owing to variations of the TCs at beam-trigger events, the larger margin was required at the caudal side in gated radiotherapy. Variations of the TCs were evaluated at the cranial and caudal sides separately. Providing some margins to compensate for tumour motion changes was a significant requirement at the caudal side in gated and non-gated radiotherapy.

Conjugate rifted margins width and asymmetry: The interplay between lithospheric strength and thermomechanical processes

AbstractNumerical experiments have been used to relate the range in the distribution and the style of deformation observed in rifted margins to localizing/delocalizing thermomechanical processes. The experiments give rise to four end‐members of margins for varying initial lithospheric strength and extension rates. The first two end‐members are narrow and asymmetric and narrow and near‐symmetric, conjugate margins. The third end‐member is asymmetric conjugate margins, wherein one side is &lt;100 km wide and the other is &gt;100–300 km wide. Lastly, we explore wide rift systems that may form very asymmetric conjugate margins with one narrow margin and a very wide conjugate, 200 km to &gt; 350 km across. With initial and boundary conditions close to that inferred from the North and South Atlantic margins, we find that not all margins experience a polyphase rifting history of stretching‐thinning‐exhumation. Instead, the stretching mode can be very short or protracted, and the thinning or the exhumation modes can be incomplete or absent. The deformation localization of the thinning mode is in places associated with the formation of a keystone block or “block H.” A new mechanism for the formation of the unstable crustal root under block H is described, wherein the bounding border faults lead to differential thinning of the crust and mantle lithosphere. Nonuniform extension also occurs in both types of wide rift systems and is related to the sequential deformation migration outward of an initial graben, associated with effective lithospheric strengthening that occurs during crustal thinning and bending.

SU-C-210-07: Assessment of Intra-/Inter-Fractional Internal Tumor and Organ Movement in Radiotherapy of Head and Neck Cancer Using On-Board Cine MRI

Purpose: Head and neck (HN similarly, (5.7%, 0.0%, 0.0%, 0.3%) and (23.3%, 0.0%, 35.7%, 1.7%) occurred for patients 2 and 3. Area overlapping coefficients with respect to the first frame were 76.3+/−6.4%, 90.3+/−0.6%, and 92.3+/−1.2% for the three patients, respectively. Conclusion: Both the resting and swallowing motions varied in frequency and amplitude among the patients and across fractions of a patient’s treatment. Patients receiving surgery that alters their respiratory and swallowing behavior can have significant intra-fractional internal motion. Patient-specific organ/tumor motion analysis may yield individualized asymmetric internal margins and improve the therapeutic ratio.« less

SU‐E‐J‐178: Towards ART ‐ Bootstrap Based Estimation of Cumulated Dose Distribution

Purpose:A lot of effort is put into diminishing the CTV‐PTV margin and providing Adaptive RT in order to spare normal structures adjacent to the PTV. The purpose of this work was to test the hypothesis that bootstrap method can be used to estimate cumulated dose distribution, representing the whole treatment and that the quality of this estimation is independent on the CTV‐PTV margin.Methods:For 25 prostate patients treated in our clinic daily set‐up errors were recorded. For each patient treatment plans for three techniques were prepared: 3D‐CRT, IMRT and VMAT. We prepared plans for different CTV‐PTV margins, for asymmetric margin (0.4, 0.7, and 0.7 cm for lateral, cranial‐caudal, and AP respectively) and than for several symmetric ones of 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 cm. For each treatment plan and each fraction the dose distribution was calculated with the isocenter corrected for the set‐up error. Cumulated Dose Distribution (CDDall) was calculated as the sum of all dose distributions for each single fraction. Estimated Dose Distribution (EDDn) was calculated with bootstrap methodology. The EDDn and CDDall were compared using the 3D gamma concept (2mm and 2%) for each pair of dose distributions. The Result was treated as acceptable if at least 95% of voxels with gamma index ≤ 1 were obtained.Results:For 3D and IMRT EDDn calculated from at least 8 (3D) and 7 (IMRT) first fractions (EDDn&gt;7 and EDDn&gt;6), for all patients, but one, acceptable results were obtained. For VMAT our requirement was fulfilled for all patients and EDDn&gt;2. Statistical tests show no significant difference between gamma results for different CTV‐PTV margins.Conclusion:The cumulated dose distribution CDDall can be estimated from at least 8 fractions regardless of the technique and CTV‐PTV margin. For VMAT technique the cumulated dose distribution may be estimated with 3 fractions.

Rift reactivation and migration during multiphase extension

Passive margins may undergo multiple phases of extension with distinct structural, petrological and sedimentary processes before achieving breakup. Observations of rift axis migration through time may reflect cooling, hardening and subsequent abandonment of the rift axis during either long-term periods of slow extension or periods of tectonic quiescence. Here, we use 2D thermo-mechanical numerical models to examine rift reactivation and migration during multiphase extension where a period of tectonic quiescence separates phases of extension. Our goals are to identify the rheological mechanism(s) controlling rift reactivation versus migration and determine if cooling phases may help explain recent interpretations of passive margin architecture and evolution. Our numerical experiments indicate that the relative integrated brittle strength between the initial rift and surrounding regions, rather than the total integrated strength, largely controls rift reactivation versus migration. The tectonic quiescence (cooling) duration required to induce rift migration ranges between 20 and 60 Myr (minimum bounds). This range reflects variations in extension velocity, magnitude of shear zone healing, crustal rheology and asthenospheric rheology. Reactivated rifts after extensive (>20 Myr) cooling periods in some cases develop asymmetric margins with deformation patterns stepping toward the future rift, such as characterizing most of the Atlantic conjugate margins.

Equivalence Tests for Interchangeability Based on Two One-Sided Probabilities

A test treatment is considered to be interchangeable with its reference treatment if they are equivalent and expected to produce the same clinical result in any given patient. To assess interchangeability, FDA Draft Guidance (1999) and Guidance for Industry (2001, 2003) recommend using individual bioequivalence (IBE) and population bioequivalence (PBE) procedures. Chow (1999) and Chow and Liu (1999) gave a discussion on the limitation of the aggregate criteria of the IBE and PBE proposed therein. They mentioned that it is not clear whether IBE or PBE can imply average bioequivalence. Alternative approaches have been proposed to address the weakness of IBE and PBE. Dong et al. (2014) discuss the tolerance interval method and an approximate test for interchangeability defined by a two-sided probability. These tests may not be able to test for the two one-sided tests (TOST) with asymmetric margins around the true mean difference. In addition, the tests of two-sided probability provide no direction when failing the equivalence in interchangeability. Thus, we reexamine the statistical properties of the two one-sided tolerance interval approaches proposed by Tsong and Shen (2007, 2008). In this project, we extend their approach for parallel arms trials and paired/crossover data without the assumption of equal sample sizes and variances. We also develop the exact power function and assess the type I error rate of our proposed approach. In addition, we study the sample size determination based on the interchangeability testing utilizing the tolerance interval method.

Study of Interfraction Movements of Tongue During Radiation Therapy in Cases of Tongue Malignancy Using Volumetric kV Cone Beam CT (CBCT) Imaging

Tongue is possibly the most mobile organ in head & neck region. Due to this, chances of geographic miss during fractionated radiation therapy are high. Several institutions use tongue immobilization devices such as mouth bite to reduce tongue movements. Drawbacks of using tongue bite include patient discomfort, increased acute radiation reactions & chances of secondary infection. This study analyzed interfraction movements of tongue using KV CBCT for patients of tongue malignancy treated without using tongue bite. We analyzed 100 Cone Beam CTs performed on 10 patients. During radiation therapy, all patients underwent KV CBCT imaging to assess tongue movements in three dimensions using 5 reference points: Point A: Tip of tongue; Point B: Point over right lateral border, 4 cm posterior to the tip of tongue; Point C: Point over left lateral border, 4 cm posterior to the tip of tongue; Point D: Point over superior most part (dorsum) of tongue, 4 cm posterior to the tip of tongue; and Point E: Point over the surface of base of tongue at the level of tip of epiglottis; Point A represents movement in anteroposterior (AP) direction, point B and C represent movements in right lateral (RL) and left lateral (LL) directions respectively, point D represents movements in supero-inferior (SI) direction & point E represents movements of base of tongue in AP direction. Movement of reference point towards the PTV (outward movement) was given +ve sign & movement in the opposite direction (inward movement) was given -ve sign. Mean movements of point A: +0.21 cm (SD: 0.12) & -0.23 (SD: 0.14), point B: +0.14 cm (SD: 0.04) & -0.19 cm (SD: 0.1), point C: +0.12 cm (SD: 0.05) & -0.14 cm (SD: 0.06), point D: +0.15 cm (SD: 0.07) & -0.29 cm (SD: 0.22) & point E: +0.23 cm (SD: 0.15) & -0.23 cm (SD: 0.14). Coverage of 95% of tongue mobility would require an asymmetric margin (IM: internal margin) of 0.33 cm anteriorly, 0.18 cm in right lateral direction, 0.17 cm in left lateral direction, 0.22 cm superiorly & 0.38 cm posteriorly to create an ITV (internal target volume). This study supports the use of asymmetric & relatively larger (>0.5 cm in anterior, posterior and superior direction) CTV to PTV margins for tongue malignancy in view of potential inter-fraction mobility of tongue. Margins of 0.3 to 0.5 cm towards left and right lateral direction can be considered safe. Application of an ITV concept with an asymmetric CTV to ITV margin (IM) can be considered. ITV to PTV margin of 0.3 cm can be safe in an IGRT setting with KV-CBCT.Scientific Abstract 3582; TablePoint locationMean +ve movement (SD)Mean -ve movement (SD)Range of movementFrequency of all absolute readings (irrespectiveof +ve or -ve sign)<1 mm (%)1-2 mm (%)2-3 mm (%)3-4 mm (%)4-5 mm (%)>5 mm (%)A0.214 (0.124)(-)0.234 (0.145)(+)0.5 to (-)0.755%20%9%11%4%1%B0.14 (0.045)(-)0.19 (0.1)(+)0.22 to (-)0.3974%16%8%2%0%0%C0.121 (0.051)(-)0.146 (0.060)(+)0.3 to (-)0.3478%16%5%1%0%0%D0.155 (0.071)(-)0.291 (0.222)(+)0.38 to (-)0.856%22%10%4%3%5%E0.236 (0.152)(-)0.236 (0.140)(+)0.9 to (-)0.5236%28%15%11%8%2%P-value.0014.0008-.0014.0008.0045.0562.1132.1596 Open table in a new tab

Flip-flop detachment tectonics at nascent passive margins in SE Afar

We propose a two-stage tectonic evolution of SE Afar in Djibouti leading to the complex development of highly asymmetric conjugate margins. From c . 8.5 to c . 2 Ma, an early mafic crust developed, associated in the upper crust with synmagmatic growth faults dipping dominantly to the SW. After an erosional stage, a new detachment fault system developed from c . 2 Ma with an opposite sense of motion (i.e. to the NE), during an amagmatic extensional event. In the Asal area, break-up occurred after c . 0.8 Ma along the footwall of an active secondary detachment fault rooted at depth above the lithospheric necking zone. This evolution suggests that flip-flop detachment tectonics is developed during extension at passive margins, in connection with the dynamics of the melting mantle and the associated magma plumbing of the crust.

Improving smile and dental esthetics: A comprehensive periodontal and restorative approach after orthodontics

AbstractThe esthetic demands in smile are continually rising, being thus, critical factors such as; dental midline, smile line, incisal embrasures, tooth position, width to length crown ratio, symmetry of contra-lateral gingival margins, and gingival display need to be taken into consideration. The aim of this case report is to describe the improvement of smile esthetics in a patient that after orthodontic therapy, presented excessive gingival display, asymmetric gingival margins, and shape altered upper right and left lateral incisors, which in turn compromised dental esthetics. The present clinical case required comprehensive treatment process, including proper diagnosis and excellent communication between the prosthodontist and periodontist in order to carry out successful esthetic crown lengthening and restorative dentistry. Results were followed-up for 2 years after the case completion. A pleasant smile was obtained, showing stability over time.

Crustal thinning in the northern Tyrrhenian Rift: Insights from multichannel and wide‐angle seismic data across the basin

AbstractExtension of the continental lithosphere leads to the formation of rift basins or rifted continental margins if breakup occurs. Seismic investigations have repeatedly shown that conjugate margins have asymmetric tectonic structures and different amount of extension and crustal thinning. Here we compare two coincident wide‐angle and multichannel seismic profiles across the northern Tyrrhenian rift system sampling crust that underwent different stages of extension from north to south and from the flanks to the basin center. Tomographic inversion reveals that the crust has thinned homogeneously from ~24 km to ~17 km between the Corsica Margin and the Latium Margin implying a β factor of ~1.3–1.5. On the transect 80 km to the south, the crust thinned from ~24 km beneath Sardinia to a maximum of ~11 km in the eastern region near the Campania Margin (β factor of ~2.2). The increased crustal thinning is accompanied by a zone of reduced velocities in the upper crust that expands progressively toward the southeast. We interpret that the velocity reduction is related to rock fracturing caused by a higher degree of brittle faulting, as observed on multichannel seismic images. Locally, basalt flows are imaged intruding sediment in this zone, and heat flow values locally exceed 100 mW/m2. Velocities within the entire crust range 4.0–6.7 km/s, which are typical for continental rocks and indicate that significant rift‐related magmatic underplating may not be present. The characteristics of the pre‐tectonic, syn‐tectonic and post‐tectonic sedimentary units allow us to infer the spatial and temporal evolution of active rifting. In the western part of the southern transect, thick postrift sediments were deposited in half grabens that are bounded by large fault blocks. Fault spacing and block size diminish to the east as crustal thinning increases. Recent tectonic activity is expressed by faults cutting the seafloor in the east, near the mainland of Italy. The two transects show the evolution from the less extended rift in the north with a fairly symmetric conjugate structure to the asymmetric margins farther south. This structural evolution is consistent with W‐E rift propagation and southward increasing extension rates.

Three independent one-dimensional margins for single-fraction frameless stereotactic radiosurgery brain cases using CBCT

Setting a proper margin is crucial for not only delivering the required radiation dose to a target volume, but also reducing the unnecessary radiation to the adjacent organs at risk. This study investigated the independent one-dimensional symmetric and asymmetric margins between the clinical target volume (CTV) and the planning target volume (PTV) for linac-based single-fraction frameless stereotactic radiosurgery (SRS). The authors assumed a Dirac delta function for the systematic error of a specific machine and a Gaussian function for the residual setup errors. Margin formulas were then derived in details to arrive at a suitable CTV-to-PTV margin for single-fraction frameless SRS. Such a margin ensured that the CTV would receive the prescribed dose in 95% of the patients. To validate our margin formalism, the authors retrospectively analyzed nine patients who were previously treated with noncoplanar conformal beams. Cone-beam computed tomography (CBCT) was used in the patient setup. The isocenter shifts between the CBCT and linac were measured for a Varian Trilogy linear accelerator for three months. For each plan, the authors shifted the isocenter of the plan in each direction by ±3 mm simultaneously to simulate the worst setup scenario. Subsequently, the asymptotic behavior of the CTV V80% for each patient was studied as the setup error approached the CTV-PTV margin. The authors found that the proper margin for single-fraction frameless SRS cases with brain cancer was about 3 mm for the machine investigated in this study. The isocenter shifts between the CBCT and the linac remained almost constant over a period of three months for this specific machine. This confirmed our assumption that the machine systematic error distribution could be approximated as a delta function. This definition is especially relevant to a single-fraction treatment. The prescribed dose coverage for all the patients investigated was 96.1% ± 5.5% with an extreme 3-mm setup error in all three directions simultaneously. It was found that the effect of the setup error on dose coverage was tumor location dependent. It mostly affected the tumors located in the posterior part of the brain, resulting in a minimum coverage of approximately 72%. This was entirely due to the unique geometry of the posterior head. Margin expansion formulas were derived for single-fraction frameless SRS such that the CTV would receive the prescribed dose in 95% of the patients treated for brain cancer. The margins defined in this study are machine-specific and account for nonzero mean systematic error. The margin for single-fraction SRS for a group of machines was also derived in this paper.

Comparative Assessment of Liver Tumor Motion Using CineMRI Versus 4DCT

Accurate radiation therapy (RT) targeting of hepatic tumors is critically dependent on organ and tumor motion assessment, most commonly performed with 4DCT. Since 4DCT captures only a snapshot of tumor motion in time and is prone to artifacts and irregular breathing, it is unclear how reliable 4DCT is in determining intra-fraction tumor motion. CineMRI allows for prolonged real-time organ and tumor motion assessment with no radiation exposure. We compared the extent of tumor motion between 4DCT and cineMRI in patients with hepatic tumors treated with RT. Ten patients with hepatocellular carcinoma (n = 6), cholangiocarcinoma (n = 3), and liver metastasis (n = 1) underwent 4DCT and 2D biplanar cineMRI scans during simulation to determine the extent of target motion in the superior/inferior (S/I), anterior/posterior (A/P) and lateral directions. CineMRI was performed over 5 minutes. Tumor motion was determined by the changes in centroid position of the gross tumor volume (GTV) as determined by deformable contour propagation. Margins were determined by using histograms of 90% confidence interval of GTV motion in each direction. Motion estimates from 4DCT was performed by evaluation of the marker positions in each phase of the CT. We compared the extent of GTV motion and resulting margin size between cineMRI and 4DCT. The difference in A/P margin and lateral motion between the 4DCT and cineMRI was minimal (Table). The S/I margin was underestimated by 4DCT in 8/10 patients by a mean of 3.4 mm (range, 1.5-7 mm). The average inferior motion (median to 10th percentile) was of greater magnitude (6.2 mm vs 4.2 mm) compared to the average superior motion (median to 90th percentile). Superior, inferior, lateral, anterior, and posterior margins of 6, 8, 3, 5, and 3 mm respectively would be needed to cover 90% of the intra-fraction GTV motion as defined by cineMRI. CineMRI detects differences in hepatic intra-fraction tumor motion when compared to 4DCT, most notably in the S/I direction. When treating without respiratory management, caution should be taken when determining tumor margin as 4DCT can underestimate target motion, particularly in the S/I direction. Asymmetric margins as mentioned above should be considered to account for intra-fraction hepatic tumor motion.Oral Scientific Abstract 108; TableComparative Assessment of Liver Tumor Motion Using CineMRI Versus 4DCTMean/Median Cine-MRI motion (range), mmMean/Median 4D-CT motion (range), mmMotion difference (range), mmSuperior-Inferior10.4/9.8 (6-18)7.7/7.5 (4-12)2.7 (-0.5-7)Anterior-Posterior5.6/4.8 (2.2-15.5)4.9/4.0 (2-7.5)0.7(-1.8-5.5)Lateral2.8/2.4 (1-4.5)2.3/2.0 (0-6)0.5 (-2-2.5) Open table in a new tab

Deglutition-Induced Dose Deformations in Head-and-Neck Cancer IMRT Treatments: Volume Shuttle Imaging Analysis

The 4D respiration-correlated imaging using external metric system is ideal way to capture range of motion for moving targets. However momentary apneic phase during swallowing leads to sorting errors and image artifacts thus deterring this modality from being used to track motion displacements in head and neck region. A special Imaging tool used for CT-Angio/joint motion study called Dynamic helical Volume shuttle (DHVS) fills this void. DVHS is continuous bidirectional scan with extended 'z' coverage and improved temporal sampling. Present study aims to assess the deglutition induced displacement and dose deformations in patients with head and neck cancer using DHVS imaging. Twenty patients of treatment naive oropharyngeal cancers following rigid immobilization were setup in treatment position on a 128 slice multidetector CT Scanner and underwent DHVS imaging (2 mts scan). Image data for single patient was acquired during voluntary effortful swallowing act (once in 20 seconds) at 2 alternating table positions with the table shuttling back and forth. Around 3000 images/per pt. were reformatted using Dynamic Pitch Cone Beam Reconstruction (RetroRecon) for 12 phases @ 1.25 mm thickness and cine looped to quantify the range of motion during swallowing. Deglutition-induced GTV and Spinal cord displacements were quantified based on position change during deglutition relative to pre-swallow structure location for anterior (A), posterior (P), superior (S), and inferior (I) directions. The image datasets in extreme bins (peak expiratory and peak inspiratory bins) were backprojected on to the original plan in QA mode (to preserve the plan integrity) and dose deformations happening consequent to swallowing were analyzed. Deglutition-induced maximal GTV displacements ranged from 3 mm to 18 mm with mean and standard deviation of 3.5 +/- 2, 2.50 +/- 1.51, 1.75 +/- 4.0, and 9.60 +/- 7.7 mm in the A, P, I, and S directions respectively. The SC displacement ranged from 2.8 mm of the prescribed dose would still be delivered to the tumor even if the tumor partly vacated the PTV during the entire swallowing event for 15 of the 20 patients during the VHS scan. However, for the other 5 patients’ cohort, which had frequent swallowing, the target coverage failed to reach 80%. This is the first study reported in literature using DHVS for deglutition displacements quantification. Asymmetric IMs and PRV margins, derived from directional displacement, using DVS imaging should be employed to account for tumor motion for high precision treatments in HNC.

Effect of unilateral diaphragmatic paralysis on postpneumonectomy lung growth.

Respiratory muscle-associated stretch has been implicated in normal lung development (fetal breathing movements) and postpneumonectomy lung growth. To test the hypothesis that mechanical stretch from diaphragmatic contraction contributes to lung growth, we performed left phrenic nerve transections (PNT) in mice with and without ipsilateral pneumonectomy. PNT was demonstrated by asymmetric costal margin excursion and confirmed at autopsy. In mice with two lungs, PNT was associated with a decrease in ipsilateral lung volume (P<0.05) and lung weight (P<0.05). After pneumonectomy, PNT was not associated with a change in activity level, measureable hypoxemia, or altered minute ventilation; however, microCT scanning demonstrated altered displacement and underinflation of the cardiac lobe within the first week after pneumonectomy. Coincident with the altered structural realignment, lung impedance measurements, fitted to the constant-phase model, demonstrated elevated airway resistance (P<0.05), but normal peripheral tissue resistance (P>0.05). Most important, PNT appeared to abrogate compensatory lung growth after pneumonectomy; the weight of the lobes of the right lung was significantly less than pneumonectomy alone (P<0.001) and indistinguishable from nonsurgical controls (P>0.05). We conclude that the cyclic stretch associated with diaphragmatic muscle contraction is a controlling factor in postpneumonectomy compensatory lung growth.

United We Stand? Coordinating Capacity Investment and Allocation in Joint Ventures

Among the recent innovative strategies for coping with product variety and market risk some firms have partnered to leverage economies of scale and risk pooling by sharing manufacturing capacity. In this paper we study how to structure such a joint venture to achieve full efficiency at low transaction costs. Specifically, we study whether capacity should be owned jointly or separately. Overall, we find that the two ownership structures have complementary strengths and weaknesses in term of their incentives for coordinating capacity allocation and investment. On the one hand, capacity allocation is simple to coordinate under joint ownership, but may entail high transaction costs under separate ownership when the joint venture consists of many firms with different profit margins. On the other hand, capacity investments remain simple to coordinate under separate ownership, but are efficient under joint ownership only in the presence of large economies of scale or asymmetric demands or asymmetric profit margins, and would otherwise entail high transaction costs. Our analysis thus characterizes the trade-off between economic benefits and transaction costs in the choice of capacity ownership structure.