Design considerations for the sides of multileaf collimator leaves

The use of multileaf collimators (MLCs) in radiation therapy is becoming increasingly widespread as more commercial vendors offer them as options on medical linear accelerators. In most of the commercially available multileaf collimators, the sides of the leaves have a 'tongue and groove' (TG) feature, which is necessary for blocking the straight through leakage when the leaves are staggered together. The dosimetric effects of the TG design have not been fully studied for applications in block replacement or in dynamic intensity modulation. This article analytically explains the TG effect by calculating a deficit in photon flux when the side of a leaf is exposed. It is shown that the maximum photon deficit occurs when the leaf thickness is shared equally by the tongue and groove of adjacent leaves. New designs are proposed based on the theoretical analysis and improvements of the new design in leakage and the TG effect are verified with Monte-Carlo simulations. Measurements of the TG effect on an MLC from one manufacturer are also conducted. The results of the measurements confirm the analytical explanations of the TG effect.     

Use of a multileaf collimator for the production of intensity-modulated beams

The continuous release of nitric oxide (NO) from the constitutive, endothelial isoform of nitric oxide synthase (e-NOS) serves mainly to keep the vasculature in a continuous state of active vasodilation. Although it has been suggested that NO production from e-NOS might also be affected by hemorrhagic shock (HS), this relationship is still controversial. Therefore, the roles of NO in the pathophysiology in hemorrhagic shock were reviewed. According to the previous reports, NO might play an important role in the pathophysioliogy of HS. In the early phase of HS, it may be possible that NO delivered from e-NOS serves a cytoprotective function in preventing shock-induced organ injury. This opinion suggests that endothelial NO production has a significant modulatory effect on vascular tone during hemorrhage, and that inhibition of NO production permits greater vasconstrictor influences leading to organ injury. NO production in the late phases of HS has an adverse effect on survival rate in the HS model. Moreover, the findings from an animal study of prolonged periods of HS suggest that excessive NO formation, including those produced from i-NOS, induces vascular hypoactivity and they have suggested that NOS inhibitors may improve the therapeutic outcome for patients suffering from HS. Therefore, it may be suggested that NO might play a biphasic role, cytoprotective during the early phase and cytotoxic late in HS.     

The optimum intensities for multiple static multileaf collimator field compensation

A method of determining the optimum beam intensities for compensation using multiple static multileaf collimator fields is presented. In this method a histogram of the number of beam pixels against beam intensity is generated for the intensity-modulated beam (IMB). The intensity of each beam to be used is chosen to minimize the mean square deviation between each bin in the histogram and the closest beam intensity. This method has been applied to sample IMBs possessing one maximum and two maxima. For both cases, the use of uniform beam intensity increments is shown to be close to optimal. In the case with two maxima, the efficacy of irradiating both peaks simultaneously, rather than separately, has been studied and shown to be of potential benefit. The optimum intensities for an IMB for breast radiotherapy are also presented.     

Characteristics of scattered electron beams shaped with a multileaf collimator

Characteristics of dual-foil scattered electron beams shaped with a multileaf collimator (MLC) (instead of an applicator system) were studied. The electron beams, with energies between 10 and 25 MeV, were produced by a racetrack microtron using a dual-foil scattering system. For a range of field sizes, depth dose curves, profiles, penumbra width, angular spread in air, and effective and virtual source positions were compared. Measurements were made when the MLC alone provided collimation and when an applicator provided collimation. Identical penumbra widths were obtained at a source-to-surface distance of 85 cm for the MLC and 110 cm for the applicator. The MLC-shaped beams had characteristics similar to other machines which use trimmers or applicators to collimate scanned or scattered electron beams. Values of the effective source position and the angular spread parameter for the MLC beams were similar to those of the dual-foil scattered beams of the Varian Clinac 2100 CD and the scanned beams of the Sagittaire linear accelerators. A model, based on Fermi-Eyges multiple scattering theory, was adapted and applied successfully to predict penumbra width as a function of collimator-surface distance.     

A method for implementing dynamic photon beam intensity modulation using independent jaws and a multileaf collimator

A mathematical model is derived for digitally controlled linear accelerators to deliver a desired photon intensity distribution by combining collimator motion and machine dose rate variations. It shows that, at any instant, the quotient of the machine dose rate and the speed of collimator motion is proportional to the gradient of the desired in-air photon fluence distribution. The model is applicable for both independently controlled collimator jaws and multileaf collimators and can be implemented by controlling different parameters to accommodate linear accelerators from different manufactures. For independent jaws, each pair of jaws creates photon fluence variations along the direction of the jaw movement. For multileaf collimators, where each leaf is independently controlled, any two-dimensional (2D) photon fluence distribution can be delivered. The model has been implemented for wedged isodose distributions using independent jaws, and 2D intensity modulation using a multileaf collimator. One-dimensional (1D) wedged isodose distributions are created by moving an independent jaw at constant speed while varying machine dose rate. 2D intensity modulation has been implemented using a 'dynamic stepping' scheme, which controls the leaf progression during irradiation at constant machine dose rate. With this automated delivery scheme, the beam delivery time for dynamic intensity modulation, which depends on the complexity of the desired intensity distribution, approaches that of conventional beam modifiers. This paper shows the derivation of the model, its application, and our delivery scheme. Examples of 1D dynamic wedges and 2D intensity modulations will be given to illustrate the versatility of the model, the simplicity of its application, and the efficiency of beam delivery. These features make this approach practical for delivering conformal therapy treatments.     

The potential for increased sparing of normal tissue in parallel opposed techniques with a multileaf collimator

A multileaf collimator (MLC) can be used in parallel opposed techniques as a direct replacement for standard-shaped beam blocks. However, improved shielding is possible if the MLC field is designed to fit a target rather than to mimic a straight-edged block. This study has compared the treatment areas produced by the MLC and by conventionally blocked fields with the target area for 43 parallel opposed treatments. It was found in every case that the MLC treated less than 10% excess tissue, and, in over 70% of patients, the excess was less than 5%. The conventional fields, however, treated more than 10% excess tissue in 70% of patients. The effect of MLC orientation and the benefits of using an MLC are discussed.     

An experimental investigation of need for cognition.

This experiment is designed "to demonstrate the existence of a need for cognition and to test the effects of differential ambiguity upon people with different degrees of strength of need cognition." Half of the Ss in each group are exposed to a structured stimulus, half to an ambiguous stimulus. Reactions to experimental manipulations are obtained by means of a post-experimental questionnaire. It was found that: (1) There is consistency between the two independent measures of cognition need. (2) The ambiguous situation produced more frustration than did the structured one. Degree of ambiguity is more important for people with a high cognition need. (3) No differences are found in the degree to which groups of Ss impose meaning upon the experimental stimulus. (4) No relationships are found between need of cognition and need for achievement. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

An experimental investigation of the safe distance for internal orbital dissection

Six 3-dialkylaminomethyl-1-azaanthraquinones and five 4-dialkylaminomethyl-1-azaanthraquinones were synthesized and evaluated in vitro cytotoxicity against four human cancer cell lines. The compounds retained much of their cytotoxic activity against the multi-drug-resistant cell line (KB-V-1) as shown by resistance index.     

Scapegoating: An experimental investigation.

A group of 48 high prejudice (HP) Ss and 48 low prejudice (LP) Ss were studied, with ? of each group exposed to an aggression including film and ? to a control film. When a measure of fantasy aggression (Objective Apperception Test) was employed, LP Ss exceeded HP Ss in expression of prejudice. However, when a verbal measure of prejudice (Triandis and Triandis Social Distance scale) was employed, an increase in prejudice always followed instigation to aggression, HP Ss always exceeded LP Ss in expression of prejudice, and scapegoating was specifically isolated towards the Chinese. (18 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

An experimental investigation of subliminal perception.

Does subliminal perception really work? 2 groups of Ss enrolled in a sales and advertising class at Purdue University were used as a control group and an experimental group. A film and a questionnaire were used. The chi square technique was used in comparing the experimental and control group data. "If subliminal perception occurred, it did not affect questionnaire responses… . The burden of proof is placed on those who insist that subliminal perception is capable of influencing behavior." (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

An experimental investigation on the kinetics of solute driven remelting

The present study is aimed at understanding the interface kinetics during solute driven remelting in metallic alloys. Solid Al is placed in contact with a liquid Al-Mg alloy. As solid and liquid compositions at the interface are out of equilibrium, remelting takes place. The remelting rate is estimated as a function of time using a simple heat balance. The estimated velocity from the heat-balance calculations shows excellent agreement with the geometric velocity, directly measured from the remelted samples in each experiment. This confirms the accuracy and reliability of the heat-balance calculations and establishes this technique as a potential method for tracing the interface velocity during remelting. The results indicate that, at a constant temperature, an increase in liquid supersaturation leads to a linear increase in remelting velocity, as a result of an increasing driving force for remelting. At a constant liquid supersaturation, an increase in temperature results in an exponential increase in the remelting velocity, due to the enhanced mass transport at the higher temperatures. Semi-empirical relations are derived from these experimental observations and a combined analysis of the effects of driving force and kinetics yields a relation for remelting velocity as a function of temperature for a variety of boundary conditions. Remelting velocities predicted by this relation are in good agreement with the experimental observations.     

An experimental investigation of the gasification of graphite by carbon dioxide

The gasification of graphite by carbon dioxide was studied under atmospheric pressure in a fixed bed reactor in the temperature range of 1173–1773?K, CO₂ partial pressures 2–10?kPa and gas flow rate 0·5–2·0?L?min^?1. Iron presented in a small amount in graphite ash had a catalytic effect on the gasification reaction at 1373?K; this effect was weaker at 1473?K due to the melting of iron saturated with carbon. The gasification rate increased with increasing CO₂ partial pressure and total gas flow rate.     

An experimental investigation of the internal methane pressure in hydrogen attack

An experimental investigation of the internal methane pressure that is the driving force for bubble growth in hydrogen attack (HA) was done on pure iron (204 ppm C) and on two low carbon steels of slightly different compositions. The methane contentN (c.c gas/g. material) in attacked specimens was measured by a vacuum extraction technique. The total void volumeV (c.c) was determined from density measurements before and after HA exposure. The two values,N andV, were then used in an equation of state to calculate an average methane pressureP for the attack stages beyond a density loss (d.l.) greater than 0.05 pct. It was determined thatN andP depend on hydrogen exposure conditions and the presence of traces of strong carbide forming alloying elements (in steel). They are independent of specimen size and grain size over a limited range.P varies as the bubble volume increases, showing a generally decreasing trend which brings it to values lower than calculated equilibrium pressures, although well within the same order of magnitude. Possible reasons for this behavior are discussed.     

An experimental investigation of deformationinduced heating during tensile testing

An experimental investigation of deformation heating during uniaxial tensile testing is presented for Armco interstitial-free steel (I. F. steel) and stainless steel type 310 (310SS). Temperature distributions were measured along the specimen length for tests conducted at various strain rates and subjected to various heat flow conditions (in stirred water, in air, and with insulation). Maximum temperature rises of 75 °C and 118 °C were recorded in the neck. I.F. steel shows a decreasing failure strain from ∼45 pct to ∼40 pct when natural temperature gradients develop. 310SS shows a decreasing failure strain from ∼60 pct to ∼42 pct when natural heating is allowed. In 310SS, the developed temperature gradient accounts for about one-third of this ductility reduction while the uniform temperature rise accounts for about two-thirds. The uniform temperature rise influences ductilityvia a temperaturedependent work-hardening rate. These results have been qualitatively compared with finite element modeling of these tensile tests using constitutive equations generated previously. This study provides the basis for understanding several modified forming processes and demonstrates that care must be taken in interpreting data from “standard” tensile tests since such tests reflect the thermal properties of the material and environment as well as mechanical behavior. Formerly Graduate Student at The Ohio State University     

An experimental investigation of deformationinduced heating during tensile testing

An experimental investigation of deformation heating during uniaxial tensile testing is presented for Armco interstitial-free steel (I. F. steel) and stainless steel type 310 (310SS). Temperature distributions were measured along the specimen length for tests conducted at various strain rates and subjected to various heat flow conditions (in stirred water, in air, and with insulation). Maximum temperature rises of 75 °C and 118 °C were recorded in the neck. I.F. steel shows a decreasing failure strain from ～45 pct to ～40 pct when natural temperature gradients develop. 310SS shows a decreasing failure strain from ～60 pct to ～42 pct when natural heating is allowed. In 310SS, the developed temperature gradient accounts for about one-third of this ductility reduction while the uniform temperature rise accounts for about two-thirds. The uniform temperature rise influences ductilityvia a temperaturedependent work-hardening rate. These results have been qualitatively compared with finite element modeling of these tensile tests using constitutive equations generated previously. This study provides the basis for understanding several modified forming processes and demonstrates that care must be taken in interpreting data from “standard” tensile tests since such tests reflect the thermal properties of the material and environment as well as mechanical behavior.     

An experimental investigation of deformationinduced heating during tensile testing

Metallurgical and Materials Transactions A - An experimental investigation of deformation heating during uniaxial tensile testing is presented for Armco interstitial-free steel (I. F. steel) and...     

The mechanics of suppression: An experimental investigation.

Ss were given a TAT-like picture with a limited number of prominent stimulus objects and asked to tell stories which excluded 1 of the stimuli. A 2nd condition involved the introduction of the "idea" of a stimulus, without a related visual image, with the subsequent instruction not to use the introduced idea in storytelling. 2 groups of Ss—a group of average high school students and 1 of intellectually superior university graduate students—were used. Responses were classified according to process utilized in suppressing the forbidden stimulus. Both higher and lower IQ Ss utilized essentially the same types of methods, which were found to be somewhat primitive in nature. In addition, however, the graduate group introduced some seemingly more refined operations to achieve successful suppression. (PsycINFO Database Record (c) 2010 APA, all rights reserved)