Implementing multiple static field delivery for intensity modulated beams.

A clinically oriented two-dimensional intensity-modulated beam delivery method is implemented using multiple static segmented fields, i.e., the "step-and-shoot" approach. Starting with a desired al" intensity distribution, it creates a multiple-level intensity approximation, and then constructs a sequence of segmented fields to deliver the multiple-level intensities using multileaf collimator (MLC) and independent backup jaws. The approach starts with a simple grouping of all the nonzero intensity values into a minimum number of clusters for a user specified deviation tolerance for the ideal plan. The k-means clustering algorithm is then employed to find the optimal levels of intensity that minimize the discrepancies between the ideal and the approximated intensities, without violating the user specified deviation tolerance. The multiple-level intensities are then decomposed into a sequence of machine deliverable segments. Apart from the first segment for each gantry angle, all the other segments are arranged to minimize the total travel distance of the leaves. The first segment covers the entire irradiated area and is used for treatment verification by electronic portal imaging. The implementation issues due to the physical constraints of the MLCs are also addressed.     

Segmentation of IMRT plans for radical lung radiotherapy delivery with the step-and-shoot technique

The purpose of this work was to determine a segmentation protocol for the treatment of localized non-small-cell lung cancer (NSCLC) with intensity-modulated radiotherapy (IMRT) that is as effective as possible while practically simple and hence robust to known practical inaccuracies. This study focused on the stratification of continuous profiles into a discrete number of intensity levels. The selection of the segmentation parameters for the delivery of the fluence profiles using multiple static fields has been considered. Five-field equispaced IMRT treatment plans of five patients with NSCLC were selected. The study comprised nine treatment plans for each patient, starting from a conformal plan, optimizing it for IMRT and then segmenting it utilizing different numbers of segments in each case and optimizing for segment weights separately. A conformal plan, optimized for beam directions, collimator and wedge angles, was also used for comparison with the IMRT plans, so as to consider the best coplanar conformal case. A dose objective for the PTV and the organs-at-risk plus a constraint for the spinal cord were set for all inverse plans. All stages were compared with the aid of dose-volume histograms, dose distributions at the plane of the isocenter, intensity maps for key beams and plots of PTV homogeneity and overall conformality versus complexity. The unsegmented IMRT plans gave the best results but cannot be realized in practice with an MLC. They were best approximated by plans that needed 106-167 segments to deliver, but did not deteriorate significantly when approximated by plans which required 26-40 segments in total. All segmented IMRT plans gave a better lung sparing than the conformal plans, indicating that the deterioration of IMRT plans following segmentation is not equivalent to that of unmodulated, conformal plans. However, optimized conformal plans have the potential to approach the lung sparing achieved by segmented IMRT plans. Among the IMRT situations examined, five-field treatment plans for the lung, utilizing a maximum of 40 segments in total, have proven to give a good approximation of the IMRT plans with continuous modulation.     

Optimization of intensity modulated radiotherapy under constraints for static and dynamic MLC delivery.

Multi-leaf collimators (MLCs) are emerging as the prevalent modality to apply intensity modulated radiotherapy (IMRT). Both the principle and the particular design of MLCs stipulate complex constraints on the practically applicable intensity modulated radiation fields. Most consequentially, the distribution of exposure times across the maximum field outline is either a piecewise constant function in the static mode or a piecewise linear function in the dynamic mode of driving an MLC. In view of clinical utility, the total leaf movement should be minimized, which requires that MLC-related constraints be considered in the dose optimization process. A method is proposed to achieve this for both static MLC fields and dynamic leaf close-in application. The method is an amendment to a generic gradient-based IMRT dose optimization algorithm and solves numerical problems related to the non-convexity of the MLC constraints, which can cause erratic behaviour of a gradient-based algorithm. It employs bistable penalty functions to select preferrable leaf configurations from the configuration space of the MLC, which is limited by specific design features. Together with an 'annealing' escape mechanism from local minima, the algorithm is capable of finding the optimum of an IMRT problem as leaf sequences with minimized leaf travel. In particular, the efficiency of static IMRT can be raised to the levels of unmodulated fields with very few field segments, thereby increasing the utility of IMRT in clinical practice.     

A comparison of the effects of mixed static and dynamic work with mainly dynamic work in hot conditions

Summary Current physiological criteria for limiting work in hot conditions are frequently based on responses to mainly dynamic work (eg treadmill walking). Their applicability to industrial situations containing mixed static and dynamic work is questioned, since the physiological responses to static work are different from those of dynamic work. Each of eight subjects attempted a one hour uphill treadmill walk (mainly dynamic work), and an uphill treadmill walk whilst intermittently carrying a 20 kg weight in the arms (mixed static and dynamic work). The external work rates in the two conditions were equal, effected by lowering the treadmill gradient in the loaded condition. Experiments were conducted in a hot climate (33‡ C dry bulb, 25‡ C wet bulb). Oxygen consumption, minute ventilation, sweat rate and rated perceived exertion were all significantly higher (p<0.001) for the mixed static and dynamic work than for the dynamic work. This was also the case for heart rate and forearm skin temperature (p<0.01), and for auditory canal temperature (p<0.05). There was no significant difference between the two types of work for mean skin temperature, calf skin temperature and chest skin temperature. These results show that for the same external work, physiological strain and perceived exertion are greater for mixed static and dynamic work (carrying a load in the arms) than for mainly dynamic work (walking on a treadmill). They suggest that it is not appropriate to make direct comparisons of laboratory studies based on dynamic work, with practical situations containing mixed static and dynamic work in the heat.     

The effects of tumor motion on planning and delivery of respiratory-gated IMRT

The purpose of this study is to investigate the effects of object motion on the planning and delivery of IMRT. Two phantoms containing objects were imaged using CT under a variety of motion conditions. The effects of object motion on axial CT acquisition with and without gating were assessed qualitatively and quantitatively. Measurements of effective slice width and position for the CT scans were made. Mutual information image fusion was adapted for use as a quantitative measure of object deformation in CT images. IMRT plans were generated on the CT scans of the moving and gated object images. These plans were delivered with motion, with and without gating, and the delivery error between the moving deliveries and a nonmoving delivery was assessed using a scalable vector-based index. Motion during CT acquisition produces motion artifact, object deformation, and object mispositioning, which can be substantially reduced with gating. Objects that vary in cross section in the direction of motion exhibit the most deformation in CT images. Mutual information provides a useful quantitative estimate of object deformation. The delivery of IMRT in the presence of target motion significantly alters the delivered dose distribution in relation to the planned distribution. The utilization of gating for IMRT treatment, including imaging, planning, and delivery, significantly reduces the errors introduced by object motion.     

An evaluation of gating window size,delivery method,and composite field dosimetry of respiratory-gated IMRT

A respiratory gating system has been developed based on a commercial patient positioning system. The purpose of this study is to investigate the ability of the gating system to reproduce normal, nongated IMRT operation and to quantify the errors produced by delivering a nongated IMRT treatment onto a moving target. A moving phantom capable of simultaneous two-dimensional motion was built, and an analytical liver motion function was used to drive the phantom. Studies were performed to assess the effect of gating window size and choice of delivery method (segmented and dynamic multileaf collimation). Additionally, two multiple field IMRT cases were delivered to quantify the error in gated and nongated IMRT with motion. Dosimetric error between nonmoving and moving deliveries is related to gating window size. By reducing the window size, the error can be reduced. Delivery error can be reduced for both dynamic and segmented delivery with gating. For the implementation of dynamic IMRT delivery in this study, dynamic delivery was found to generate larger delivery errors than segmented delivery in most cases studied. For multiple field IMRT delivery, the largest errors were generated in regions where high field modulation was present parallel to the axis of motion. Gating was found to reduce these large errors to clinically acceptable levels.     

Analysis of organ motion effects on the effective fluences for liver IMRT

An effective fluence concept was employed to make forward dose calculations to investigate the effects of a distorted fluence map on dose plans. Fluence changes caused by organ motion were calculated using Chui's algorithm (2003 Med. Phys. 30 1736). In two test cases with various fluence maps, the effects of motion were simulated using a maximal displacement from 5 mm to 25 mm; 108 fluence maps that were calculated from 16 IMRT plans for eight liver cancer patients were analyzed and compared with and without gating. Fluoroscopic measurements were made of a moving diaphragm in this study. Fluence changes associated with superior-inferior organ motion, perpendicular to the moving MLC, were also examined. The effects of motion on the fluence maps were evaluated from both the fluence differences between static and motion and the chi function. The maximum displacements of the organs in all of these cases were analyzed and correlated with the change in fluence generated from the liver IMRT plans. The dosimetric effects on the target coverage were evaluated for each plan. The results indicate that, for the same fluence map, the mean fluence intensity error or the percentage of the fluence points that have an unacceptable error is linearly related to the extent of motion. For different fluence maps, the degree to which the fluence is distorted by motion is strongly related to the product of the motion extent and the fluence gradient in the direction of diaphragm motion. For eight liver patients and 16 IMRT plans in this work (with gated technique, motion extent from 0.5 cm to 1.0 cm; without gated technique, motion extent from 0.9 cm to 1.8 cm), the fluence modulations are mild, such that the respiratory motion of each patient did not strongly affect the CTV coverage. The mean dose error is 1.5% for free motion (0.9-1.8 cm) and is around 1% for gated motion (0.5-1 cm).     

IMRT delivery to a moving target by dynamic MLC tracking: delivery for targets moving in two dimensions in the beam's eye view

A new modification of the dynamic multileaf collimator (dMLC) delivery technique for intensity-modulated therapy (IMRT) is outlined. This technique enables the tracking of a target moving through rigid-body translations in a 2D trajectory in the beam's eye view. The accuracy of the delivery versus that of deliveries with no tracking and of 1D tracking techniques is quantified with clinically derived intensity-modulated beams (IMBs). Leaf trajectories calculated in the target-reference frame were iteratively synchronized assuming regular target motion. This allowed the leaves defined in the lab-reference frame to simultaneously follow the target motion and to deliver the required IMB without violation of the leaf maximum-velocity constraint. The leaves are synchronized until the gradient of the leaf position at every instant is less than a calculated maximum. The delivered fluence in the target-reference frame was calculated with a simple primary-fluence model. The new 2D tracking technique was compared with the delivered fluence produced by no-tracking deliveries and by 1D tracking deliveries for 33 clinical IMBs. For the clinical IMBs normalized to a maximum fluence of 200 MUs, the rms difference between the desired and the delivered IMB was 15.6 +/- 3.3 MU for the case of a no-tracking delivery, 7.9 +/- 1.6 MU for the case where only the primary component of motion was corrected and 5.1 +/- 1.1 MU for the 2D tracking delivery. The residual error is due to interpolation and sampling effects. The 2D tracking delivery technique requires an increase in the delivery time evaluated as between 0 and 50% of the unsynchronized delivery time for each beam with a mean increase of 13% for the IMBs tested. The 2D tracking dMLC delivery technique allows an optimized IMB to be delivered to moving targets with increased accuracy and with acceptable increases in delivery time. When combined with real-time knowledge of the target motion at delivery time, this technique facilitates improved target conformality relative to no-tracking deliveries and allows PTV margin reduction.     

Guidance document on delivery,treatment planning,and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee

Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy since the advent of the medical linear accelerator. It allows the clinical implementation of highly conformal nonconvex dose distributions. This complex but promising treatment modality is rapidly proliferating in both academic and community practice settings. However, these advances do not come without a risk. IMRT is not just an add-on to the current radiation therapy process; it represents a new paradigm that requires the knowledge of multimodality imaging, setup uncertainties and internal organ motion, tumor control probabilities, normal tissue complication probabilities, three-dimensional (3-D) dose calculation and optimization, and dynamic beam delivery of nonuniform beam intensities. Therefore, the purpose of this report is to guide and assist the clinical medical physicist in developing and implementing a viable and safe IMRT program. The scope of the IMRT program is quite broad, encompassing multileaf-collimator-based IMRT delivery systems, goal-based inverse treatment planning, and clinical implementation of IMRT with patient-specific quality assurance. This report, while not prescribing specific procedures, provides the framework and guidance to allow clinical radiation oncology physicists to make judicious decisions in implementing a safe and efficient IMRT program in their clinics.     

Elongated beamlets: a simple technique for segment and MU reduction for sMLC IMRT delivery on accelerators utilizing 5 mm leaf widths

The focus of this work is to demonstrate the effects of using an elongated beamlet to achieve similar dose conformity as achieved with a square beamlet while reducing the number of segments and subsequent MU required. A series of 10 patients were planned for IMRT delivery to the prostate using minimum beamlet sizes of 5x5 mm2 (default scheme), 10x5 mm2 with the short axis parallel to the prostate-rectum interface (scheme 1), and 10x5 mm2 with the short axis perpendicular to the prostate-rectum interface (scheme 2). All other parameters between plans were left unchanged. Plans were appropriately normalized and evaluated for R65, R40, conformity index, total number of segments and MU. All plans were generated using the Corvus inverse planning system. The average number of segments in this study decreased by approximately 49% for both schemes 1 and 2. The subsequent number of MU required decreased by approximately 34.6%. The resultant modified modulation scaling factor (MSFmod) decreased by approximately 34.3%. Additionally, we found that each isodose distribution using scheme 2 would still meet our clinical acceptance criteria with no visible degradation in the dose distribution as compared with the default scheme. In conclusion, we have demonstrated that it is possible to achieve similar results as those obtained using a 5x5 mm2 beamlet with respect to target coverage and critical structure sparing by using strategically oriented elongated beamlets. This technique directly translates to a decreased MSF(mod) allowing for decreased leakage dose to the patient, a decreased risk of exceeding secondary shielding limits in pre-existing vaults, and shorter treatment times.     

A review of the acute effects of static and dynamic stretching on performance

An objective of a warm-up prior to an athletic event is to optimize performance. Warm-ups are typically composed of a submaximal aerobic activity, stretching and a sport-specific activity. The stretching portion traditionally incorporated static stretching. However, there are a myriad of studies demonstrating static stretch-induced performance impairments. More recently, there are a substantial number of articles with no detrimental effects associated with prior static stretching. The lack of impairment may be related to a number of factors. These include static stretching that is of short duration (<90 s total) with a stretch intensity less than the point of discomfort. Other factors include the type of performance test measured and implemented on an elite athletic or trained middle aged population. Static stretching may actually provide benefits in some cases such as slower velocity eccentric contractions, and contractions of a more prolonged duration or stretch-shortening cycle. Dynamic stretching has been shown to either have no effect or may augment subsequent performance, especially if the duration of the dynamic stretching is prolonged. Static stretching used in a separate training session can provide health related range of motion benefits. Generally, a warm-up to minimize impairments and enhance performance should be composed of a submaximal intensity aerobic activity followed by large amplitude dynamic stretching and then completed with sport-specific dynamic activities. Sports that necessitate a high degree of static flexibility should use short duration static stretches with lower intensity stretches in a trained population to minimize the possibilities of impairments.     

The knee adduction angle of the osteo-arthritic knee: A comparison of 3D supine,static and dynamic alignment

BackgroundEnd-stage knee osteoarthritis (OA) commonly results in knee arthroplasty. Three dimensional (3D) supine imaging is often used for pre-operative planning to optimise post-operative knee adduction angles (KAA). However, supine imaging may not represent loaded knee alignment. The aim of this study was to investigate differences in knee alignment under supine, static and dynamic conditions in healthy subjects and subjects with knee OA.MethodsNine healthy subjects and 15 subjects with end-stage knee OA were recruited. All subjects underwent supine imaging and motion capture during gait. KAAs were calculated from supine images (SUPINE), upright standing (STATIC) and at the first peak ground reaction force during gait (DYNAMIC), and were compared.ResultsKAAs were significantly higher (more varus) during gait compared with static (loaded and unloaded) in healthy subjects (p < 0.01) but not in subjects with knee OA. There was a good correlation between SUPINE and DYNAMIC for both healthy and OA subjects (R² > 0.58), with differences in the two relationships; healthy knees had a higher KAA during gait for any given KAA in the supine position, whereas OA knees that were valgus in imaging became more valgus during gait, and the opposite occurred for varus knees.ConclusionsFactors that may contribute to the noted differences between healthy and OA subjects include morphological changes in the joint as a result of OA, and gait compensation strategies in people with end-stage OA. Dynamic 3D motion capture provides important information about functional alignment that is not provided by supine imaging or static motion capture.Clinical RelevanceGait analysis may provide useful information to the surgeon during surgical planning of knee arthroplasties.     

Effects of the intensity levels and beam map resolutions on static IMRT plans

In this study we focus on how the intensity level and multileaf collimator (MLC) resolution affect the quality of IMRT plans using the static MLC delivery technique. The planning process is based on a least-square dose-based quadratic function and uses a simulated annealing algorithm to sample the discrete variables. Three clinical cases are studied empirically: a medulloblastoma, a prostate, and an oropharyngeal carcinoma. The intensity levels used are 3, 5, 10, 20, and continuous; the map resolution varies from 0.15-1.5 cm, with the leaf width equal to the step size. The influence of these two parameters are studied by comparing the cost value and the cost of delivery time from a trade-off point of view. An "efficient frontier" is drawn by connecting the plans with the lowest cost value at any given resolutions. For each case, a practical delivery region is defined by doubling the delivery time needed at a normal setting (five levels, 1.0 cm). Within this region, the "efficient frontier" demonstrates that the plans with five intensity levels are the most efficient comparing with plans with higher levels. This is a confirmation of the conclusion from Keller-Reichenbecher et al. [Int. J. Radiat. Oncol., Biol., Phys. 45, 1315-1324 (1999)]. It indicates that to further improve the plan quality with the minimal cost of extra delivery time, the most economical way is to improve the resolution rather than using higher intensity levels.     

Pairwise multiple comparison test procedures: an update for clinical child and adolescent psychologists.

Locating pairwise differences among treatment groups is a common practice of applied researchers. Articles published in this journal have addressed the issue of statistical inference within the context of an analysis of variance (ANOVA) framework, describing procedures for comparing means, among other issues. In particular, 1 article (Jaccard & Guilamo-Ramos, 2002b) presented some new methods of performing contrasts of means whereas another presented a framework for obtaining robust tests within this same context (Jaccard & Guilamo-Ramos, 2002a). The purpose of this article is to add to these contributions by presenting some newer methods for conducting pairwise comparisons of means, that is by extending the contributions of the first article and applying the framework of the second article to pairwise multiple comparisons. The newer methods are intended to provide additional sensitivity to detect treatment group differences and provide tests that are robust to the effects of variance heterogeneity, nonnormality, or both.     

Effect of static electric field treatment on multiple antibiotic-resistant pathogenic strains of Escherichia coli and Staphylococcus aureus.

This study evaluated the effect of a (4.5 kV/cm, 50 Hz) static electric field (SEF) on pathogenic strains of Escherichia coli and Staphylococcus aureus with multiple antibiotic resistance. The bacteria were grown overnight at 37 degrees C in a nutrient broth medium, then inoculated in 5 mL fresh nutrient broth medium and incubated for 2 h at 25 degrees C with continuous shaking at 190 rpm. 10 x colony-forming units/mL of these bacteria were subjected to a 4.5 kV/cm, 50 Hz, SEF for various time periods. The effects of 5 different SEF exposure times (30, 60, 90, 120 and 150 min) on the bacteria were evaluated by the plate count agar method. The growth percentages of SEF treatment groups were significantly less than that of the control group. Inactivation significantly increased with the duration of SEF exposure. The results indicate that growth inhibition by SEF in the Gram-negative bacteria, E. coli, was greater than that in the Gram-positive bacteria, S. aureus. This study has demonstrated the antimicrobial effects of SEF treatment on 2 important pathogens, suggesting its potential for application as a method for controlling microbial population growth within in a variety of environments.     

Development of a static and dynamic simulator for osteosyntheses of the mandible.

The development of new methods and materials for osteosynthesis requires in vitro testing prior to clinical use to exclude major problems or difficulties. In the field of oral and maxillofacial surgery no standardized testing procedures have so far been established. From a biomechanical point of view the lower jaw can be described as a lever that during the chewing cycle various forces at various points of the bone act upon. Standardized solid foam polyurethane mandibles (Sawbones, Malm?, Sweden) were used for the testing of various types of screws and different types of fractures. Via linkages that were connected to hydraulic cylinders defined forces were exerted on the polyurethane mandibles and the deformation was registered depending on the force. Monitoring was carried out contactless and therefore without back coupling by the use of a video system that recorded well-defined points on both sides of the fracture line. The photographs were then evaluated by special software (SIMI MOTION CAPTURE). The control of the cylinders was performed through a processor that besides static forces also allowed dynamic testing (e.g. sinusoid oscillation with defined amplitude and number of cycles). For the standardized and realistic three-dimensional static or dynamic testing of new methods and materials for osteosynthesis of the lower jaw this test stand has proved to be optimally suitable. Independent on the type of fracture and the material used the field of application for our test stand is the complete lower jaw. Therefore an ethically correct and economically reasonable testing of industrial products and clinical methods of osteosynthesis can be performed with this simulator.     

Effects of intra-fraction motion on IMRT dose delivery: statistical analysis and simulation

There has been some concern that organ motion, especially intra-fraction organ motion due to breathing, can negate the potential merit of intensity-modulated radiotherapy (IMRT). We wanted to find out whether this concern is justified. Specifically, we wanted to investigate whether IMRT delivery techniques with moving parts, e.g., with a multileaf collimator (MLC), are particularly sensitive to organ motion due to the interplay between organ motion and leaf motion. We also wanted to know if, and by how much, fractionation of the treatment can reduce the effects. We performed a statistical analysis and calculated the expected dose values and dose variances for volume elements of organs that move during the delivery of the IMRT. We looked at the overall influence of organ motion during the course of a fractionated treatment. A linear-quadratic model was used to consider fractionation effects. Furthermore, we developed software to simulate motion effects for IMRT delivery with an MLC, with compensators, and with a scanning beam. For the simulation we assumed a sinusoidal motion in an isocentric plane. We found that the expected dose value is independent of the treatment technique. It is just a weighted average over the path of motion of the dose distribution without motion. If the treatment is delivered in several fractions, the distribution of the dose around the expected value is close to a Gaussian. For a typical treatment with 30 fractions, the standard deviation is generally within 1% of the expected value for MLC delivery if one assumes a typical motion amplitude of 5 mm (1 cm peak to peak). The standard deviation is generally even smaller for the compensator but bigger for scanning beam delivery. For the latter it can be reduced through multiple deliveries ('paintings') of the same field. In conclusion, the main effect of organ motion in IMRT is an averaging of the dose distribution without motion over the path of the motion. This is the same as for treatments with conventional beams. Additional effects that are specific to the IMRT delivery technique appear to be relatively small, except for the scanning beam.     

Dose optimization for the MRI-accelerator: IMRT in the presence of a magnetic field

A combined system of a 6 MV linear accelerator and a 1.5 T MRI scanner is currently being developed. In this system, the patient will be irradiated in the presence of a 1.5 T magnetic field. This causes a strong dose increase at tissue-air interfaces. Around air cavities in the patient, these effects may become problematic. Homogeneous dose distributions can be obtained around regularly shaped symmetrical cavities using opposing beams. However, for more irregularly shaped cavities this approach may not be sufficient. This study will investigate whether IMRT can be used to cope with magnetic field dose effects, in particular for target volumes adjacent to irregularly shaped air cavities. Therefore, an inverse treatment planning approach has been designed based on pre-calculated beamlet dose distribution kernels. Using this approach, optimized dose distributions were calculated for B = 1.5 T and for B = 0 T. Investigated target sites include a prostate cancer, a laryngeal cancer and an oropharyngeal cancer. Differences in the dose distribution between B = 0 and 1.5 T were minimal; only the skin dose increased for B = 1.5 T. Homogeneous dose distributions were obtained for target structures adjacent to air cavities without the use of opposing beams. These results show that a 1.5 T magnetic field does not compromise the ability to achieve desired dose distributions with IMRT.