Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Radiotherapy, Intensity-Modulated”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22Linked to original sources
PubMed2001

Concepts for shuttling multileaf collimators for intensity-modulated radiation therapy.

Shuttling multileaf collimators (SMLCs) can increase the MU efficiency of intensity-modulated radiation therapy compared with the multiple-static-field (MSF-MLC) technique or dynamic MLC (DMLC) technique with conventional MLCs. In a previous paper (Phys. Med. Biol. 45 3343-58) a particular SMLC was shown, for highly modulated intensity distributions, to increase the MU efficiency compared with the MSF-MLC technique. In this companion paper, two new arrangements similar to that described in the earlier paper, but with less mechanical complexity, are shown to be constructionally simpler but less MU efficient. Additionally another new concept of SMLC is shown which also increases the MU efficiency compared with the MSF-MLC technique and often improves the MU efficiency compared with the previously reported SMLC for highly modulated intensity distributions. It also leads to zero tongue-and-groove underdose in the direction orthogonal to that of the shuttling elements (so-called across-the-rows).

Equipment Design↗
PubMed2001

A method of simulating dynamic multileaf collimators using Monte Carlo techniques for intensity-modulated radiation therapy.

A method of modelling the dynamic motion of multileaf collimators (MLCs) for intensity-modulated radiation therapy (IMRT) was developed and implemented into the Monte Carlo simulation. The simulation of the dynamic MLCs (DMLCs) was based on randomizing leaf positions during a simulation so that the number of particle histories being simulated for each possible leaf position was proportional to the monitor units delivered to that position. This approach was incorporated into an EGS4 Monte Carlo program, and was evaluated in simulating the DMLCs for Varian accelerators (Varian Medical Systems, Palo Alto. CA, USA). The MU index of each segment, which was specified in the DMLC-control data, was used to compute the cumulative probability distribution function (CPDF) for the leaf positions. This CPDF was then used to sample the leaf positions during a real-time simulation, which allowed for either the step-shoot or sweeping-leaf motion in the beam delivery. Dose intensity maps for IMRT fields were computed using the above Monte Carlo method, with its accuracy verified by film measurements. The DMLC simulation improved the operational efficiency by eliminating the need to simulate multiple segments individually. More importantly, the dynamic motion of the leaves could be simulated more faithfully by using the above leaf-position sampling technique in the Monte Carlo simulation.

Biophysical Phenomena↗
PubMed2001

Computer-assisted selection of coplanar beam orientations in intensity-modulated radiation therapy.

In intensity-modulated radiation therapy (IMRT), the incident beam orientations are often determined by a trial and error search. The conventional beam's-eye view (BEV) tool becomes less helpful in IMRT because it is frequently required that beams go through organs at risk (OARs) in order to achieve a compromise between the dosimetric objectives of the planning target volume (PTV) and the OARs. In this paper, we report a beam's-eye view dosimetrics (BEVD) technique to assist in the selection of beam orientations in IMRT. In our method, each beam portal is divided into a grid of beamlets. A score function is introduced to measure the 'goodness' of each beamlet at a given gantry angle. The score is determined by the maximum PTV dose deliverable by the beamlet without exceeding the tolerance doses of the OARs and normal tissue located in the path of the beamlet. The overall score of the gantry angle is given by a sum of the scores of all beamlets. For a given patient. the score function is evaluated for each possible beam orientation. The directions with the highest scores are then selected as the candidates for beam placement. This procedure is similar to the BEV approach used in conventional radiation therapy, except that the evaluation by a human is replaced by a score function to take into account the intensity modulation. This technique allows one to select beam orientations without the excessive computing overhead of computer optimization of beam orientation. It also provides useful insight into the problem of selection of beam orientation and is especially valuable for complicated cases where the PTV is surrounded by several sensitive structures and where it is difficult to select a set of 'good' beam orientations. Several two-dimensional (2D) model cases were used to test the proposed technique. The plans obtained using the BEVD-selected beam orientations were compared with the plans obtained using equiangular spaced beams. For all the model cases investigated, the use of BEVD-selected beam orientations improved the dose distributions significantly. These examples indicate that the technique has considerable potential for simplifying the IMRT treatment planning process and allows for better utilization of the technical capacity of IMRT.

Humans↗
PubMed2002

Possibilities for intensity-modulated brachytherapy: technical limitations on the use of non-isotropic sources.

An investigation was undertaken into possible dose conformity advantages and technical limitations of utilizing radially asymmetric internally applied radiation sources for intensity-modulated brachytherapy (IMBT). A feasible form of a source for IMBT would be a linear source with a high-intensity angular region, with some fractional transmission through the remainder of the source, which inhibits the resolution achievable in intensity modulation. Indexed rotation of the source about its axis would provide radial intensity modulation, which could compensate for variations in the spatial relationship between the source position and location of the target edge. Two treatment situations were simulated--one two-dimensional and one three-dimensional--both utilizing a single source (single catheter). The optimal intensity distribution of the source was determined by simulated annealing optimization using a conformality-based objective. The parameters in the optimization included the angular size of the source high-intensity region, and the fractional transmission through the low-intensity part of the source. Results indicate that limitations in source design suggest an optimal high-intensity resolution of approximately pi/4 to pi/8. The advantages of IMBT are rapidly reduced when fractional transmission through the low-intensity side of the source is increased.

Algorithms↗
PubMed2002

Minimizing delivery time and monitor units in static IMRT by leaf-sequencing.

Intensity-modulated radiation therapy (IMRT) requires the determination of the appropriate multileaf collimator settings to deliver an intensity map. The purpose of this work was to attempt to reduce the number of segments required for IMRT delivery and the number of monitor units required to deliver an intensity map. An intensity map may be written as a matrix. Leaf sequencing was formulated as a problem of decomposing the matrix into a series of sub-matrices. Sets of random intensity matrices were created and the segmentations produced by applying different algorithms were compared. The number of segments, important if verification and record (VR) overhead is significant, and beam on times were examined. It is shown that reducing the value of the matrix entries by the maximum amount at each stage results in the smallest number of steps. Reducing the 2-norm (sum of the squares) of the matrix entries by the maximum amount at each step results in the smallest beam on time. Three new algorithms are introduced, two of which produce results that are superior to those generated by the algorithms of other researchers. The resulting methods can be expanded upon to include tongue and groove effects and leaf inter-digitization. With square random matrices of the order 15, the reduction in beam time and segmentation is up to 30-40%. Compared to previous algorithms, those presented here have demonstrated a reduction in the beam on time required to deliver an intensity map by 30-40%. Similarly, the number of segments needed to deliver an intensity map is also reduced.

Algorithms↗
PubMed2002

Optimization of intensity-modulated very high energy (50-250 MeV) electron therapy.

This work evaluates the potential of very high energy (50-250 MeV) electron beams for dose conformation and identifies those variables that influence optimized dose distributions for this modality. Intensity-modulated plans for a prostate cancer model were optimized as a function of the importance factors, beam energy and number of energy bins, number of beams, and the beam orientations. A trial-and-error-derived constellation of importance factors for target and sensitive structures to achieve good conformal dose distributions was 500, 50, 10 and I for the target, rectum, bladder and normal tissues respectively. Electron energies greater than 100 MeV were found to be desirable for intensity-modulated very high energy electron therapy (VHEET) of prostate cancer. Plans generated for lower energy beams had relatively poor conformal dose distributions about the target region and delivered high doses to sensitive structures. Fixed angle beam treatments utilizing a large number of fields in the range 9-21 provided acceptable plans. Using more than 21 beams at fixed gantry angles had an insignificant effect on target coverage, but resulted in an increased dose to sensitive structures and an increased normal tissue integral dose. Minor improvements in VHEET plans utilizing a 'small' number (< or =9) of beams may be achieved if, in addition to intensity modulation, energy modulation is implemented using a small number (< or =3) of beam energies separated by 50 to 100 MeV. Rotation therapy provided better target dose homogeneity but unfortunately resulted in increased rectal dose, bladder dose and normal tissue integral dose relative to the 21-field fixed angle treatment plan. Modulation of the beam energy for rotation therapy had no beneficial consequences on the optimized dose distributions. Lastly, selection of beam orientations influenced the optimized treatment plan even when a large number of beams (approximately 15) were employed.

Electrons↗
PubMed2002

Treatment of recurrent malignant gliomas with stereotactic intensity modulated radiation therapy.

Malignant gliomas are usually refractory to aggressive combined-modality therapy, and the incidence of recurrence and death after treatment is very high. State-of-the-art techniques such as stereotactic intensity-modulated radiation therapy (IMRT) are now available to deliver a high dose of radiation to the tumor with relative preservation of surrounding tissues to achieve optimal tumor coverage with minimal toxicity. We report 10 patients (median age 48 years) with recurrent malignant gliomas that were treated with stereotactic directed IMRT. Initial tumor histologies included one low grade glioma (upgraded to anaplastic astrocytoma at recurrence), four anaplastic astrocytomas, and four glioblastomas multiforme. One patient was originally presumed to have a brain metastasis secondary to renal cell carcinoma but was pathologically confirmed as having glioblastoma multiforme at the time of recurrence. Before recurrence, all patients had been treated with external beam radiation therapy (median 59.7 Gy). All recurrences were confirmed by a subtotal resection (5/10) or by imaging (5/10). The median Karnofsky performance score at the time of IMRT was 80. The median tumor volume was 34.69 cm. Treatment was delivered on a 10-MV linear accelerator with a mini-multileaf collimator, MIMiC, and planned with Peacock/Corvus software. Radiation was delivered in daily fractions of 5 Gy, to a total median dose of 30 Gy at the 71% to 93% median isodose line. Median overall survival time was 10.1 months from the date of stereotactic treatment, with 1- and 2-year survival rates of 50% and 33.3%, respectively. Fractionated stereotactic intensity modulated radiation therapy is a novel technique used in the treatment of recurrent malignant gliomas, which produces results comparable to other currently used stereotactic techniques.

Adult↗
PubMed2000

Dependence of linac output on the switch rate of an intensity-modulated tomotherapy collimator.

The electro-mechanical, multivane intensity modulated collimator ("MIMiC") slit collimator with 40 vanes has been applied in the delivery of inversely planned sequential tomotherapy to over 4,000 patients. The collimator is binary in that each vane switches between fully open or closed status. Resulting beamlet patterns provide the intensity distributions imparting dose to the patient. The bouncing and damping of vanes at the two ends of their travel cause transient dose perturbations near and at the borders of the treatment field. These perturbations are not explicitly modeled by the planning system. Clinical beamlet profiles and output factors may then differ from those in the planning system and as a function of the vane switch period. A mechanical model of vane switching was developed to describe this dependency. Dose output and distribution of seven simple vane patterns with different switch times were measured with ionization chambers and radiographic films in polystyrene and anthropomorphic phantoms. Linac output dependence on switch time relative to vane open time was determined for four intensity modulated radiotherapy (IMRT) patients from measurements of an ionization chamber embedded in a cylindrical polystyrene phantom. Results demonstrate output dependence on switch time and, accordingly, on the servo mechanism for monitor units, arc length, dose rate, and gantry speed. In conclusion, the output dependence borders on clinical significance-improvements to collimator, dose calculation, commissioning, and quality assurance (QA) are suggested.

Humans↗
PubMed2001

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.

Algorithms↗
PubMed2001

Minimizing the number of segments in a delivery sequence for intensity-modulated radiation therapy with a multileaf collimator.

This paper proposes a sequencing algorithm for intensity-modulated radiation therapy with a multileaf collimator in the static mode. The algorithm aims to minimize the number of segments in a delivery sequence. For a machine with a long verification and recording overhead time (e.g., 15 s per segment), minimizing the number of segments is equivalent to minimizing the delivery time. The proposed new algorithm is based on checking numerous candidates for a segment and selecting the candidate that results in a residual intensity matrix with the least complexity. When there is more than one candidate resulting in the same complexity, the candidate with the largest size is selected. The complexity of an intensity matrix is measured in the new algorithm in terms of the number of segments in the delivery sequence obtained by using a published algorithm. The beam delivery efficiency of the proposed algorithm and the influence of different published algorithms used to calculate the complexity of an intensity matrix were tested with clinical intensity-modulated beams. The results show that no matter which published algorithm is used to calculate the complexity of an intensity matrix, the sequence generated by the algorithm proposed here is always more efficient than that generated by the published algorithm itself. The results also show that the algorithm used to calculate the complexity of an intensity matrix affects the efficiency of beam delivery. The delivery sequences are frequently most efficient when the algorithm of Bortfeld et al. is used to calculate the complexity of an intensity matrix. Because no single variation is most efficient for all beams tested, we suggest implementing multiple variations of our algorithm.

Algorithms↗
PubMed2001

Smoothing intensity-modulated beam profiles to improve the efficiency of delivery.

Intensity-modulated beam profiles are generated by an inverse planning or optimization algorithm, a process that, being computationally complex and intensive, is inherently susceptible to noise and numerical artifacts. These artifacts make delivery of the beams more difficult, oftentimes for little, if any, observable improvement in the dose distributions. In this work we examine two approaches for smoothing the beam profiles. The first approach is to smooth the beam profiles subsequent to each iteration in the optimization process (method A). The second approach is to include a term within the objective function that specifies the smoothness of the profiles as an optimization criterion (method B). The two methods were applied to a phantom study as well as three clinical sites: paraspinal, nasopharynx, and prostate. For the paraspinal and nasopharynx cases, which have critical organs with low tolerance doses in close proximity, method B produced sharper dose gradients, better target dose homogeneity, and more critical organ sparing. In the less demanding prostate case, the two methods give similar results. In addition, method B is more efficient during optimization, requiring fewer iterations, but less efficient during DMLC delivery, requiring a longer beam-on time.

Algorithms↗
PubMed2001

Verification of dynamic intensity-modulated beam deliveries in canine subjects.

Our objective in this work was to assess the precision and degree of accuracy with which intensity modulated radiation therapy (IMRT) can deliver highly localized dose distributions to tumors near critical structures using the dynamic sliding window technique. Measurements of dose distribution were performed both in vivo and in vitro using a combination of dosimeters [thermoluminescent dosimeters (TLD's), films, and diodes]. In vivo measurements were performed in two groups of purpose-bred dogs: one receiving four-field three-dimensional (3D) conformal treatment and the other receiving IMRT. The algorithms used in the inverse planning process included the Macro Pencil Beam (MPB) model and Projections onto Convex Sets (POCS). Single beam measurements were performed in phantoms to verify the accuracy of monitor unit settings required for delivering the desired doses. The composite doses from the delivery of the seven beam intensity modulated plans were measured in phantoms and cadavers, Biological end points (spinal cord toxicity and neurologic deficits due to irradiation) were evaluated at the end of one year to determine the spatial accuracy of the IMRT treatments over a fractionated course in live subjects. Results in single beam measurements were used at first to improve the dose calculation and translation algorithms. Results of the measurements for the delivery of all seven beams in phantoms confirmed that the system was capable of accurate spatial and dosimetric IMRT delivery. The in vivo results showed dramatic differences between control and IMRT-treated dogs, with the IMRT group showing no adverse effects and the control animals showing severe spinal cord injuries due to irradiation. The measurements presented in this paper have helped to verify the successful and accurate delivery of IMRT in a clinically related model using the University of Washington Medical Center (UWMC) system.

Algorithms↗
PubMed2001

Delivery of intensity-modulated radiation therapy with a conventional multileaf collimator: comparison of dynamic and segmental methods.

Intensity-modulated radiation therapy (IMRT) can be delivered with a conventional multileaf collimator (MLC), either in dynamic mode (DMLC) or in segmental mode (SMLC, also known as "step-and-shoot"). The advantage of DMLC is its ability to deliver the desired intensity profile produced by inverse planning with a high degree of fidelity. The SMLC method, on the other hand, resembles treatment with multiple static fields, and can be more easily verified. However, the use of SMLC requires that the desired profile be approximated by discrete levels of intensity, which may lead to degradation in the delivered dose distribution. Clearly, the results of SMLC delivery depend on the number of levels and the spatial resolution of the intensity distribution. In this work, we compare the DMLC method and the SMLC method employing different numbers of levels and different spatial resolutions. Three disease sites were studied: prostate, nasopharynx, and breast, with three cases for each. In general, a 5- to 10-level SMLC plan produced results comparable to that from a DMLC plan. The target coverage is improved by increasing the number of levels while critical organs are better protected with finer spatial resolutions. The beam-on-time (MUs) requirement for SMLC was approximately 20% less than DMLC, but the delivery time (in minutes) was about twice as long. Thus, the choice depends on many factors including machine capability, quality assurance, target coverage, critical organ protection, beam-on-time, delivery time, and other clinical considerations.

Breast Neoplasms↗
PubMed2002

Incorporation of realistic delivery limitations into dynamic MLC treatment delivery.

The clinical implementation of IMRT involves the use of a number of complex software-based systems, typically including an inverse planning system, a leaf sequencer, and a computer-controlled treatment delivery system. The inverse planning system determines the desired fluence patterns, the leaf sequencer translates those fluence maps into leaf trajectories, and the control system delivers those trajectories. While verification of intensity-modulated treatment fields has focused primarily on the dosimetric aspects of delivery, accurate delivery of the intended fluence distribution is dependent upon both the leaf sequencer and delivery control systems. Leaf sequencing algorithms typically do not incorporate many control system limitations, and this can lead to discrepancies between planned and delivered sequences. In this work, simple and complex fields were sequenced for the dynamic sliding window technique using different leaf speeds and tolerance settings to identify various limitations of the accelerator control system. This work was conducted on a Varian 2100 EX equipped with a Millennium 120 leaf MLC. The identified limitations were then incorporated into the sequencing algorithm using a limiting leaf velocity (less than the maximum leaf velocity), the leaf position tolerance, and the communications delay in the control system. Collision avoidance in leaf pairs was found to depend on a control system-enforced minimum gap between leaves and led to acceleration effects. By incorporating these effects into the leaf sequencing algorithm, dynamic sliding-window leaf sequences were produced which did not require beam interruptions or dose rate modulations for the parameter values used in calculating the sequence (dose rate, tolerance, leaf speed, and total monitor units). Incorporation of control system limitations into the leaf sequencing algorithm results in IMRT fields that are delivered with the prescribed constant dose rate, require less time to deliver, and have well-defined, calculable transmission dose characteristics.

Acceleration↗
PubMed2002

Multiple local minima in IMRT optimization based on dose-volume criteria.

Multiple local minima traps are known to exist in dose-volume and dose-response objective functions. Nevertheless, their presence and consequences are not considered impediments in finding satisfactory solutions in routine optimization of IMRT plans using gradient methods. However, there is often a concern that a significantly superior solution may exist unbeknownst to the planner and that the optimization process may not be able to reach it. We have investigated the soundness of the assumption that the presence of multiple minima traps can be ignored. To find local minima, we start the optimization process a large number of times with random initial intensities. We investigated whether the occurrence of local minima depends upon the choice of the objective function parameters and the number of variables and whether their existence is an impediment in finding a satisfactory solution. To learn about the behavior of multiple minima, we first used a symmetric cubic phantom containing a cubic target and an organ-at-risk surrounding it to optimize the beam weights of two pairs of parallel-opposed beams using a gradient technique. The phantom studies also served to test our software. Objective function parameters were chosen to ensure that multiple minima would exist. Data for 500 plans, optimized with random initial beam weights, were analyzed. The search process did succeed in finding the local minima and showed that the number of minima depends on the parameters of the objective functions. It was also found that the consequences of local minima depended on the number of beams. We further searched for the multiple minima in intensity-modulated treatment plans for a head-and-neck case and a lung case. In addition to the treatment plan scores and the dose-volume histograms, we examined the dose distributions and intensity patterns. We did not find any evidence that multiple local minima affect the outcome of optimization using gradient techniques in any clinically significant way. Our study supports the notion that multiple minima should not be an impediment to finding a good solution when gradient-based optimization techniques are employed. Changing the parameters for the objective function had no observable effect on our findings.

Dose-Response Relationship, Radiation↗
PubMed2002

Dosimetric performance of an enhanced dose range radiographic film for intensity-modulated radiation therapy quality assurance.

Film-based quality assurance (QA) is an important element of any intensity modulated radiation therapy (IMRT) program. XV2 film is often used for IMRT QA, however, it has saturation and energy response limitations which hinder accurate film dosimetry. A new commercially released ready-pack film has been introduced that has an extended dose range (EDR2), reportedly allowing measured doses above 600 cGy without saturation. Also, this film may have less energy dependence due to its composition. The purpose of this paper is to study and compare the two types of film with respect to absolute dose accuracy for IMRT plans, percent depth dose accuracy for square fields between 2 and 20 cm, ability to measure composite plan isodoses and single beam fluence maps for IMRT cases, and sensitivity to processor variations over time. In 19 IMRT patient QA tests, the EDR2 film was able to achieve an absolute dose accuracy of better than 2% vs over 4% for XV2 film. The EDR2 film was able to reproduce ionization chamber and diode-measured percent depth doses to 20 cm depth generally to within 1% over the range of field sizes tested compared to about 10% for the XV2 film. When compared to calculations, EDR2 film agreed better than XV2 film for both composite plan isodoses and single beam fluence intensity maps. The EDR2 film was somewhat more resistant to processor changes over time than the XV2 film, with a standard deviation of dose reproducibility of less than 2% compared to 6%, respectively.

Equipment Failure Analysis↗
PubMed2002

Smoothing intensity-modulated treatment delivery under hardware constraints.

A method is developed to smooth intensity-modulated beam delivery while considering hardware constraints. The method uses matrix algebra to model MLC leaf motion and hardware constraints such as interleaf digitization, synchronization requirements, and intersegment field abutment to deliver intensity modulated beam sequences. It was implemented and demonstrated for simulated and clinical intensity modulated cases. The simulated cases included intensity distributions from 3 x 3 to 30 x 30 in dimension with 1,000 intensity levels. The clinical examples included four treatments sites: prostate, head and neck, lung, and esophagus cancers. In particular, we studied the relationships between the intensity levels, segment numbers and their influence on the dose-volume histograms of the inversely optimized IMRT treatment plans. It was found that a significant reduction in the leaf segment numbers was achieved by applying the smoothing approach. The effects were found to be more significant for the unconstrained delivery than for the constrained delivery. The effects were also found to be more sensitive for the complex intensity distributions such as the head-and-neck cases than for the less complex ones such as the prostate cases. These results demonstrated the feasibility of maintaining inversely optimized treatment plans under acceptable tolerance levels while smoothing IMRT treatments with hardware constraints.

Algorithms↗
PubMed1997

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.

Humans↗