Evaluation of linear accelerator gating with real-time electromagnetic tracking.

PURPOSE: Intrafraction organ motion can produce dosimetric errors in radiotherapy. Commonly, the linear accelerator is gated using real-time breathing phase obtained by way of external sensors. However, the external anatomy does not always correlate well with the internal position. We examined a beam gating technique using signals from implanted wireless transponders that provided real-time feedback on the tumor location without an imaging dose to the patient. METHODS AND MATERIALS: An interface was developed between Calypso Medical's four-dimensional electromagnetic tracking system and a Varian Trilogy linear accelerator. A film phantom was mounted on a motion platform programmed with lung motion trajectories. Deliveries were performed when the beam was gated according to the signal from the wireless transponders. The dosimetric advantages of beam gating and the system latencies were quantified. RESULTS: Beam gating using on internal position monitoring provided up to a twofold increase in the dose gradients. The percentage of points failing to be within +/-10 cGy of the planned dose (maximal dose, approximately 200 cGy) was 3.4% for gating and 32.1% for no intervention in the presence of motion. The mean latencies between the transponder position and linear accelerator modulation were 75.0 +/-12.7 ms for beam on and 65.1 +/- 12.9 ms for beam off. CONCLUSION: We have presented the results from a novel method for gating the linear accelerator using trackable wireless internal fiducial markers without the use of ionizing radiation for imaging. The latencies observed were suitable for gating using electromagnetic fiducial markers, which results in dosimetric improvements for irradiation in the presence of motion.

An arc-sequencing algorithm for intensity modulated arc therapy.

Intensity modulated arc therapy (IMAT) is an intensity modulated radiation therapy delivery technique originally proposed as an alternative to tomotherapy. IMAT uses a series of overlapping arcs to deliver optimized intensity patterns from each beam direction. The full potential of IMAT has gone largely unrealized due in part to a lack of robust and commercially available inverse planning tools. To address this, we have implemented an IMAT arc-sequencing algorithm that translates optimized intensity maps into deliverable IMAT plans. The sequencing algorithm uses simulated annealing to simultaneously optimize the aperture shapes and weights throughout each arc. The sequencer enforces the delivery constraints while minimizing the discrepancies between the optimized and sequenced intensity maps. The performance of the algorithm has been tested for ten patient cases (3 prostate, 3 brain, 2 head-and-neck, 1 lung, and 1 pancreas). Seven coplanar IMAT plans were created using an average of 4.6 arcs and 685 monitor units. Additionally, three noncoplanar plans were created using an average of 16 arcs and 498 monitor units. The results demonstrate that the arc sequencer can provide efficient and highly conformal IMAT plans. An average sequencing time of approximately 20 min was observed.

Jaws-only IMRT using direct aperture optimization.

Using direct aperture optimization, we have developed an inverse planning approach that is capable of producing efficient intensity modulated radiotherapy (IMRT) treatment plans that can be delivered without a multileaf collimator. This "jaws-only" approach to IMRT uses a series of rectangular field shapes to achieve a high degree of intensity modulation from each beam direction. Direct aperture optimization is used to directly optimize the jaw positions and the relative weights assigned to each aperture. Because the constraints imposed by the jaws are incorporated into the optimization, the need for leaf sequencing is eliminated. Results are shown for five patient cases covering three treatment sites: pancreas, breast, and prostate. For these cases, between 15 and 20 jaws-only apertures were required per beam direction in order to obtain conformal IMRT treatment plans. Each plan was delivered to a phantom, and absolute and relative dose measurements were recorded. The typical treatment time to deliver these plans was 18 min. The jaws-only approach provides an additional IMRT delivery option for clinics without a multileaf collimator.

An examination of the number of required apertures for step-and-shoot IMRT.

We have examined the degree to which step-and-shoot IMRT treatment plans can be simplified (using a small number of apertures) without sacrificing the dosimetric quality of the plans. A key element of this study was the use of direct aperture optimization (DAO), an inverse planning technique where all of the multi-leaf collimator constraints are incorporated into the optimization. For seven cases (1 phantom, 1 prostate, 3 head-and-neck and 2 lung), DAO was used to perform a series of optimizations where the number of apertures per beam direction varied from 1 to 15. In this work, we attempt to provide general guidelines for how many apertures per beam direction are sufficient for various clinical cases using DAO. Analysis of the optimized treatment plans reveals that for most cases, only modest improvements in the objective function and the corresponding DVHs are seen beyond 5 apertures per beam direction. However, for more complex cases, some dosimetric gain can be achieved by increasing the number of apertures per beam direction beyond 5. Even in these cases, however, only modest improvements are observed beyond 9 apertures per beam direction. In our clinical experience, 38 out of the first 40 patients treated using IMRT plans produced using DAO were treated with 9 or fewer apertures per beam direction. The results indicate that many step-and-shoot IMRT treatment plans delivered today are more complex than necessary and can be simplified without sacrificing plan quality.

A toolbox for intensity modulated radiation therapy optimization.

We have designed a toolbox that provides an environment for testing radiotherapy optimization techniques, objective functions, and constraints. A set of three-dimensional (3D) pencil beam dose distributions have been computed for a cylindrical phantom. The 6 MV pencil beams were computed using a superposition-based dose engine commissioned for an Elekta SL20 linear accelerator. Due to the cylindrical symmetry of the phantom, the pencil beam dose distributions for any arbitrary beam angle can be determined by simply rotating the pencil beam data sets. Thus, the full accuracy is maintained without the need for additional dose calculations or large data storage requirements. In addition to the pencil beam data sets, tools are included for (1) rotating the pencil beams, (2) calculating the beam's eye view, (3) drawing structures, (4) writing the pencil beam dose data out to the optimizer, and (5) visualizing the optimized results. The pencil beam data sets and the corresponding tools are available for download at http://medschool.umaryland.edu/departments/radiationoncology/pencilbeam/. With this toolbox, researchers will have the ability to rapidly test new optimization techniques and formulations for intensity modulated radiation therapy and 3D conformal radiotherapy.

Inverse planning for intensity-modulated arc therapy using direct aperture optimization.

Intensity-modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines gantry rotation with dynamic multi-leaf collimation (MLC). With IMAT, the benefits of rotational IMRT can be realized using a conventional linear accelerator and a conventional MLC. Thus far, the advantages of IMAT have gone largely unrealized due to the lack of robust automated planning tools capable of producing efficient IMAT treatment plans. This work describes an inverse treatment planning algorithm, called 'direct aperture optimization' (DAO) that can be used to generate inverse treatment plans for IMAT. In contrast to traditional inverse planning techniques where the relative weights of a series of pencil beams are optimized, DAO optimizes the leaf positions and weights of the apertures in the plan. This technique allows any delivery constraints to be enforced during the optimization, eliminating the need for a leaf-sequencing step. It is this feature that enables DAO to easily create inverse plans for IMAT. To illustrate the feasibility of DAO applied to IMAT, several cases are presented, including a cylindrical phantom, a head and neck patient and a prostate patient.

Direct aperture optimization: a turnkey solution for step-and-shoot IMRT.

IMRT treatment plans for step-and-shoot delivery have traditionally been produced through the optimization of intensity distributions (or maps) for each beam angle. The optimization step is followed by the application of a leaf-sequencing algorithm that translates each intensity map into a set of deliverable aperture shapes. In this article, we introduce an automated planning system in which we bypass the traditional intensity optimization, and instead directly optimize the shapes and the weights of the apertures. We call this approach "direct aperture optimization." This technique allows the user to specify the maximum number of apertures per beam direction, and hence provides significant control over the complexity of the treatment delivery. This is possible because the machine dependent delivery constraints imposed by the MLC are enforced within the aperture optimization algorithm rather than in a separate leaf-sequencing step. The leaf settings and the aperture intensities are optimized simultaneously using a simulated annealing algorithm. We have tested direct aperture optimization on a variety of patient cases using the EGS4/BEAM Monte Carlo package for our dose calculation engine. The results demonstrate that direct aperture optimization can produce highly conformal step-and-shoot treatment plans using only three to five apertures per beam direction. As compared with traditional optimization strategies, our studies demonstrate that direct aperture optimization can result in a significant reduction in both the number of beam segments and the number of monitor units. Direct aperture optimization therefore produces highly efficient treatment deliveries that maintain the full dosimetric benefits of IMRT.

Two recruitment strategies for a clinical trial of physically frail community-living older persons.

OBJECTIVES: To compare the efficiency and cost of two distinct but complementary recruitment strategies for a clinical trial of physically frail, community-living persons, age 75 and older. DESIGN: In the first recruitment strategy, potential participants were identified and screened for physical frailty during office visits to their primary care physicians; in the second, potential participants were identified from the patient rosters of primary care physicians and were screened for physical frailty in their home. Physical frailty was defined on the basis of slow gait speed and inability to stand from a chair with one's arms folded. SETTING: General community in greater Bridgeport, Connecticut. PARTICIPANTS: Community-living persons, age 75 and older, who met criteria for physical frailty. MEASUREMENTS: Measures of efficiency included the number of persons screened for each participant randomized, the number of persons eligible for each participant randomized, and the refusal rate. Costs were calculated per randomized participant. RESULTS: One hundred eighty-eight participants, with a mean age of 83.2 years, were enrolled over 22 months. One hundred and one participants were enrolled via the office-based strategy; 87 were enrolled via the roster-based strategy. Participants in each group had considerable quadriceps weakness and performed poorly on objective measures of gait and upper- and lower-extremity function. For each participant randomized, the number of persons screened, number of persons eligible, and cost were 15.2, 1.2, and $868, respectively, for the office-based strategy and 11.6, 1.1, and $764, respectively, for the roster-based strategy. The corresponding refusal rates for the two strategies were 14.9% and 10.1%, respectively (P < .001). CONCLUSIONS: Although each recruitment strategy successfully identified older persons who were physically frail, the roster-based strategy was less expensive and performed modestly better on each measure of efficiency than the office-based strategy.

Iterative approaches to dose optimization in tomotherapy.

This paper will present the results of an investigation into three iterative approaches to inverse treatment planning. These techniques have been examined in the hope of developing an optimization algorithm suitable for the large-scale problems that are encountered in tomotherapy. The three iterative techniques are referred to as the ratio method, iterative least-squares minimization and the maximum-likelihood estimator. Our results indicate that each of these techniques can serve as a useful tool in tomotherapy optimization. As compared with other mathematical programming techniques, the iterative approaches can reduce both memory demands and time requirements. In this paper, the results from small- and large-scale optimizations will be analysed. It will also be demonstrated that the flexibility of the iterative techniques can be greatly enhanced through the use of dose-volume histogram based penalty functions and/or through the use of weighting factors assigned to each region of the patient. Finally, results will be presented from an investigation into the stability of the iterative techniques.

Inverse treatment planning for Gamma Knife radiosurgery.

An inverse treatment planning system for Gamma Knife radiosurgery has been developed using nonlinear programming techniques. The system optimizes the shot sizes, locations, and weights for Gamma Knife treatments. In the patient's prescription, the user can specify both the maximum number of shots of radiation and a minimum isodose line that must surround the entire treatment volume. After satisfying all of the constraints included in the prescription, the system maximizes the conformity of the dose distribution. This automated approach to treatment planning has been applied retrospectively to a series of patient cases, and each optimized plan has been compared to the corresponding manual plan produced by an experienced user. The results demonstrate that this tool can often improve the tumor dose homogeneity while using fewer shots than were included in the original plan. Therefore, inverse treatment planning should improve both the quality and the efficiency of Gamma Knife treatments.

Application of constrained optimization to radiotherapy planning.

Essential for the calculation of photon fluence distributions for intensity modulated radiotherapy (IMRT) is the use of a suitable objective function. The objective function should reflect the clinical aims of tumor control and low side effect probability. Individual radiobiological parameters for patient organs are not yet available with sufficient accuracy. Some of the major drawbacks of some current optimization methods include an inability to converge to a solution for arbitrary input parameters, and/or a need for intensive user input in order to guide the optimization. In this work, a constrained optimization method was implemented and tested. It is closely related to the demanded clinical aims, avoiding the drawbacks mentioned above. In a prototype treatment planning system for IMRT, tumor control was guaranteed by setting a lower boundary for target dose. The aim of low complication is fulfilled by minimizing the dose to organs at risk. If only one type of tissue is involved, there is no absolute need for radiobiological parameters. For different organs, threshold dose, relative seriality of the organs or an upper dose limit could be set. All parameters, however, were optional, and could be omitted. Dose-volume constraints were not used, avoiding the possibility of local minima in the objective function. The approach was benchmarked through the simulation of both a head and neck and a lung case. A cylinder phantom with precalculated dose distributions of individual pencil beams was used. The dose to regions at risk could be significantly reduced using at least seven ports of beam incidence. Increasing the number of ports beyond seven produced only minor further gain. The relative seriality of organs was modeled through the use of an added exponent to the dose. This approach however increased calculation time significantly. The alternative of setting an upper limit is much faster and allows direct control of the maximum dose. Constrained optimization guarantees high tumor control probability, it is computationally more efficient than adding penalty terms to the objective function, and the input parameters are dose limits known in clinical practice.

A simple model for examining issues in radiotherapy optimization.

Convolution/superposition software has been used to produce a library of photon pencil beam dose matrices. This library of pencil beams is designed to serve as a tool for both education and investigation in the field of radiotherapy optimization. The elegance of this pencil beam model stems from its cylindrical symmetry. Because of the symmetry, the dose distribution for a pencil beam from any arbitrary angle can be determined through a simple rotation of a pre-computed dose matrix. Rapid dose calculations can thus be performed while maintaining the accuracy of a convolution/superposition based dose computation. The pencil beam data sets have been made publicly available. It is hoped that the data sets will facilitate a comparison of a variety of optimization and delivery approaches. This paper will present a number of studies designed to demonstrate the usefulness of the pencil beam data sets. These studies include an examination of the extent to which a treatment plan can be improved through either an increase in the number of beam angles and/or a decrease in the collimator size. A few insights into the significance of heterogeneity corrections for treatment planning for intensity modulated radiotherapy will also be presented.

Maximum likelihood as a common computational framework in tomotherapy.

Tomotherapy is a dose delivery technique using helical or axial intensity modulated beams. One of the strengths of the tomotherapy concept is that it can incorporate a number of processes into a single piece of equipment. These processes include treatment optimization planning, dose reconstruction and kilovoltage/megavoltage image reconstruction. A common computational technique that could be used for all of these processes would be very appealing. The maximum likelihood estimator, originally developed for emission tomography, can serve as a useful tool in imaging and radiotherapy. We believe that this approach can play an important role in the processes of optimization planning, dose reconstruction and kilovoltage and/or megavoltage image reconstruction. These processes involve computations that require comparable physical methods. They are also based on equivalent assumptions, and they have similar mathematical solutions. As a result, the maximum likelihood approach is able to provide a common framework for all three of these computational problems. We will demonstrate how maximum likelihood methods can be applied to optimization planning, dose reconstruction and megavoltage image reconstruction in tomotherapy. Results for planning optimization, dose reconstruction and megavoltage image reconstruction will be presented. Strengths and weaknesses of the methodology are analysed. Future directions for this work are also suggested.

Sodium in drinking water in South Carolina.

The potential contribution of drinking water sodium to total consumption and to blood pressure levels is explored in a South Carolina survey. Most (96 per cent) of the population was served by drinking water containing less than 100 mg/l Na (median 14.8 mg/l). For households with water sources containing greater than or equal to 100 mg/l Na, drinking water accounted for approximately 8.5 per cent of total estimated consumption. After adjustment for age, sex, race, body mass index, education, and dietary sodium, a negative association between diastolic blood pressure and drinking water sodium levels was found.