Multileaf Collimator Control Research Articles

A high‐speed scintillation‐based electronic portal imaging device to quantitatively characterize IMRT delivery

We have developed an electronic portal imaging device (EPID) employing a fast scintillator and a high-speed camera. The device is designed to accurately and independently characterize the fluence delivered by a linear accelerator during intensity modulated radiation therapy (IMRT) with either step-and-shoot or dynamic multileaf collimator (MLC) delivery. Our aim is to accurately obtain the beam shape and fluence of all segments delivered during IMRT, in order to study the nature of discrepancies between the plan and the delivered doses. A commercial high-speed camera was combined with a terbium-doped gadolinium-oxy-sulfide (Gd2O2S:Tb) scintillator to form an EPID for the unaliased capture of two-dimensional fluence distributions of each beam in an IMRT delivery. The high speed EPID was synchronized to the accelerator pulse-forming network and gated to capture every possible pulse emitted from the accelerator, with an approximate frame rate of 360 frames-per-second (fps). A 62-segment beam from a head-and-neck IMRT treatment plan requiring 68 s to deliver was recorded with our high speed EPID producing approximately 6 Gbytes of imaging data. The EPID data were compared with the MLC instruction files and the MLC controller log files. The frames were binned to provide a frame rate of 72 fps with a signal-to-noise ratio that was sufficient to resolve leaf positions and segment fluence. The fractional fluence from the log files and EPID data agreed well. An ambiguity in the motion of the MLC during beam on was resolved. The log files reported leaf motions at the end of 33 of the 42 segments, while the EPID observed leaf motions in only 7 of the 42 segments. The static IMRT segment shapes observed by the high speed EPID were in good agreement with the shapes reported in the log files. The leaf motions observed during beam-on for step-and-shoot delivery were not temporally resolved by the log files.

TU‐C‐T‐6C‐04: Quantifying the Tradeoff Between Complexity and Conformality

Purpose: Complex intensity maps in an IMRT plan may have several undesirable effects: increased leakage and integral dose, higher sensitivity to delivery errors, longer treatment time. On the other hand, the potentially negative impact of a reduction of the complexity on the dose conformality is not a priori clear. We propose to study the interplay between complexity and dose conformality in an interactive multi‐objective planning approach. Method and Materials: Two surrogate measures of complexity have been studied in previous approaches: The number of multileaf collimator (MLC) segments (in step and shoot IMRT delivery), and the number of monitor units (MUs). The first is difficult to incorporate into optimization strategies because it is an “NP hard” problem and the calculation would be time prohibitive. Fortunately, one MLC vendor has proven that the MLC hardware and control software can be designed such that the pure number of MLC segments is not a limiting factor anymore. We therefore use the number of MUs as the measure of complexity. Mathematically, the number of MUs is given by the sum of positive gradients of the intensity map along the direction of leaf motion. We include the sum of positive gradients as a linear objective function in our plan optimization framework. In our approach we find solutions that are Pareto optimal, i.e., their complexity cannot be further reduced without compromising conformality. The results are incorporated into an interactive plan navigator by means of a “complexity slider”, which allows the user to interactively explore the impact of a change of the plan complexity on the dose distributions. Results: Examples of optimized IMRT plans in the head and neck region and in the prostate will be presented. The potential of reducing complexity is case dependent but in comparison with current IMRT planning approaches a substantial reduction of the number of MUs (in the order of 20%) is often achievable without compromising the dose distribution. The complexity slider allows one to explore the whole spectrum of plans including the extrema, i.e. simple but not conformal or highly conformal but complex plans. Conclusion: The complexity of an IMRT plan can be limited or reduced with various methods. In general there is a price to be paid for this. Multi‐objective optimization can help to find the most suitable tradeoff. Supported by the NCI (1 R01 CA103904)

A compact multileaf collimator for conventional and intensity modulated fast neutron therapy

A 120 leaf collimator of high resolution has been constructed to shape a fast neutron therapy (FNT) beam produced from a superconducting cyclotron using the d(48.5)+ Be reaction. The computer controlledmultileaf collimator(MLC) replaces an aging, manually operated multirod collimator (MRC). The MLC was built to address two problems: The need to increase the efficiency of FNT at the Gershenson RadiationOncology Center of the Karmanos Cancer Institute at Detroit, MI, and the desire to implement intensity modulated neutron radiotherapy for which a suitable computer controlled beam shaping device of high resolution and rapid shape changing not currently exist. The specific aims were to build a neutronMLC that would solve these problems and then verify its radiological performance as being clinically acceptable. The MLC leaves project 5 mm in the iso-centric plane perpendicular to the beam axis. A taper has been included on the leaves matching the beam divergence along one axis. A 5 mm leaf projection width was chosen to give high resolution conformality across the entire field. The maximum field size provided is 30×30 cm 2 . To reduce the interleaf transmission a 0.254 mm blocking step has been included. End-leaf steps totaling 0.762 mm were also included allowing adjacent leaf pairs to close off within the primary radiation beam. The neutronMLC also includes individual 45° and 60° automated universal tungsten wedges. All electro-mechanical functions of the MLC are governed by the multileaf collimatorcontrol system (MLCCS). The MLCCS control functions are leaf and wedge motion control, leaf and wedge position verification, switching on and off the radiation beam, and inputting certain cyclotron interlocks statuses as well as outputting MLC interlocks. The MLCCS is responsible for setting required shapes on the MLC, and verifying those shapes to be accurate to within ±1 mm for each leaf’s projection in the plane perpendicular to the beam axis at the position of the iso-center. The position verification function uses a machine vision system that images optical targets on the leaves to verify set field shape accuracy within ±1 mm at the level of iso-center. The MLC transmission was measured to be 3.9±0.5% with an 11±2% gamma component which is slightly lower than the MRC measured transmission of 4.6±0.5% with a 17±2% gamma component. The difference in the measured gamma component ratio for the closed collimators agrees within 40% to a theoretical approximation of ∼2.4 for tungsten to steel when considering equal attenuation lengths. The actual MLC penumbral measurements in water for a 10×10 cm 2 field at 5 cm depth were 9.1±0.2 mm and 11.8±0.5 mm along the focused and unfocused axes, respectively. The MRC penumbra measured 8.6±0.3 mm under the same conditions along both of its focused axes suggesting penumbral equivalence between the MLC and MRC along the focused axis with slight degradation in the unfocused direction of the MLC. The many benefits of the fully automatic MLC over the semi-manual MRC are considered to justify this compromise.

Verification of step-and-shoot IMRT delivery using a fast video-based electronic portal imaging device.

We present an investigation into the use of a fast video-based electronic portal-imaging device (EPID) to study intensity modulated radiation therapy (IMRT) delivery. The aim of this study is to test the feasibility of using an EPID system to independently measure the orchestration of collimator leaf motion and beam fluence; simultaneously measuring both the delivered field fluence and shape as it exits the accelerator head during IMRT delivery. A fast EPID that consists of a terbium-doped gadolinium oxysulphide (GdO2S:Tb) scintillator coupled with an inexpensive commercial 30 frames-per-second (FPS) CCD-video recorder (16.7 ms shutter time) was employed for imaging IMRT delivery. The measurements were performed on a Varian 2100 C/D linear accelerator equipped with a 120-leaf multileaf-collimator (MLC). A characterization of the EPID was performed that included measurements of spatial resolution, linac pulse-rate dependence, linear output response, signal uniformity, and imaging artifacts. The average pixel intensity for fields imaged with the EPID was found to be linear in the delivered monitor units of static non-IMRT fields between 3x3 and 15x15 cm2. A systematic increase of the average pixel intensity was observed with increasing field size, leading to a maximum variation of 8%. Deliveries of a clinical step-and-shoot mode leaf sequence were imaged at 600 MU/min. Measurements from this IMRT delivery were compared with experimentally validated MLC controller log files and were found to agree to within 5%. An analysis of the EPID image data allowed identification of three types of errors: (1) 5 out of 35 segments were undelivered; (2) redistributing all of the delivered segment MUs; and (3) leaf movement during segment delivery. Measurements with the EPID at lower dose rates showed poor agreement with log files due to an aliasing artifact. The study was extended to use a high-speed camera (1-1000 FPS and 10 micros shutter time) with our EPID to image the same delivery to demonstrate the feasibility of imaging without aliasing artifacts. High-speed imaging was shown to be a promising direction toward validating IMRT deliveries with reasonable image resolution and noise.

Acceptance tests and quality control (QC) procedures for the clinical implementation of intensity modulated radiotherapy (IMRT) using inverse planning and the sliding window technique: experience from five radiotherapy departments

Background and purpose: An increasing number of radiotherapy centres is now aiming for clinical implementation of intensity modulated radiotherapy (IMRT), but – in contrast to conventional treatment – no national or international guidelines for commissioning of the treatment planning system (TPS) and acceptance tests of treatment equipment have yet been developed. This paper bundles the experience of five radiotherapy departments that have introduced IMRT into their clinical routine. Methods and materials: The five radiotherapy departments are using similar configurations since they adopted the commercially available Varian solution for IMRT, regarding treatment planning as well as treatment delivery. All are using the sliding window technique. Different approaches towards the derivation of the multileaf collimator (MLC) parameters required for the configuration of the TPS are described. A description of the quality control procedures for the dynamic MLC, including their respective frequencies, is given. For the acceptance of the TPS for IMRT multiple quality control plans were developed on a variety of phantoms, testing the flexibility of the inverse planning modules to produce the desired dose pattern as well as assessing the accuracy of the dose calculation. Regarding patient treatment verification, all five centres perform dosimetric pre-treatment verification of the treatment fields, be it on a single field or on a total plan procedure. During the actual treatment, the primary focus is on patient positioning rather than dosimetry. Intracavitary in vivo measurements were performed in special cases. Result and conclusion: The configurational MLC parameters obtained through different methods are not identical for all centres, but the observed variations have shown to be of no significant clinical relevance. The quality control (QC) procedures for the dMLC have not detected any discrepancies since their initiation, demonstrating the reliability of the MLC controller. The development of geometrically simple QC plans to test the inverse planning, the dynamic MLC modules and the final dose calculation has proven to be useful in pointing out the need to remodel the single pencil beam scatter kernels in some centres. The final correspondence between calculated and measured dose was found to be satisfactory by all centres, for QC test plans as well as for pre-treatment verification of clinical IMRT fields. An intercomparison of the man hours needed per patient plan verification reveals a substantial variation depending on the type of measurements performed.

Calibration and quality assurance for rounded leaf‐end MLC systems

Multileaf collimator (MLC) systems are available on most commercial linear accelerators, and many of these MLC systems utilize a design with rounded leaf ends and linear motion of the leaves. In this kind of system, the agreement between the digital MLC position readouts and the light field or radiation field edges must be achieved with software, since the leaves do not move in a focused motion like that used for most collimator jaw systems. In this work we address a number of the calibration and quality assurance issues associated with the acceptance, commissioning, and routine clinical use of this type of MLC system. These issues are particularly important for MLCs used for various types of intensity modulated radiation therapy (IMRT) and small, conformal fields. For rounded leaf end MLCs, it is generally not possible to make both the light and radiation field edges agree with the digital readout, so differences between the two kinds of calibrations are illustrated in this work using one vendor's MLC system. It is increasingly critical that the MLC leaf calibration be very consistent with the radiation field edges, so in this work a methodology for performing accurate radiation field size calibration is discussed. A system external to the vendor's MLC control system is used to correct or handle limitations in the MLC control system. When such a system of corrections is utilized, it is found that the MLC radiation field size can be defined with an accuracy of approximately 0.3 mm, much more accurate than most vendor's specifications for MLC accuracy. Quality assurance testing for such a calibration correction system is also demonstrated.

Optimization of the step-and-shoot leaf sequence for delivery of intensity modulated radiation therapy using a variable division scheme

To deliver an intensity modulated radiation therapy (IMRT) plan by multileaf collimator (MLC) it is necessary to convert beam profiles, generated from the inverse treatment planning algorithm, into a series of instructions that the MLC control system can execute. An idealized IMRT beam profile can be regarded as a continuously varying two-dimensional function and is usually represented by an intensity map, i.e., a discretized description in space and in intensity of the beam profile. It is common to assume that the intensity map be defined over a regular grid with N steps and equal increments of intensity levels. In reality, this may not be the optimal representation of the beam profile and may introduce unnecessary discrepancies between the intensity pattern delivered and that ideally required. We have implemented an algorithm capable of minimizing the difference between the two patterns on a beam specific basis. In other words, it can produce optimized intensity maps, individually produced to suit the (continuous function) intensity profile they are intended to approximate. This enhancement in conformation is achieved by allowing variable step size and unconstrained intensity levels in the final leaf sequence.

Rectangular edge synchronization for intensity modulated radiation therapy with dynamic multileaf collimation

The implementation of intensity modulated radiotherapy by dynamic multileaf collimator control involves the use of interpreter software which creates leaf trajectory plans for each leaf pair. Interpreter software for use with an Elekta SL15 linear accelerator and dedicated multileaf collimator has been written and tested. In practice the ideal trajectory plans often predict contact between leaves from opposing leaf banks, but this is prohibited by control software on the Elekta system as it could lead to mechanical damage. If the modulation within the geometric limits of a shaped field is not to be compromised then strategies to avoid leaf contact result in additional unwanted doses outside the geometric edge. The magnitude of any such additional dose can be reduced to acceptable levels by a technique which we have called rectangular edge synchronization. The performance of interpreter software which incorporates rectangular edge synchronization has been compared with that of potentially more efficient software which does not. The option containing the rectangular edge synchronization algorithm was shown to work consistently well at high monitor unit rates, and without incurring leaf contacts, under a wide range of test conditions. It therefore provides a sound basis for using intensity modulation to replace mechanical wedges, to simulate customized patient shape compensators, or to implement the results of inverse treatment planning processes that require superimposed intensity modulated beams.

A multifeaf collimator field prescription preparation system for conventional radiotherapy

The purpose of this work is to develop a prescription preparation system for efficient field shaping using a multileaf collimator that can be used in community settings as well as research institutions. The efficiency advantage of the computer-controlled multileaf collimator, over cerrobend blocks, to shape radiation fields has been shown in conformal treatments, which typically require complete volumetric computerized tomographic data for three-dimensional radiation treatment planning--a utility not readily available to the general community. As a result, most patients today are treated with conventional radiation therapy. Therefore, we believe that it is very important to fully use the same efficiency advantage of multileaf collimator as a block replacement in conventional practice. The multileaf collimator prescription preparation system developed by us acquires prescription images from different sources, including film scanner and radiation treatment planning systems. The multileaf collimator angle and leaf positions are set from the desired field contour defined on the prescription image, by minimizing the area discrepancies. Interactive graphical tools include manual adjustment of collimator angle and leaf positions, and definition of portions of the field edges that require maximal conformation. Data files of the final leaf positions are transferred to the multileaf collimator controller via a dedicated communication link. We have implemented the field prescription preparation system and a network model for integrating the multileaf collimator and other radiotherapy modalities for routine treatments. For routine plan evaluation, isodose contours measured with film in solid water phantom at prescription depth are overlaid on the prescription image. Preliminary study indicates that the efficiency advantage of the MLC over cerrobend blocks in conformal therapy also holds true for conventional treatments. Our model of computer-controlled prescription, evaluation, and treatment using multileaf collimators can be effectively implemented in both community settings and research institutions. The resultant increase in treatment efficiency and accuracy is now available for conventional radiotherapy.

A multileaf collimator field prescription preparation system for conventional radiotherapy

Purpose: The purpose of this work is to develop a prescription preparation system for efficient field shaping using a multileaf collimator that can be used in community settings as well as research institutions. The efficiency advantage of the computer-controlled multileaf collimator, over cerrobend blocks, to shape radiation fields has been shown in conformal treatments, which typically require complete volumetric computerized tomographic data for three-dimensional radiation treatment planning—a utility not readily available to the general community. As a result, most patients today are treated with conventional radiation therapy. Therefore, we believe that it is very important to fully use the same efficiency advantage of multileaf collimator as a block replacement in conventional practice. Methods and Material: The multileaf collimator prescription preparation systems developed by us acquires prescription images from different sources, including film scanner, and radiation treatment planning systems. The multileaf collimator angle and leaf positions are set from the desired field contour defined on the prescription image, by minimizing the area discrepancies. Interactive graphical tools include manual adjustment of collimator angle and leaf positions, and definition of portions of the field edges that require maximal conformation. Data files of the final leaf positions are transferred to the multileaf collimator controller via a dedicated communication link. Results: We have implemented the field prescription preparation system and a network model for integrating the multileaf collimator and other radiotherapy modalities for routine treatments. For routine plan evaluation, isodose contours measured with film in solid water phantom at prescription depth are overlaid on the prescription image. Preliminary study indicates that the efficiency advantage of the MLC over cerrobend blocks in conformal therapy also holds true for conventional treatments. Conclusion: Our model of computer-controlled prescription, evaluation and treatment using multileaf collimators can be effectively implemented in both community settings and research institutions. The resultant increase in treatment efficiency and accuracy is now available for conventional radiotherapy.