Dynamic Multileaf Collimator Delivery Research Articles

Increased accuracy of planning tools for optimization of dynamic multileaf collimator delivery of radiotherapy through reformulated objective functions

The purpose of this study is to examine in a clinical setting a novel formulation of objective functions for intensity-modulated radiotherapy treatment plan multicriteria optimization (MCO) that we suggested in a recent study. The proposed objective functions are extended with dynamic multileaf collimator (DMLC) delivery constraints from the literature, and a tailored interior point method is described to efficiently solve the resulting optimization formulation. In a numerical planning study involving three patient cases, DMLC plans Pareto optimal to the MCO formulation with the proposed objective functions are generated. Evaluated based on pre-defined plan quality indices, these DMLC plans are compared to conventionally generated DMLC plans. Comparable or superior plan quality is observed. Supported by these results, the proposed objective functions are argued to have a potential to streamline the planning process, since they are designed to overcome the methodological shortcomings associated with the conventional penalty-based objective functions assumed to cause the current need for time-consuming trial-and-error parameter tuning. In particular, the increased accuracy of the planning tools imposed by the proposed objective functions has the potential to make the planning process less complicated. These conclusions position the proposed formulation as an alternative to existing methods for automated planning.

Primo software as a tool for Monte Carlo simulations of intensity modulated radiotherapy: A feasibility study

Introduction Intensity Modulated Radiation Therapy (IMRT) allows creating complex dose distributions. The Monte Carlo (MC) is considered the gold standard method to perform dose calculations. A recent new MC user-friendly interface named PRIMO was developed targeting External Radiotherapy environment. Nevertheless, advanced features such as IMRT are not yet introduced. Purpose To assess the capability of PRIMO to simulate IMRT procedures. Materials and methods Two Radiotherapy units, both using Varian 2300CD LINAC head were simulated. While one uses Millennium120, the other has 120HD as Multi Leaf Collimator (MLC). Static simulations were performed to validate LINAC heads and MLC models. Homemade software was developed to configure PRIMO to reproduce MLC modulation as planned by the Treatment Planning System and create dose distribution images at specific planes from PRIMO output files. For each unit, a dynamic MLC delivery was split into a number of static segments and simulated on solid water phantom. Gafchromic films were used to measure the actual dose and make comparisons with the simulations. Assessments were performed using the 2D Gamma analysis. Results More than 95% of Gamma points Conclusion A workflow was found to drive PRIMO to simulate IMRT procedures. The results of simulations agree with the experimental measurements, indicating PRIMO software as a potential tool for clinical implementation of MC simulations of IMRT technique. Disclosure The author has no relevant financial or nonfinancial relationships to disclose.

Lung tumor tracking in fluoroscopic video based on optical flow.

Respiratory gating and tumor tracking for dynamic multileaf collimator delivery require accurate and real-time localization of the lung tumor position during treatment. Deriving tumor position from external surrogates such as abdominal surface motion may have large uncertainties due to the intra- and interfraction variations of the correlation between the external surrogates and internal tumor motion. Implanted fiducial markers can be used to track tumors fluoroscopically in real time with sufficient accuracy. However, it may not be a practical procedure when implanting fiducials bronchoscopically. In this work, a method is presented to track the lung tumor mass or relevant anatomic features projected in fluoroscopic images without implanted fiducial markers based on an optical flow algorithm. The algorithm generates the centroid position of the tracked target and ignores shape changes of the tumor mass shadow. The tracking starts with a segmented tumor projection in an initial image frame. Then, the optical flow between this and all incoming frames acquired during treatment delivery is computed as initial estimations of tumor centroid displacements. The tumor contour in the initial frame is transferred to the incoming frames based on the average of the motion vectors, and its positions in the incoming frames are determined by fine-tuning the contour positions using a template matching algorithm with a small search range. The tracking results were validated by comparing with clinician determined contours on each frame. The position difference in 95% of the frames was found to be less than 1.4 pixels (approximately 0.7 mm) in the best case and 2.8 pixels (approximately 1.4 mm) in the worst case for the five patients studied.

Variable beam dose rate and DMLC IMRT to moving body anatomy

Derivation of formulas relating leaf speeds and beam dose rates for delivering planned intensity profiles to static and moving targets in dynamic multileaf collimator (DMLC) intensity modulated radiation therapy (IMRT) is presented. The analysis of equations determining algorithms for DMLC IMRT delivery under a variable beam dose rate reveals a multitude of possible delivery strategies for a given intensity map and for any given target motion patterns. From among all equivalent delivery strategies for DMLC IMRT treatments specific subclasses of strategies can be selected to provide deliveries that are particularly suitable for clinical applications providing existing delivery devices are used. Special attention is devoted to the subclass of beam dose rate variable DMLC delivery strategies to moving body anatomy that generalize existing techniques of such deliveries in Varian DMLC irradiation methodology to static body anatomy. Few examples of deliveries from this subclass of DMLC IMRT irradiations are investigated to illustrate the principle and show practical benefits of proposed techniques.

Poster - Thurs Eve-36: Use of multileaf collimator as a replacement of physical missing tissue compensator.

Missing tissue compensators are used to improve dose uniformity for some patients undergoing radiation therapy. Currently, our practice is to machine compensators out of lead alloy plate. Replacing this physical filter with a segmented multileaf collimator (MLC) delivery sequence is beneficial in terms of work flow and delivery efficiency. The purpose of this work is to compare the dose uniformity achieved by fields that are either (A) conventionally compensated, compensated by segmenting the physical compensator thickness map into either (B) step-and-shoot or (C) dynamic MLC delivery sequences using an in-house sequencer, (D) compensated using Pinnacle sequencer, or (E) compensated using IMRT optimization. A computer program was developed to construct both step-and-shoot and dynamic MLC sequence files from mechanical thickness maps of our current compensators. In addition, the Pinnacle sequencer and IMRT optimization were used to generate step-and-shoot MLC sequences. Planar doses were measured for each at the isocenter depth with an ion chamber array to compare the five methods. A comparison of the relative dose distribution shows that the compensation achieved by method (E) is in close agreement with that achieved using method (A), that is, dose uniformity within 4%. Method (D) resulted in the shortest delivery time and achieved dose uniformity to within 5%. Methods (B) and (C) need additional refinement to be of practical use. The results support the feasibility of replacing physical compensators with MLC delivery sequences. Compensation by MLC segments provides more flexibility and efficiency in design and delivery than by physical compensators while maintaining or improving the uniformity of dose to the plane of compensation.

Effect of beam number on organ‐at‐risk sparing in dynamic multileaf collimator delivery of intensity modulated radiation therapy

Due to practical limitations such as inter- and intraleaf transmission, nondivergent leaf end design, and leaf scatter, multileaf collimators (MLCs) are unable to accurately produce the ideal fluence patterns generated by inverse planning systems. Consequently, low dose regions receive substantially more radiation than they would with an ideal MLC that could generate the desired fluence pattern. Previous work by others has found that the discrepancy between desired and actual fluence patterns produced by an MLC increases rapidly with increasing complexity of the desired fluence map. In addition to the complexity of individual fluence maps, other parameters can contribute to the overall complexity of a treatment plan, most notably the number of beams. In this work, we investigate the effect of beam number on critical structure sparing for dynamic MLC delivered intensity modulated radiation therapy. Six cases from each of two challenging clinical sites, previously irradiated head and neck and paraspinal metastasis, were planned with the goal of minimizing the spinal cord dose. Plans were developed for five to 27 beams. All plans were renormalized such that the target volume receiving the prescription dose was the same for all plans of each site. For each case, we calculated the spinal cord D0.5 cm3 (the dose such that 0.5 cm3 of normal tissue receives greater than or equal to D0.5 cm3), normal tissue D1 cm3, the normal tissue mean dose, and the standard deviation of dose in the planning target volume (PTV). For the head and neck cases, the mean increase in spinal cord D0.5 cm3 between seven and 27 beam plans was 10% of the prescription dose, whereas for the paraspinal case, the increase was 2.6%. For the head and neck cases, the mean decrease in normal tissue D1 cm3 between seven and 11 beam plans was 2.6% and was constant for more than 11 beams. For the paraspinal cases, the mean decrease in normal tissue D1 cm3 between seven and 27 beam plans was 3.7%. The mean normal tissue dose was approximately independent of the number of beams for both sites. For the head and neck cases, the PTV standard deviation was independent of the number of beams, while for the paraspinal cases it decreased by an average of 1.8% from seven to 27 beams. Calculations for seven and 27 beams in which the MLC transmission was varied from 0% to 2% demonstrated that the increase in spinal cord D0.5 cm3 with increasing number of beams is largely due to MLC transmission, which is not included in the optimization. Increasing the number of beams increased the critical structure dose, although decreasing beam number results in increasing normal tissue D1 cm3 and target dose heterogeneity. The optimal tradeoff is dependent on the clinical situation, but seems to be seven to nine beams. Beam geometry optimization may reduce the number of beams required to provide adequate target coverage, thus limiting critical structure dose.

SU‐FF‐T‐426: Treatment Planning Comparison of Dynamic Multileaf Collimation and Helical Tomotherapy for Spinal Cord Sparing in Head and Neck Re‐Irradiation

Purpose: To compare dynamic multileaf collimator (DMLC) and helical tomotherapy (TOMO) IMRT delivery techniques for head and neck re‐irradiation. Method and Materials: Five cases were selected. For each case, a planning target volume (PTV) and the spinal cord were contoured. A 5 mm margin was added to the spinal cord to create a planning organ‐at‐risk volume (PRV). Plans were developed to deliver 60 Gy to 95% of the PTV while minimizing PRV dose. For DMLC delivery, 7 fields in the transverse plane were used. Plans were developed for two secondary collimator configurations, one in which the collimators were set by the optimization engine (DMLC STD) and one in which the PRV was always blocked by a collimator jaw (DMLC BLOCK). For both DMLC STD and TOMO, the optimization parameters were adjusted to minimize the PRV dose without exceeding 69 Gy to a volume of tissue >1 cc. DMLC BLOCK plans were optimized using the same parameters as the DMLC STD plans. The PTV D1cc, spinal cord D0.5cc, and non‐specified normal tissue D1cc were calculated. Results: The TOMO plans were able to both minimize the spinal cord dose and limit the hotspot. The DMLC plans were not able to limit the hotspot to the extent of the TOMO plans. The DMLC BLOCK plans achieved equivalent cord sparing, but with increased dose heterogeneity. The mean spinal cord D0.5cc, was 8.8, 16, and 8.4 Gy for the TOMO, DMLC STD, and DMLC BLOCK plans respectively. The mean PTV D1cc was 67.7, 68.6, and 71.4 Gy and the normal tissue D1cc was 65.2, 68.2, and 71.5 Gy. Conclusion: Dynamic MLC IMRT is capable of producing spinal cord sparing equivalent to helical tomotherapy. However, with current technology, the sparing is at the expense of PTV heterogeneity and larger doses to small volumes of normal tissue.

An independent dose calculation algorithm for MLC‐based stereotactic radiotherapy

We have developed an algorithm to calculate dose in a homogeneous phantom for radiotherapy fields defined by multi-leaf collimator (MLC) for both static and dynamic MLC delivery. The algorithm was developed to supplement the dose algorithms of the commercial treatment planning systems (TPS). The motivation for this work is to provide an independent dose calculation primarily for quality assurance (QA) and secondarily for the development of static MLC field based inverse planning. The dose calculation utilizes a pencil-beam kernel. However, an explicit analytical integration results in a closed form for rectangular-shaped beamlets, defined by single leaf pairs. This approach reduces spatial integration to summation, and leads to a simple method of determination of model parameters. The total dose for any static or dynamic MLC field is obtained by summing over all individual rectangles from each segment which offers faster speed to calculate two-dimensional dose distributions at any depth in the phantom. Standard beam data used in the commissioning of the TPS was used as input data for the algorithm. The calculated results were compared with the TPS and measurements for static and dynamic MLC. The agreement was very good (<2.5%) for all tested cases except for very small static MLC sizes of 0.6 cm x 0.6 cm (<6%) and some ion chamber measurements in a high gradient region (<4.4%). This finding enables us to use the algorithm for routine QA as well as for research developments.

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Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy. In the dynamic multileaf collimator (MLC) method of IMRT delivery, because of the relatively small gaps between opposed leaves and because most regions are shielded by leaves most of the time, the delivered dose is very sensitive to MLC leaf positional accuracy. A variation of +/-0.2 mm in the gap width can result in a dose variation of +/-3% for each clinical dynamic MLC field. Most often the effects of leaf motion are inferred from dose deviations on film or from variations in ionization measurements. These techniques provide dosimetric information but do not provide detailed information for diagnosing delivery problems. Therefore, a dynamic log file (Dynalog file) was used to verify dynamic MLC leaf positional accuracy. Measuring for narrow gaps using the thickness gauge could detect a log file accuracy of approximately 0.1 mm. The accuracy of dynamic MLC delivery depends on the accuracy with which the velocity of each leaf is controlled. We studied the relationship between leaf positional accuracy and leaf velocity. Leaf velocity of 0.7 cm/sec caused approximately 0.2 mm leaf positional variation. We then analyzed leaf positional accuracy for the clinical dynamic MLC field using Dynalog File Viewer (Varian Medical Systems, Inc., Palo Alto, CA), and developed a new program that can analyze more detailed leaf motions. Using this program, we can obtain more detailed information, and therefore can determine the source of dose uncertainties for the dynamic MLC field.

Po-Poster - 04: A daily quality assurance measurement for dynamic MLC treatments

A daily test was developed to assure intensity modulated fields are delivered with a high degree of accuracy and precision using the dynamic multileaf collimator delivery mode. Quality assurance of the MLC, particularly for dynamic mode of delivery, requires positional accuracy of individual leafs, as well as gap width defined by opposing leaf pairs. The daily test was designed to be quick, simple to use, and easy to analyze. With the secondary collimating jaws at 25 cm wide and 39 cm long, a 5mm sliding gap over 5 cm at isocenter placed between two static fields 4 cm off axis was measured dosimetrically using Kodak XV film. The sliding gap allows us to test dosimetrically the positional accuracy defined by Varian in motor encoder counts, as a function of beam on time. The speed of the leaves is controlled by the MUs delivered. Relative film dosimetry with respect to a baseline film obtained after MLC alinment is performed daily and analyzed using RIT software. In order to facilitate the registration of the QA film to the baseline film, a mounting device utilizing the upper wedge block was used. The QA test was tested using beam shaper files with controlled errors introduced and dose differences of 5% are easily detected. In addition to the daily film QA, as part of morning photon output checks a sliding gap output is measured and these results will be presented as well.

SU‐FF‐T‐193: Dosimetric Responses at Different Gantry and Collimator Angles in Dynamic MLC Beam Delivery

Purpose: IMRT uses Dynamic Multi‐Leaf Collimator (DMLC). This study is to investigate the DMLC performance at different gantry and collimator angles based on dosimetric measurements. Conformity index and symmetry index were introduced to quantify the performance. Method and Materials: We developed a technique to measure the dosimetric impact of DMLC delivery at different gantry and collimator angles. Various DMLC patterns were designed to observe the discrepancies of the DMLC delivery at different gantry collimator angles. The discrepancies were quantified by acquiring dosimetric information under the corresponding radiation delivery conditions. The designed dynamic fields were delivered using 6 MV photon beams to a Solid Water TM phantom with an ion chamber at the isocenter. Phantom was carefully set up so that both the phantom and the chamber remained the same, geometrically, with respect to the beam's coordinates when the gantry angle was changed. The measurements were carried out for two Varian's 23Ex Liancs. Conformity index was defined to measure the output ratios at different gantry angles for a same DMLC pattern. Symmetry index was defined to assess the dosimetric discrepancies of the same pattern with opposite collimator setting at a same gantry angle. Results: The same measurements were performed for each machine for five consecutive days. It was observed that conformity index varied within between 98% and 100%. And the results varied on daily basis, which may imply the slight instability of DMLC performance. A consistent value less than 100% for the conformity index may indicate a gravity effect on the DMLC performance. In the meantime at a fixed gantry angle and at different collimator angle with a same DMCL pattern, the symmetry index varied randomly from 97.5% to 102.2%. Conclusion: The gravity may affect the DMLC dosimetric performance. This impact on IMRT dynamic delivery warrants further investigation.

Clinical implementation of dynamic and step-and-shoot IMRT to treat prostate cancer with high risk of pelvic lymph node involvement

Background and purpose: Two systems have been developed for treating patients with locally advanced prostate cancer using intensity-modulated radiotherapy (IMRT): one using dynamic multi-leaf collimator delivery and the other using step-and-shoot. This paper describes the clinical implementation of these two techniques, and presents results from the first 14 patients treated in a clinical setting (nine dynamic, five step-and-shoot). Patients and methods: Dynamic treatments were planned using Corvus, and step-and-shoot using Helax-TMS; all were delivered using Elekta accelerators. Prior to the first clinical treatments, validation measurements were carried out for each system, including measurements for a complete IMRT treatment. The reproducibility of dynamic delivery and the characteristics of the accelerator for low-monitor-unit (MU) deliveries were also assessed. An extensive quality assurance (QA) program was performed for each of the patients. Additionally, timing measurements were carried out to assess the practicalities of the technique. Results: The planning objectives were met in most cases. Absolute doses for complete IMRT treatments were within 2%, on average, with dose distributions generally showing agreement within 3% or 3 mm. Beam modulation measurements made throughout each patient's treatment indicated that both delivery methods were reproducible. The dynamic plans required an average of 765 MU per beam, with a treatment delivery time of 14 min; corresponding results for step-and-shoot plans were 105 MU and 10 min. Conclusions: Two IMRT techniques for this group of patients have been successfully implemented in the clinic. The more complex dynamic treatments showed no advantages over the step-and-shoot approach. QA results have shown accurate and reproducible delivery for both techniques, giving increased confidence in the techniques and allowing a reduction in the QA program.

Method of identifying dynamic multileaf collimator irradiation that is highly sensitive to a systematic MLC calibration error.

In intensity modulated radiotherapy (IMRT), radiation is delivered in a multiple of multileaf collimator (MLC) subfields. A subfield with a small leaf-to-leaf opening is highly sensitive to a leaf-positional error. We introduce a method of identifying and rejecting IMRT plans that are highly sensitive to a systematic MLC gap error (sensitivity to possible random leaf-positional errors is not addressed here). There are two sources of a systematic MLC gap error: centerline mechanical offset (CMO) and, in the case of a rounded end MLC, radiation field offset (RFO). In IMRT planning system, using an incorrect value of RFO introduces a systematic error ARFO that results in all leaf-to-leaf gaps that are either too large or too small by (2*DeltaRFO), whereas assuming that CMO is zero introduces systematic error DeltaCMO that results in all gaps that are too large by DeltaCMO=CMO. We introduce a concept of the average leaf pair Opening (ALPO) that can be calculated from a dynamic MLC delivery file. We derive an analytic formula for a fractional average fluence error resulting from a systematic gap error of Deltax and show that it is inversely proportional to ALPO; explicitly it is equal to Deltax/(ALPO+ 2 * RFO+ epsilon), in which epsilon is generally of the order of 1 mm and Deltax =2 * Delta RFO + CMO. This analytic relationship is verified with independent numerical calculations.

Computer verification of fluence map for intensity modulated radiation therapy.

In a treatment planning system for intensity modulated radiation therapy (IMRT), the time sequence of multileaf collimator (MLC) settings are derived from an optimal fluence map as a postoptimization process using a software module called a "leaf sequencer." The dosimetric accuracy of the dynamic delivery depends on the functionality of the module and it is important to verify independently the correctness of the leaf sequences for each field of a patient treatment. This verification is unique to the IMRT treatment and has been done using radiographic film, electronic portal imaging device (EPID) or electronic imaging system (BIS). The measurement tests both the leaf sequencer and the dynamic multileaf collimator (MLC) delivery system, providing a reliable assurance of clinical IMRT treatment. However, this process is labor intensive and time consuming. In this paper, we propose to separate quality assurance (QA) of the leaf sequencer from the dynamic MLC delivery system. We describe a simple computer algorithm for the verification of the leaf sequences. The software reads in the leaf sequences and simulates the motion of the MLC leaves. The generated fluence map is then compared quantitatively with the reference map from the treatment planning system. A set of pre-defined QA indices is introduced to measure the "closeness" between the computed and the reference maps. The approach has been used to validate the CORVUS (NOMOS Co., Sewickley, PA) treatment plans. The results indicate that the proposed approach is robust and suitable to support the complex IMRT QA process.