Multileaf Collimator Motion Research Articles

The dosimetric impact of inversely optimized arc radiotherapy plan modulation for real-time dynamic MLC tracking delivery

Real-time dynamic multileaf collimator (MLC) tracking for management of intrafraction tumor motion can be challenging for highly modulated beams, as the leaves need to travel far to adjust for target motion perpendicular to the leaf travel direction. The plan modulation can be reduced by using a leaf position constraint (LPC) that reduces the difference in the position of adjacent MLC leaves in the plan. The purpose of this study was to investigate the impact of the LPC on the quality of inversely optimized arc radiotherapy plans and the effect of the MLC motion pattern on the dosimetric accuracy of MLC tracking delivery. Specifically, the possibility of predicting the accuracy of MLC tracking delivery based on the plan modulation was investigated. Inversely optimized arc radiotherapy plans were created on CT-data of three lung cancer patients. For each case, five plans with a single 358° arc were generated with LPC priorities of 0 (no LPC), 0.25, 0.5, 0.75, and 1 (highest possible LPC), respectively. All the plans had a prescribed dose of 2 Gy × 30, used 6 MV, a maximum dose rate of 600 MU/min and a collimator angle of 45° or 315°. To quantify the plan modulation, an average adjacent leaf distance (ALD) was calculated by averaging the mean adjacent leaf distance for each control point. The linear relationship between the plan quality [i.e., the calculated dose distributions and the number of monitor units (MU)] and the LPC was investigated, and the linear regression coefficient as well as a two tailed confidence level of 95% was used in the evaluation. The effect of the plan modulation on the performance of MLC tracking was tested by delivering the plans to a cylindrical diode array phantom moving with sinusoidal motion in the superior-inferior direction with a peak-to-peak displacement of 2 cm and a cycle time of 6 s. The delivery was adjusted to the target motion using MLC tracking, guided in real-time by an infrared optical system. The dosimetric results were evaluated using gamma index evaluation with static target measurements as reference. The plan quality parameters did not depend significantly on the LPC (p ≥ 0.066), whereas the ALD depended significantly on the LPC (p < 0.001). The gamma index failure rate depended significantly on the ALD, weighted to the percentage of the beam delivered in each control point of the plan (ALD(w)) when MLC tracking was used (p < 0.001), but not for delivery without MLC tracking (p ≥ 0.342). The gamma index failure rate with the criteria of 2% and 2 mm was decreased from > 33.9% without MLC tracking to <31.4% (LPC 0) and <2.2% (LPC 1) with MLC tracking. The results indicate that the dosimetric robustness of MLC tracking delivery of an inversely optimized arc radiotherapy plan can be improved by incorporating leaf position constraints in the objective function without otherwise affecting the plan quality. The dosimetric robustness may be estimated prior to delivery by evaluating the ALD(w) of the plan.

Investigation of the feasibility of a simple method for verifying the motion of a binary multileaf collimator synchronized with the rotation of the gantry for helical tomotherapy

In this paper, we suggest a new method for verifying the motion of a binary multileaf collimator (MLC) in helical tomotherapy. For this we used a combination of a cylindrical scintillator and a general‐purpose camcorder. The camcorder records the light from the scintillator following photon irradiation, which we use to track the motion of the binary MLC. The purpose of this study is to demonstrate the feasibility of this method as a binary MLC quality assurance (QA) tool. First, the verification was performed using a simple binary MLC pattern with a constant leaf open time; secondly, verification using the binary MLC pattern used in a clinical setting was also performed. Sinograms of simple binary MLC patterns, in which leaves that were open were detected as “open” from the measured light, define the sensitivity which, in this case, was 1.000. On the other hand, the specificity, which gives the fraction of closed leaves detected as “closed”, was 0.919. The leaf open error identified by our method was −1.3±7.5%. The 68.6% of observed leaves were performed within ± 3% relative error. The leaf open error was expressed by the relative errors calculated on the sinogram. In the clinical binary MLC pattern, the sensitivity and specificity were 0.994 and 0.997, respectively. The measurement could be performed with −3.4±8.0% leaf open error. The 77.5% of observed leaves were performed within ± 3% relative error. With this method, we can easily verify the motion of the binary MLC, and the measurement unit developed was found to be an effective QA tool.PACS numbers: 87.56.Fc, 87.56.nk

A phantom-based evaluation of a real-time tracking micro MLC delivery

A cutting edge real-time tracking delivery system was evaluated for respiratory-correlated radiation therapy. The performed evaluations included Multi-Leaf Collimator (MLC) leaf position accuracy, MLC motion time delay, dosimetric performance, and tracking failure. The experiment-based results indicated that the dynamic MLC was capable of achieving the position accuracy in 1~2 mm and the MLC time delay in the range of 100 μs. The dosimetric comparison evidenced the significant advantage when utilising real-time tracking. The analysis of experiments also indicated the potential fail of tracking due to the impact of patient breathing patterns and the visibility of tumour.

Experimental investigation of a moving averaging algorithm for motion perpendicular to the leaf travel direction in dynamic MLC target tracking

In dynamic multileaf collimator (MLC) motion tracking with complex intensity-modulated radiation therapy (IMRT) fields, target motion perpendicular to the MLC leaf travel direction can cause beam holds, which increase beam delivery time by up to a factor of 4. As a means to balance delivery efficiency and accuracy, a moving average algorithm was incorporated into a dynamic MLC motion tracking system (i.e., moving average tracking) to account for target motion perpendicular to the MLC leaf travel direction. The experimental investigation of the moving average algorithm compared with real-time tracking and no compensation beam delivery is described. The properties of the moving average algorithm were measured and compared with those of real-time tracking (dynamic MLC motion tracking accounting for both target motion parallel and perpendicular to the leaf travel direction) and no compensation beam delivery. The algorithm was investigated using a synthetic motion trace with a baseline drift and four patient-measured 3D tumor motion traces representing regular and irregular motions with varying baseline drifts. Each motion trace was reproduced by a moving platform. The delivery efficiency, geometric accuracy, and dosimetric accuracy were evaluated for conformal, step-and-shoot IMRT, and dynamic sliding window IMRT treatment plans using the synthetic and patient motion traces. The dosimetric accuracy was quantified via a tgamma-test with a 3%/3 mm criterion. The delivery efficiency ranged from 89 to 100% for moving average tracking, 26%-100% for real-time tracking, and 100% (by definition) for no compensation. The root-mean-square geometric error ranged from 3.2 to 4.0 mm for moving average tracking, 0.7-1.1 mm for real-time tracking, and 3.7-7.2 mm for no compensation. The percentage of dosimetric points failing the gamma-test ranged from 4 to 30% for moving average tracking, 0%-23% for real-time tracking, and 10%-47% for no compensation. The delivery efficiency of moving average tracking was up to four times higher than that of real-time tracking and approached the efficiency of no compensation for all cases. The geometric accuracy and dosimetric accuracy of the moving average algorithm was between real-time tracking and no compensation, approximately half the percentage of dosimetric points failing the gamma-test compared with no compensation.

SU‐E‐T‐406: Use of Portal Dosimetry with EPID for Quality Control of RapidArc

Purpose: Develop a methodology for RapidArc QA using Eletronic Portal Dosimetry Device (EPID) Methods: The RapidArc delivers intensity‐modulated radiation with different monitor units (MUs) at varying dose rates while multileaf collimator (MLC) leaves are continuously moving as the gantry is simultaneously rotating with varying speed. The quality assurance before treatment should be performed to confirm that the accelerator will reproduce what was planned.We developed a technique for using the EPID to quality assurance of RapidArc. This technique consists of converting the original motion of the MLC in a fixed gantry, keeping the MLC movements, monitor units, and fractional doses of each control point as the original plan, which can then be calculated and measured with EPID. Absolute dose measurements with ionization chamber in a water equivalent phantom were performed for validation of this technique. For planar dose distribution two comparisons were performed using as reference the gamma function (3 % / 2 mm): EPID (static field × original arc) and static field (EPID × Matrix − IBA). We evaluated the dependence of the EPID response with dose rate and with the moving gantry. Our score to approve are gamma maximum less than 3 and gamma area with value greater than 1 is less then 5%. For absolute dosimetry the score is below 3%. Results: After analyzing 120 cases, the maximum gamma was 2,8 and the mean value 1,6 and for gamma area, the maximum gamma was 4,2%. For absolute dosimetry all plans were below 3%. Was also observed that these values of gamma maximum accured in tongue‐and‐groove region, showing that the planning system is not properly modeled this effect. Conclusions: After analyzing 120 cases, the results show this methodology is appropriate for quality control of RapidArc plans.

SU-E-T-477: Multileaf Collimator Quality Assurance for Volumetric Modulated Arc Therapy

Purpose: To develop a quality assurance procedure for the multileaf collimator(MLC) in volumetric modulated arc therapy (VMAT) beam delivery. Method and Materials: A general pattern for MLC QA is proposed in this study to test the interaction between gantry speed and the MLC traveling speed on different linacs for VMAT dose delivery. An independently mounted film was used to catch the entire arc movement. To utilize similar concept for picket‐fence test, we divided the gantry 360degrees into two counterclockwise fields, 180 to 0 and 0 to 180degrees, and dynamic abutted segments were delivered on every 15 degree gantry rotation. The MLC traveled 3 cm distance in each segment dynamically, from starting opening at 0 and ending at 3cm. Starting point for each 15 degree segment was from 0th, 3rd, 6th, 9th…to 36th cm with MLC bank B moving. But each segment was delivered twice, first in counterclockwise and second time in clockwise directions with MLC travelling in opposite directions with starting point 3rd, 6th, 9th, 12th …to 39th cm with MLC bank A moving. These two fields had the same MLC segments. The accumulated dose on film was analyzed. Results: With perfect speed combination between gantry and MLC, the total dose profile along the leaf‐ motion direction is uniform. All leaves in each bank move together that will be reflected in the width of the strip and conjunction lines, while one leaf is out of range by 1 mm, the profile will show non‐uniform profile only at that leaf position. The conjunction lines of each segment also represent the reproducibility of the combination of gantry and MLC positions. Conclusion: We have investigated a quality assurance procedure for the MLC used in VMAT treatment. It can detect the mismatch between gantry speed and MLC motion to quantify the accuracy of VMAT dose delivery.

MLC mediated beam hardening effects in IMRT

Abstract Purpose Beam hardening effects due to photons passing through the multi-leaf collimator (MLC) frequently exist in Intensity Modulated Radiotherapy (IMRT) fields. A fast online dose transmission verification system, MBMC (Measurement Based Monte Carlo), can verify IMRT delivery by using a fluence efficiency map to replace MLC geometry and movement simulation. This system, however, ignores beam hardening effects, and assumes that dose disturbances through the MLC are not significant for IMRT fields. This assumption has to be justified before it can be applied in the clinic. Methods In this study, we simulated several field sizes (0.5 × 0.5, 1 × 1, 3 × 3, 5 × 5, and 10 × 10 cm2) to evaluate the dose influence of beam hardening effects under clinical conditions. In addition, a LATCH technique was used during simulation processes, which can record each particle interaction with specific gantry components, to distinguish between dose contribution from the total beam and MLC mediated beam. Results The MLC indeed caused significant beam hardening effects, but the dose contribution fraction from the MLC was noticeable only for field sizes less than 1 × 1 cm2. Furthermore, in mixed fields containing both the total beam and MLC mediated beam, the maximum dose deviation due to the presence of the MLC is small even for the 0.5 × 0.5 cm2 field size (∼2%). Conclusions The MLC causes noticeable beam hardening effects, but this effect results in only slight dose differences that are only noticeable for small field sizes in IMRT delivery. The use of a fluence efficiency map was feasible in our MBMC system.

Comparative analysis of volumetric modulated arc therapy versus intensity modulated radiation therapy for radiotherapy of anal carcinoma

PurposeVolumetric modulated arc therapy (VMAT), an evolution of intensity modulated radiation therapy (IMRT), utilizes the dynamic modulation of angular dose rate through changes in gantry speed, linear accelerator dose rate, and multileaf collimator motion to deliver a treatment fraction in rotational fashion with improved efficiency and in shorter time. In general, target coverage relative to adjacent organ-at-risk sparing is highly dependent on the complexity of the treatment site. Therefore, we critically evaluated VMAT compared to IMRT for definitive treatment of anal carcinoma with respect to dosimetry and efficiency. Methods and MaterialsUsing SmartArc (Philips Healthcare, Andover, MA), VMAT treatment plans were generated for 10 patients treated at our institution for anal carcinoma, and compared to the IMRT plans used for clinical treatment. The patients were all female, had T classification TX/1 (n = 5), T2 (n = 3) or T3 (n = 2), were node-negative (n = 6) or node–positive (n = 4), and were treated to a total dose of 50 to 58 Gy. Pairwise comparisons were made between VMAT and IMRT plans with respect to dose-volume histogram parameters relating the dose received by target volumes, relevant organs at risk, and normal tissues. The plans were machine-delivered, with actual beam delivery times measured. ResultsVMAT plans had superior planning target volume coverage and dose homogeneity, with improved conformality in treatment of the elective nodal volume, in comparison to IMRT. Mean dose to the small bowel, genitalia, and femoral heads were significantly lower with VMAT, and similar with respect to bladder, pelvic bones, and normal tissues. Integral dose was comparable between the 2 techniques. VMAT plans required 36.8% fewer monitor units, and beam delivery time was shorter by 9 minutes. ConclusionsOur results indicate that VMAT represents an ideal treatment modality for anal carcinoma, generating plans with excellent target coverage, lower doses to organs at risk, and shorter treatment times, in comparison to IMRT.

Comparative Analysis of Volumetric Modulated Arc Therapy (VMAT) vs. Intensity Modulated Radiation Therapy (IMRT) for Radiotherapy of Anal Carcinoma

Comparative Analysis of Volumetric Modulated Arc Therapy (VMAT) vs. Intensity Modulated Radiation Therapy (IMRT) for Radiotherapy of Anal Carcinoma

Analyses of the Mechanical Accuracy of RapidArc Treatment using Log File Data

RapidArc technology is a novel delivery method that enables high-dose conformality by optimizing the dose rate, gantry speed, and multi-leaf collimator (MLC) shapes. In evaluating RapidArc using radiographic film, it has been reported that the MLC movement, variable dose rate, and gantry speed can be controlled precisely during RapidArc. However, the gravity effect due to the gantry angle could not be taken into account because the radiographic film rotated with the gantry. Therefore, this study evaluated the accuracy of the MLC leaf position, gantry angle, and monitor units (MUs) using log files.

Radio frequency noise from an MLC: a feasibility study of the use of an MLC for linac-MR systems

Currently several groups are actively researching the integration of a megavoltage teletherapy unit with magnetic resonance (MR) imaging for real-time image-guided radiotherapy. The use of a multileaf collimator (MLC) for intensity-modulated radiotherapy for linac-MR units must be investigated. The MLC itself will likely reside in the fringe field of the MR and the motors will produce radio frequency (RF) noise. The RF noise power spectral density from a Varian 52-leaf MLC motor, a Varian Millennium MLC motor and a brushless fan motor has been measured as a function of the applied magnetic field using a near field probe set. For the Varian 52-leaf MLC system, the RF noise produced by 13 of 52 motors is studied as a function of distance from the MLC. Data are reported in the frequency range suitable for 0.2–1.5 T linac-MR systems. Below 40 MHz the Millennium MLC motor tested showed more noise than the Varian 52-leaf motor or the brushless fan motor. The brushless motor showed a small dependence on the applied magnetic field. Images of a phantom were taken by the prototype linac-MR system with the MLC placed in close proximity to the magnet. Several orientations of the MLC in both shielded and non-shielded configurations were studied. For the case of a non-shielded MLC and associated cables, the signal-to-noise ratio (SNR) was reduced when 13 of 52 MLC leaves were moved during imaging. When the MLC and associated cables were shielded, the measured SNR of the images with 13 MLC leaves moving was experimentally the same as the SNR of the stationary MLC image. When the MLC and cables are shielded, subtraction images acquired with and without MLC motion contains no systematic signal. This study illustrates that the small RF noise produced by functioning MLC motors can be effectively shielded to avoid SNR degradation. A functioning MLC can be incorporated into a linac-MR unit.

An integral quality monitoring system for real‐time verification of intensity modulated radiation therapy

To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy. The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22 x 22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX. For a 10 x 10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3 x 3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3 X 3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%. The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.

Development and Evaluation of a Target-tracking Radiation-therapySystem Using a Multileaf Collimator (MLC) Synchronized with MovingOrgans

A target-tracking radiation-therapy (RT) system that tracks the movement of a treatment target resulting from internal organ movement has been developed, and the system was evaluated for effectiveness. The developed radiation-therapy system determines the limit of the MLC (multileaf collimator) movement range with an acquired maximum displacement value of target movement during the radiotherapy planning stage and moves the MLC to continuously detect and synchronize the displacement of the abdominal by using a CCD camera monitoring system during real-time RT treatment. The system consists of a Co-60 teletherapy unit, an abdominal displacement detection system to correlate the movement of internal organs, and a MLC moving stage synchronized with the abdominal displacement. The effectiveness of the RT system was evaluated by analyzing the beam penumbra from the dose distribution on films (Gafchromic EBT) installed within a moving phantom. Penumbra widths (80%/20%) were measured for the nominal left (perpendicular to leaf motion) and right (parallel to leaf motion) directions of the x-axis and the y-axis for a fixed phantom and fixed MLC (3.3 mm/3.5 mm and 3.7 mm/3.1 mm), a moving phantom and a fixed MLC (9.0 mm/8.2 mm and 9.1 mm/8.8 mm), and a moving phantom and a moving MLC (3.7 mm/3.5 mm and 3.7 mm/3.8 mm). Percent area deviations of the 100% to 80% field of the maximum dose for the exposure condition of a fixed phantom and a fixed MLC were a relatively low value of 0.64% and a relatively high value of 15.04%, as compared to the values for a moving phantom and a fixed MLC and for a moving phantom and a moving MLC. With the method to move the MLC by passive synchronization with organ motion, the target-tracking radiation-therapy system was successfully implemented and was evaluated for performance. In future studies, various verification processes should be performed prior to the application of the system in a clinical situation.

SU‐FF‐T‐101: Volumetric Modulated Arc Therapy for the Treatment of Paraspinal Lesions

Purpose: Volumetric Modulated Arc Therapy (VMAT) is a recent treatment modality that synchronizes gantry rotation, multileaf collimator motion and dose rate modulation, usually in a single arc. In this feasibility study, we evaluate the efficacy of VMAT, using an optimization algorithm developed at our institution, for paraspinal stereotactic body radiation therapy (SBRT). Method and Materials: Ten patients with paraspinal lesions were retrospectively planned with VMAT and results were compared to 5 – 8 field treated IMRT plans. Tumor locations varied from C3 to L1. The prescription dose was 24 Gy and cord was constrained to 14 Gy. To compare plan quality, PTV D(99%), D(95%), V(22.8Gy), Dmax, cord Dmax and Dmean, esophagus D(2cc), and lung Dmean were reviewed. Monitor units were assessed to evaluate plan efficiency. Results: Patient averaged PTV D(99%), D(95%), Dmax and V(22.8Gy) were 18.7, 21.7, 29.6 Gy and 92.8% respectively for VMAT and 18.5, 21.8, 28.4 Gy and 93.5% respectively for IMRT. Average cord Dmax was 13.8 and 13.9 Gy for VMAT and IMRT, respectively, but the cord Dmean with VMAT was 34% less than with IMRT. The average esophagus D(2cc) was 14.4 Gy for VMAT and 17.5 Gy for IMRT. The average mean lung dose was 2.8 Gy for VMAT and 2.3 Gy for IMRT. The average MUs were 4817 and 12343 for VMAT and IMRT plans, respectively. Conclusion: PTV coverage was similar for VMAT and IMRT with the exception of slightly higher hot spots with VMAT. For critical structures near the PTV such as cord and esophagus, maximum doses were similar but mean doses were lower with VMAT. Structures farther way from the PTV (e.g. lung) had higher mean doses with VMAT. VMAT plans were more efficient with substantially lower MUs than IMRT. Conflict of Interest (only if applicable): Research agreement with Varian Corporation

SU-FF-T-247: Influence of Gantry Angle, Multi-Leaf Collimator (MLC), and Jaw Position Errors to Dose Distribution in Volumetric Modulated Arc Therapy (VMAT)

Purpose: To establish a machine quality assurance procedure during VMAT delivery.Method and Materials: A single‐arc VMAT plan with 73 control points (CP) and 5‐degree gantry angle spacing for a prostate cancer patient has been created by ERGO++ TPS (Elekta, 3DLine). Actual multi‐leaf collimator(MLC) and jaw positions, gantry angle and dose rates during prostate VMAT delivery have been recorded in every 0.25 seconds. The dose re‐calculation using these recorded data has been performed and compared with the original plan using the gamma index. Calculated dose distributions in the TPS were also compared with irradiated GafChromic films placed inside a plastic phantom on axial, sagittal, and coronal isocenter planes using the same index. Result: Typical peak errors of MLC position, jaw position, and gantry angle were 3 degrees, 0.6 mm, and 1 mm, respectively. The cumulative MU error was practically negligible because Elekta VMAT delivery was based on MU based servo control. Recalculated dose and original plan agreed well under 2 %–2 mm gamma index criteria. Even in a plan that requires rapid motion of MLCs and jaws, their dynamical parameters were sufficiently well controlled, which was confirmed by comparing film dosimetry and dose calculation with sufficiently small gantry‐angle resampling. Conclusions: Machine quality assurance for VMAT delivery has been performed with a satisfied result.

Commissioning of Volumetric Modulated Arc Therapy (VMAT)

Volumetric modulated arc therapy (VMAT) involves the simultaneous use of dynamic multileaf collimator (DMLC) techniques and gantry arcing; appropriate quality assurance is therefore required. This article describes the development and implementation of procedures for commissioning VMAT on a commercial linear accelerator (Elekta PreciseBeam VMAT with MLCi and Beam Modulator heads). Tests for beam flatness and symmetry at the variable dose rates required for VMAT were performed. Multileaf collimator (MLC) calibration was investigated using dynamic prescriptions. The cumulative dose delivered by a sliding window aperture was measured and compared with calculated values. Rotational accuracy was evaluated using dynamic prescriptions which required accurate correlated motion of both gantry and MLC leaves. Finally, measured and calculated dose distributions for complete VMAT treatment plans were compared and evaluated. Beam symmetry was found to be better than 3% down to dose rates of 75 MU/min. MLC calibration provided continuity of dose at match planes of better than 4%, which was comparable to interleaf leakage effects. Integrated sliding window doses were within 3% of those calculated. Tests for rotational accuracy showed uniformity of peripheral dose mostly within +/-4% of local control point dose, or approximately +/-0.2% of total central dose. A two-arc prostate case showed an absolute dose difference between calculations and measurements of less than 3%, with gamma (3% and 3 mm) of better than 95%. VMAT has been successfully commissioned and has been introduced into clinical use. The Elekta DMLC has also been shown to be suitable for sliding window delivery.

Real‐time tumor tracking with preprogrammed dynamic multileaf‐collimator motion and adaptive dose‐rate regulation

The authors have developed a new method for real-time tumor tracking with dynamic multileaf-collimator (MLC) motion under condition of free breathing. Unlike other previously proposed tumor-tracking methods, their new method uses a preprogrammed dynamic MLC sequence in combination with real-time dose-rate control. This new scheme circumvents the technical challenge in MLC-based tumor tracking of having to control the MLC motion in real time, based on real-time detected tumor motion. With their new method, the movement of the tumor, as a function of breathing phase, amplitude, or tidal volume, is reflected in the preprogrammed MLC sequence. The irregularity of breathing during treatment is handled by real-time regulation of the machine dose rate, which effectively speeds up or slows down the delivery of radiation as needed. This method is based on the fact that all of the parameters in dynamic radiation delivery, including MLC motion, are enslaved to the cumulative dose, which, in turn, can be accelerated or decelerated by varying the dose rate. Because commercially available MLC systems do not allow the MLC delivery sequence to be modified in real time based on the patient's breathing signal, previously proposed tumor-tracking techniques using a MLC cannot be readily implemented in the clinic today. By using a preprogrammed MLC sequence to handle the required motion, the task for real-time control is greatly simplified. With their new scheme, which they call dose-rate-regulated tracking (DRRT), it is possible to use existing linear accelerators that have dynamic MLC capability to achieve real-time tumor tracking, provided that the beam dose rate can be controlled externally. Tracking-error evaluation for 13 patients out of 14 resulted in a tracking error of less than 1 mm (1 sigma), if the effect of the response time of the treatment machine on the dose-rate modulation can be neglected. Film measurements on a moving phantom with variable breathing patterns and DRRT delivery showed that 97% of the measurement points have gamma values less than 1 (for 3% and 2-mm criteria), while non-DRRT delivery showed only 87%. This study shows that real-time tracking is feasible with DRRT even when the patient breathing frequency is irregular. Effects of the variation of breathing amplitude and of base line drift on the tracking error with DRRT are discussed; pending further study, a criterion is suggested for patient selection in the application of this new technique in the clinic.

Poster - Thurs Eve-01: Comparison of clinical IMRT plan quality and delivery accuracy: Few large segments vs many small segments.

Commercial radiation treatment planning systems for intensity modulation use optimization algorithms that can vary multi-leaf collimator (MLC) segment sizes, segment number and the minimum number of monitor units (MU) per segment. These parameters are varied according to the treatment site, size, location, and proximity to the organs at risk. This study compares the utility of optimization using (Case A) few large segments and a higher minimum MU per segment to that of (case B) using many smaller segments with a lower minimum MU per segment. For Case A, the patient benefits from a reduced treatment time associated with fewer MUs and fewer MLC movements and an increased accuracy in dose delivery. Also, shorter treatment times may lead to fewer patient movement uncertainties. The accumulated MLC leakage dose is reduced, the patient specific quality assurance (QA) is more manageable and small field modeling inaccuracies are reduced. Pinnacle-3 (v8) plans are generated with direct machine parameter optimization (DMPO) for both scenarios. Three dimensional dose distributions and dose volume histograms are used to compare plan quality. We compare plans using few large MLC segments with those using many small MLC segments for some clinical cases. Improved plan quality is demonstrated using fewer MLC segments. Dose QAs are performed and compared for each scenario using MapCheck and film. When comparing dose delivery accuracy between different MU per segment settings, a decrease in delivery errors with minimum MU size is observed. In conclusion, few large MLC segments with larger area should be used when possible.

SU‐GG‐T‐501: Dosimetric Effects Caused by Interplay Between MLC‐Delivery Dynamics and Organ Breathing‐Motion in IMRT for Lung

Purpose: Interplay between IMRT multi‐leaf collimator (MLC) delivery and patient's breathing motion can affect the delivered dose distribution. Interplay is the correlated effect between MLC‐delivery and organ‐motion, due to either MLC motion or initial breathing phase dependence of MLC‐delivery. This study investigated the clinical relevance of interplay for lung IMRT plans assuming free‐breathing delivery. Method and Materials: An in‐house developed 4D Monte Carlo system was used for patient dose calculations. Three patients with lung carcinoma were studied, where motion‐amplitude was 1–2cm. A voxel‐displacement‐map was used to quantify the non‐rigid motion of organs in the CT‐geometry. In order to quantify the MLC position relative to the breathing phase, each photon going through the linac head is tagged to indicate the associated breathing phase during which it will enter the patient. 4DCT data was sorted into 6‐phases, inhale, exhale, 2 phases from inhale to exhale, and 2 phases from exhale to inhale. The exhale phase was used as reference phase. Interplay effects were studied on a beam‐by‐beam basis for a treatment in 30 fractions. Different starting initial MLC phases relative to the breathing motion were selected per daily fraction. Results: The volume of ITV covered by 5–15Gy isodose lines can vary up to 10% relative to the average volume covered per beam over 30‐fractions. Organ motion causes in some instances the dose to be primarily delivered to phases close to inhale and in other cases delivered to phases close to exhale, i.e. the reference phase. For hypo‐fractionation (10–15 fractions), the volume of ITV covered by 5–15Gy isodose lines can vary up to 12–15% relative to mean volume covered over 30‐fractions. Conclusion: Interplay between MLC‐delivery and organ motion can lead to under‐dosed regions in the ITV volume.

Investigation of the dosimetric effect of respiratory motion using four-dimensional weighted radiotherapy

We have developed a four-dimensional weighted radiotherapy (4DW-RT) technique. This method involves designing the motion of the linear accelerator beam to coincide with the tumour motion determined from 4D-CT imaging while including a weighting factor to account for irregular motion and limitations of the delivery system. Experiments were conducted with a moving phantom to assess limitations of the delivery system when applying this method. Although the multi-leaf collimator motion remains within the tolerance of the linear accelerator, the extent of motion was less than 1 mm larger than the designed one, and there was a net system latency of approximately 0.2 s. The dose distributions were measured and simulated using different weighting factors and motion scenarios. The breathing characteristics (period, extent of motion, drift and standard deviations) of 32 patients were evaluated using the Varian RPM system. Breathing variability was assessed by plotting the average breathing motion as a function of the breathing phase. Simulations were carried out to determine the optimal weighting factor based on typical patient breathing characteristics. These results establish that the 4DW-RT method demonstrates potential for dose escalation without increasing exposure to healthy tissue.