Intrafraction Motion Management Research Articles

OC-0062: Imaging dose assessment for intrafraction motion management in ion beam therapy

OC-0062: Imaging dose assessment for intrafraction motion management in ion beam therapy

Intrafraction Motion Management for Prostate SBRT: Clinical Experience and Motion Analysis

collimator (0 ). This misalignment (X,Y shifts) was applied to dose distributions generated at C Z 90 by 10 DMLC fields after they were rotated -90 . Comparisons were made to dose distributions generated at C Z 0 and their differences were evaluated using Gamma Index (2 mm/2%). Results: The collimators of both Linacs have little center misalignment (<1 mm) at four gantry angles. The uncorrected matching rate for two static MLC fields (C Z 0 and C Z 90 ) is over 99.9%. However, the corresponding uncorrected matching rate for 10 DMLC fields at two collimator angles is on an average of 79% (65-92%), with the best matching rate of 92.3% at G Z 0 . Another way to quantify the discrepancies between the two dose distributions is the “additional shift” needed to obtain the “best match” between them. It was found that these “additional shifts” were between 1-5 mm, with an average of 2.9 mm, depending on both machine and gantry angle. Conclusions: EPIDs are a useful QA tool to assess dose distributions produced by DMLC. Significant dose variations were observed from collimator rotation. Those dose variations at C Z 90 were not caused by the conventional mechanical inaccuracies and could be described by an average 2.9mm systemic shift from those produced at CZ 0 . The clinical significance of these variations needs to be further investigated. Author Disclosure: J. Li: None. C. Burman: None. K. Schupak: None. M.F. Chan: None.

Aggressive Intrafraction Motion Management Is Required for SBRT for Localized Prostate Cancer

Aggressive Intrafraction Motion Management Is Required for SBRT for Localized Prostate Cancer

Incorporation of treatment plan spatial and temporal dose patterns into a prostate intrafractional motion management strategy

Periodic MV∕KV radiographs taken during volumetric modulated arc therapy (VMAT) for hypofractionated treatment provide guidance in intrafractional motion management. The choice of imaging frequency and timing are key components in delivering the desired dose while reducing associated overhead such as imaging dose, preparation, and processing time. In this project the authors propose a paradigm with imaging timing and frequency based on the spatial and temporal dose patterns of the treatment plan. A number of control points are used in treatment planning to model VMAT delivery. For each control point, the sensitivity of individual target or organ-at-risk dose to motion can be calculated as the summation of dose degradations given the organ displacements along a number of possible motion directions. Instead of acquiring radiographs at uniform time intervals, MV∕KV image pairs are acquired indexed to motion sensitivity. Five prostate patients treated via hypofractionated VMAT are included in this study. Intrafractional prostate motion traces from the database of an electromagnetic tracking system are used to retrospectively simulate the VMAT delivery and motion management. During VMAT delivery simulation patient position is corrected based on the radiographic findings via couch movement if target deviation violates a patient-specific 3D threshold. The violation rate calculated as the percentage of traces failing the clinical dose objectives after motion correction is used to evaluate the efficacy of this approach. Imaging indexed to a 10 s equitime interval and correcting patient position accordingly reduces the violation rate to 19.5% with intervention from 44.5% without intervention. Imaging indexed to the motion sensitivity further reduces the violation rate to 12.1% with the same number of images. To achieve the same 5% violation rate, the imaging incidence can be reduced by 40% by imaging indexed to motion sensitivity instead of time. The simulation results suggest that image scheduling according to the characteristics of the treatment plan can improve the efficiency of intrafractional motion management. Using such a technique, the accuracy of delivered dose during image-guided hypofractionated VMAT treatment can be improved.

SU-E-J-214: Target Intrafraction Motion Dosimetric Impact on 5-Fraction Proton Prostate SBRT.

To investigate dosimetric impact of prostate intra-fraction motion to five fraction hypofractionated proton treatment with uniform scanning (US) and double scattering (DS) techniques using real-time prostate tracking data from electromagnetic transponder system. Prostate intra-fraction motion can have spatiotemporal interplay with proton treatment delivery. Five fraction (7.25Gy/fraction) prostate proton stereotactic body radiotherapy (SBRT) treatments were simulated for total 14 patients using in-house proton treatment simulation program. The US treatment was simulated by rigidly moving CTV through a series of temporal-spatial dose matrices indexed by energy layers, according to prostate motion traces. The CTV temporal doses of the whole treatment fraction were obtained and summed as final prostate CTV dose. The DS treatment was simulated by moving CTV through the energy layer summed dose matrix. For all patients, the fraction doses and the total dose to the CTV were presented for both DS and US treatments. The CTV dose of different fractions indicated that its dose degradation depends on magnitude and direction of prostate intra-fraction motion and is patient specific. For one of the prostate motion traces investigated, only 70% of CTV received 100% prescribed dose for a simulated US treatment and 79% CTV had 100% dose for a DS treatment. Furthermore, DVH and isodose graphs of both treatments revealed that intra-fraction motion caused significant CTV cold and hot spots in US treatment whereas only cause CTV underdose in DS treatment. Intra-fraction prostate motion causes dose uncertainty to CTV. In the 5 fraction prostate SBRT, prostate intra-fraction motion causes significant target dose degradation. In US treatment, spatiotemporal interplay between energy layers delivery and prostate motion leads to hot and cold spots in CTV for some patients with severe prostate intra-fraction motion. Further investigation of intra-fraction motion management and its impact on CTV dose is necessary.

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.

A Study to Quantify the Effectiveness of Daily Endorectal Balloon for Prostate Intrafraction Motion Management

To quantify intrafraction prostate motion between patient groups treated with and without daily endorectal balloon (ERB) employed during prostate radiotherapy and establish the effectiveness of the ERB. Real-time intrafraction prostate motion from 29 non-ERB (1,061 sessions) and 30 ERB (1,008 sessions) patients was evaluated based on three-dimensional (3D), left, right, cranial, caudal, anterior, and posterior displacements. The average percentage of time with 3D and unidirectional prostate displacements >2, 3, 4, 5, 6, 7, 8, 9, and 10 mm in 1-min intervals was calculated for up to 6 min of treatment time. The Kolmogorov-Smirnov method was used to evaluate the intrafraction prostate motion pattern between both groups. Large 3D motion (up to 1 cm or more) was only observed in the non-ERB group. The motion increased as a function of elapsed time for displacements >2-8 mm for the non-ERB group and >2-4 mm for the ERB group (p < 0.05). The percentage time distributions between the two groups were significantly different for motion >5 mm (p < 0.05). The 3D symmetrical internal margin (IM) can be reduced from 5 to 3 mm (40% reduction), whereas the asymmetrical IM can be reduced from 3 to 2 mm (33% reduction) in cranial, caudal, anterior, and posterior for 6 min of treatment, when ERB is used. Beyond 6 min, the symmetrical 3D and asymmetrical cranial, caudal, anterior, and posterior IMs can be reduced from 9, 4, 7, 7, and 8 to 5, 2, 5, 3, and 4 mm, respectively (up to 57% reduction). The percentage of time that the prostate was displaced in any direction was less in the ERB group for almost all magnitudes of motion considered. The directional analysis shows that the ERB reduced IMs in almost all directions, especially the anterior-posterior direction.

SU‐E‐J‐145: Complete Study to Characterize the Effectiveness of Daily Endorectal Balloon (ERB) for Prostate Intrafraction Motion Management

Purpose: To quantify intrafraction prostate motion between patient groups treated with and without daily endorectal balloon (ERB) employed during prostate radiotherapy and establish the effectiveness of the ERB. Methods: Real time intrafraction prostate motion from 29 non‐ERB (1061 sessions) and 30 ERB (1008 sessions) patients was evaluated based on three‐dimensional (3D), left, right, cranial, caudal, anterior and posterior displacements. The average percentage of time with 3D and unidirectional prostate displacements &gt; 2, 3, 4, 5, 6, 7, 8, 9, and 10mm in 1 minute intervals was calculated for up to 6 minutes of treatment time. The Kolmogorov‐Smirnov method was used to evaluate the intrafraction prostate motion pattern between both groups. Results: 3D motion &gt;=1 cm was observed for the non‐ERB group only. The motion increased as a function of elapsed time for displacements &gt; 2 to 8mm for the non‐ERB group and &gt; 2 to 4 mm for the ERB group (p &lt; 0.05). The percentage time distributions between the two groups were significantly different for motion &gt; 5 mm (p &lt; 0.05). The 3D internal margin (IM) covering 95% treatment time can be reduced from 5 to 3 mm (40% reduction) while the asymmetrical IM can be reduced from 3 to 2 mm (33% reduction) in cranial, caudal, anterior, and posterior for 6 minutes treatment, when ERB is employed. Beyond 6 minutes, the 3D and cranial, caudal, anterior, and posterior IMs can be reduced from 9, 4, 7, 7, and 8 to 5, 2, 5, 3, and 4 mm, respectively (up to 57% reduction). The calculated IMs apply to the worst case scenario patient as well. Conclusions: The ERB effectively reduces prostate motion and allows for smaller IMs. Therefore, daily ERB has the potential to further improve toxicity profiles in prostate radiotherapy.

Target tracking using DMLC for volumetric modulated arc therapy: A simulation study

Target tracking using dynamic multileaf collimator (DMLC) is a promising approach for intrafraction motion management in radiation therapy. The purpose of this work is to develop a DMLC tracking algorithm capable of delivering volumetric-modulated arc therapy (VMAT) to the targets that experience two-dimensional (2D) rigid motion in the beam's eye view. The problem of VMAT delivery to moving targets is formulated as a control problem with constraints. The relationships between gantry speed, gantry acceleration, MLC leaf-velocity, dose rate, and target motion are derived. An iterative search algorithm is developed to find numerical solutions for efficient delivery of a specific VMAT plan to the moving target using 2D DMLC tracking. The delivery of five VMAT lung plans is simulated. The planned and delivered fluence maps in the target-reference frame are calculated and compared. The simulation demonstrates that the 2D tracking algorithm is capable of delivering the VMAT plan to a moving target fast and accurately without violating the machine constraints and the integrity of the treatment plan. The average delivery time is only 29 s longer than that of no-tracking delivery, 101 versus 72 s, respectively. The fluence maps are normalized to 200 MU and the average root-mean-square error between the desired and the delivered fluence is 2.1 MU, compared to 14.8 MU for no-tracking and 3.6 MU for one-dimensional tracking. A locally optimal MLC tracking algorithm for VMAT delivery is proposed, aiming at shortest delivery time while maintaining treatment plan invariant. The inconsequential increase of treatment time due to DMLC tracking is clinically desirable, which makes VMAT with DMLC tracking attractive in treating moving tumors.

First Prospective Study to Determine the Role of Daily Endorectal Balloon on Prostate Intrafraction Motion Management during External Beam Radiotherapy

First Prospective Study to Determine the Role of Daily Endorectal Balloon on Prostate Intrafraction Motion Management during External Beam Radiotherapy

Prostate Intrafraction Motion Assessed by Simultaneous kV Fluoroscopy at MV Delivery II: Adaptive Strategies

To investigate potential benefits of adaptive strategies for managing prostate intrafractional uncertainties when interfraction motion is corrected online. Prostate intrafraction motion was measured using kV fluoroscopy during MV delivery for 571 fractions from 30 hypofractionated radiotherapy patients. We evaluated trending over treatment course using analysis of variance statistics, and we evaluated the ability to correct patient-specific systematic error and apply patient-specific statistical margins after 2 to 15 fractions to compensate 90% of motion. We also evaluated the ability to classify patients into small- and large-motion subgroups based on the first 2 to 20 fractions using discriminant analysis. No time trend was observed over treatment course, and intrafraction motion was patient specific (p < 0.0001). Systematic error in the first week correlated well with that in subsequent weeks, with correlation coefficients of 0.53, 0.50, and 0.41 in right-left (RL), anterior-posterior (AP), and superior-inferior (SI), respectively. After 5 fractions, the adaptive strategy resulted in average margin reductions of 0.3, 0.7, and 0.7 mm in RL, AP, and SI, respectively, with margins ranging from 1 to 3.2 mm in RL, 2 to 7.0 mm in AP, and 2 to 6.6 mm in SI. By contrast, population margins to include the same percentage of motion were 1.7, 4.0, and 4.1 mm. After 2 and 5 fractions, patients were classified into small- and large-motion groups with ~77% and ~83% accuracy. Adaptive strategies are feasible and beneficial for intrafraction motion management in prostate cancer online image guidance. Patients may be classified into large- and small-motion groups in early fractions using discriminant analysis.

A Failure Detection Strategy for Intrafraction Prostate Motion Monitoring With On-Board Imagers for Fixed-Gantry IMRT

To develop methods to monitor prostate intrafraction motion during fixed-gantry intensity-modulated radiotherapy using MV treatment beam imaging together with minimal kV imaging for a failure detection strategy that ensures prompt detection when target displacement exceeds a preset threshold. Real-time two-dimensional (2D) marker position in the MV image plane was obtained by analyzing cine-MV images. The marker's in-line movement, and thus its time-varying three-dimensional (3D) position, was estimated by combining the 2D projection data with a previously established correlative relationship between the directional components of prostate motion. A confirmation request for more accurate localization using MV-kV triangulation was triggered when the estimated prostate displacement based on the cine-MV data was greater than 3 mm. An interventional action alert followed on positive MV-kV confirmation. To demonstrate the feasibility and accuracy of the proposed method, simulation studies of conventional-fraction intensity-modulated radiotherapy sessions were done using 536 Calypso-measured prostate trajectories from 17 radiotherapy patients. A technique for intrafraction prostate motion management has been developed. The technique, using "freely available" cine-MV images and minimum on-board kV imaging (on average 2.5 images/fraction), successfully limited 3D prostate movement to within a range of 3 mm relative to the MV beam for 99.4% of the total treatment time. On average, only approximately one intervention/fraction was needed to achieve this level of accuracy. Instead of seeking to accurately and continuously localize the prostate target as existing motion tracking systems do, the present technique effectively uses cine-MV data to provide a clinically valuable way to minimize kV usage, while maintaining high targeting accuracy.

Monoscopic Imaging for Intrafraction Motion Management

Purpose: Image-guidance allows for accurate positioning of spinal radiosurgery patients; however, due to long treatment times an accurate and efficient intra-fraction motion management is necessary. An analysis tool, Snap Verification (SV, BrainLAB AG, Feldkirchen, Germany) has been implemented to monitor intra-fraction motion based on single oblique digital radiographs. We investigate the feasibility of the method in a clinical setting and its performance in detecting spinal radiosurgery patient movements in 3D space.

SU-GG-T-133: Increasing the Delivery Efficiency of IMRT DAO-Based Plans through Segment Reordering

Purpose: To develop and test an algorithm intended to optimize with regard to time the ordering of segments for DAO‐based step‐and‐shoot IMRT plans. Methods and Materials: After DAO‐based step‐and‐shoot IMRT plans are developed in our commercial treatment planning system (TPS), they are exported as rtp files that are processed by an in‐house software. For each gantry position, all segments are imported and the maximum distance traveled by a leaf pair is determined for each segment. The program then semi‐exhaustively explores the solution space to reorder the segments such that the sum of the maximum distance traveled by leaves is minimized. We assume that this will lead to delivery time reductions. Once all segments are ordered, the program writes a new rtp file. A second, in‐house developed software program checks this new file to ensure that every segment in the newly re‐ordered rtp file corresponds exactly to one in the original file. The delivery time for plans corresponding to both rtp files was measured and compared to assess the gain achieved through plan re‐ordering. Results: In all 12 cases investigated here, we observed reductions in treatment time (i.e. efficiency gains), ranging from 6 s to 1 minute; with an average time reduction of 0.55 minutes per treatment. A fairly strong correlation (R2=0.744) was seen between the decrease in delivery time and the total number of segments in the plan, suggesting that the ‘longest’ treatment plans have the most opportunity for reduction in treatment delivery time. Conclusion: We successfully developed and tested an in‐house computer program for re‐ordering segments of a DAO‐based IMRT step and shoot treatment plan. The re‐ordering was measured on clinical plans to provide average reductions of delivery time of 0.55 minutes per treatment. Such improved efficiency is beneficial for both intrafractional motion management and increase of patient throughput.

TU‐E‐204B‐04: DMLC Implementation of a Prostate Intrafraction Motion Correction Strategy Based on Failure Detection Concept

Purpose: To develop a dynamic MLC‐based real‐time motion tracking system through stereoscopic MV‐kV target localization and to utilize a failure detection concept to reduce kV imaging dose while maintaining high targeting accuracy. Methods and Materials: Continuous MV‐kV target localization, although informative, imposes excessive kV imaging dose. In order to reduce kV usage, we attempted for a first step to only detect potential motion beyond a pre‐defined threshold using MV images and in a second step, through applying as‐needed MV‐kV imaging, to confirm the over‐threshold event and obtain accurate marker position information which could be used to instruct instantaneous MLC adjustment. This technique is particularly useful for slow non‐periodic motion. A Varian Trilogy linac with onboard imaging and a Millennium 120‐leaf MLC was used to examine selected typical prostate trajectories using a 4D motion platform. Geometric and dosimetric tracking performances were evaluated by tracking a moving gold marker during dose delivery from 7 gantry angles. In‐house designed software was used for marker extraction, decision making, and MLC adjustment. Results: Total system latency from occurrence of an over‐threshold motion to the end of MLC repositioning was found to be 0.5 – 0.6 sec. The target was kept within a preset 2.5mm range 99% and 96% of the beam‐on time requiring only 1 and 13 kV‐on for a slow drifting and a high‐frequency motion case, respectively. The MLC repositioning accuracy is ∼0.5mm with system calibration. Dosimetric benefits are seen in reduced failure rate of the gamma‐test applied to dose measurements. Conclusions: Cine MV imaging during therapy delivery provides a valuable tool for detecting non‐negligible intrafraction prostate motion and facilitates intelligent kV use to significantly reduce imaging dose. Successful integration of this monitoring strategy and DMLC motion compensation is demonstrated. The developed system is intriguing for prostate intrafraction motion management with low kV imaging overhead.

Adaptive Management of Intrafraction Motion in Online Image Guided Prostate Cancer Radiotherapy

Adaptive Management of Intrafraction Motion in Online Image Guided Prostate Cancer Radiotherapy

Sci-Wed PM: Delivery-06: Management of Intra-Fraction Organ Motion: First Performance Evaluation of an Experimental Dynamic Tumor Tracking System

The management of intra‐fraction organ motion is an area of active research in medicalradiation physics. We present the development of a technical platform that allows a dynamic real‐time adaptation of radiation fields to moving radiation targets with a multi‐leaf collimator. The system consists of a dynamical control‐loop for an experimental MLC160 integrated with an ARTISTE linear accelerator. The information about the real‐time tumor position is taken directly from potentiometers that steer the movement of phantoms in the iso‐centric reference frame of the linear accelerator. As a second input the real‐time signal of the 4D‐tumor tracking device of CalypsoMedical Inc. can be employed. The performance of the system is evaluated by experimental studies with two moving phantoms: i) 2‐dimensional movements of the radiation target are simulated by a block of RW3 slabs that moves perpendicular to the treatment beam with different amplitudes for each direction of motion ii) an anatomically shaped lung phantom for which a solid tumor of RW3 is embedded in a low‐density material can be moved periodically in the cranial‐caudal direction with different amplitudes and frequencies. The irradiation of both phantoms can be analyzed by film dosimetry. The geometrical accuracy of the radiation field adaptation was measured with an EPID and gafchromic films imbedded into the phantoms. In a first test we have determined the latency of the complete system to approximately 500 ms; however a geometrical tracking accuracy within few millimeters could be established for periodical target movements of 2–4 cm amplitudes and characteristic breathing frequencies.

TU-E-BRC-04: Strategies for Real-Time MR Imaging for Integrated MRI+Linac Systems

Purpose: Integrated MRI+linac systems can potentially yield complete spatio-temporal knowledge of the irradiated anatomy during beam-on — representing the ideal guidance strategy for 4D radiotherapy delivery. In this work, we investigate rapid imaging strategies for such devices to enable real-time, MR-guided, motion-adaptive radiation delivery. Method and Materials: SNR in MR images is described by: SNR μ B0×(Dx×Dy×Dz)× Tacq, where 0 B is the primary field, Dx, Dy and Dz are voxel dimensions and acq T is the acquisition time. We investigated trade-offs between SNR and two important design and operational parameters for MRI+linac systems — (i) 0 B, which impacts design complexity, and (ii) acq T, which impacts the spatio-temporal accuracy of real-time guidance. In the first study, SSFP and SPGR sequences were employed to acquire 3D volumes (1.2 s/volume) and 2D coronal slices (0.3 s/slice) of the thoracic region from five human subjects (1.5T scanner). To simulate lower fieldstrength, image SNR was progressively degraded by adding increasing levels of gaussian noise. A fat deposit on the diaphragm was segmented in the noise-free and the degraded images and the error in the estimated position was computed. In the second study, faster acquisition through partial k-space scanning was simulated. A cylindrical water-filled phantom containing seven oilfilled cylinders was imaged and a progressively increasing number of mid-frequency phase encode lines were zeroed prior to reconstruction. The centers of oil-filled cylinders were auromatically segmented in the full and partial k-space acquisitions, and the positional error was computed with respect to the full k-space image. Results: Positional errors of the anatomic feature were within 1.5 mm for a factor-of-6 SNR degradation, corresponding to B0 = 0.25T. Partial k-space acquisition could be performed to increase acquisition speed by over a factor of 5, while maintaining sub-1 mm accuracy. Conclusion: These initial studies indicate the feasibility of low-field, real-time MRI for intrafraction motion management using integrated MRI+linac systems.

First Demonstration of Combined kV/MV Image-Guided Real-Time Dynamic Multileaf-Collimator Target Tracking

For intrafraction motion management, a real-time tracking system was developed by combining fiducial marker-based tracking via simultaneous kilovoltage (kV) and megavoltage (MV) imaging and a dynamic multileaf collimator (DMLC) beam-tracking system. The integrated tracking system employed a Varian Trilogy system equipped with kV/MV imaging systems and a Millennium 120-leaf MLC. A gold marker in elliptical motion (2-cm superior-inferior, 1-cm left-right, 10 cycles/min) was simultaneously imaged by the kV and MV imagers at 6.7 Hz and segmented in real time. With these two-dimensional projections, the tracking software triangulated the three-dimensional marker position and repositioned the MLC leaves to follow the motion. Phantom studies were performed to evaluate time delay from image acquisition to MLC adjustment, tracking error, and dosimetric impact of target motion with and without tracking. The time delay of the integrated tracking system was approximately 450 ms. The tracking error using a prediction algorithm was 0.9 +/- 0.5 mm for the elliptical motion. The dose distribution with tracking showed better target coverage and less dose to surrounding region over no tracking. The failure rate of the gamma test (3%/3-mm criteria) was 22.5% without tracking but was reduced to 0.2% with tracking. For the first time, a complete tracking system combining kV/MV image-guided target tracking and DMLC beam tracking was demonstrated. The average geometric error was less than 1 mm, and the dosimetric error was negligible. This system is a promising method for intrafraction motion management.

Toward Submillimeter Accuracy in the Management of Intrafraction Motion: The Integration of Real-Time Internal Position Monitoring and Multileaf Collimator Target Tracking

We report on an integrated system for real-time adaptive radiation delivery to moving tumors. The system combines two promising technologies-three-dimensional internal position monitoring using implanted electromagnetically excitable transponders and corresponding real-time beam adaptation using a dynamic multileaf collimator (DMLC). In a multi-institutional academic and industrial collaboration, a research version of the Calypso position monitoring system was integrated with a DMLC-based four-dimensional intensity-modulated radiotherapy delivery system using a Varian 120-leaf multileaf collimator (MLC). Two important determinants of system performance-latency (i.e., elapsed time between target motion and MLC response) and geometric accuracy-were investigated. Latency was quantified by acquiring continuous megavoltage X-ray images of a moving phantom (with embedded transponders) that was tracked in real time by a circular MLC field. The latency value was input into a motion prediction algorithm within the DMLC tracking system. Geometric accuracy was calculated as the root-mean-square positional error between the target and the centroid of the MLC aperture for patient-derived three-dimensional motion trajectories comprising two lung tumor traces and one prostate trace. System latency was determined to be approximately 220 milliseconds. Tracking accuracy was observed to be sub-2 mm for the respiratory motion traces and sub-1 mm for prostate motion. We have developed and characterized a research version of a novel four-dimensional delivery system that integrates nonionizing radiation-based internal position monitoring and accurate real-time DMLC-based beam adaptation. This system represents a significant step toward achieving the eventual goal of geometrically ideal dose delivery to moving tumors.