Raghu Venkat

Management of three-dimensional intrafraction motion through real-time DMLC tracking

Tumor tracking using a dynamic multileaf collimator (DMLC) represents a promising approach for intrafraction motion management in thoracic and abdominal cancer radiotherapy. In this work, we develop, empirically demonstrate, and characterize a novel 3D tracking algorithm for real-time, conformal, intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT)-based radiation delivery to targets moving in three dimensions. The algorithm obtains real-time information of target location from an independent position monitoring system and dynamically calculates MLC leaf positions to account for changes in target position. Initial studies were performed to evaluate the geometric accuracy of DMLC tracking of 3D target motion. In addition, dosimetric studies were performed on a clinical linac to evaluate the impact of real-time DMLC tracking for conformal, step-and-shoot (S-IMRT), dynamic (D-IMRT), and VMAT deliveries to a moving target. The efficiency of conformal and IMRT delivery in the presence of tracking was determined. Results show that submillimeter geometric accuracy in all three dimensions is achievable with DMLC tracking. Significant dosimetric improvements were observed in the presence of tracking for conformal and IMRT deliveries to moving targets. A gamma index evaluation with a 3%-3 mm criterion showed that deliveries without DMLC tracking exhibit between 1.7 (S-IMRT) and 4.8 (D-IMRT) times more dose points that fail the evaluation compared to corresponding deliveries with tracking. The efficiency of IMRT delivery, as measured in the lab, was observed to be significantly lower in case of tracking target motion perpendicular to MLC leaf travel compared to motion parallel to leaf travel. Nevertheless, these early results indicate that accurate, real-time DMLC tracking of 3D tumor motion is feasible and can potentially result in significant geometric and dosimetric advantages leading to more effective management of intrafraction motion.

Toward submillimeter accuracy in the management of intrafraction motion: the integration of real-time internal position monitoring and multileaf collimator target tracking.

PURPOSE 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). METHODS AND MATERIALS 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. RESULTS 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. CONCLUSION 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.

Development and preliminary evaluation of a prototype audiovisual biofeedback device incorporating a patient-specific guiding waveform.

The aim of this research was to investigate the effectiveness of a novel audio-visual biofeedback respiratory training tool to reduce respiratory irregularity. The audiovisual biofeedback system acquires sample respiratory waveforms of a particular patient and computes a patient-specific waveform to guide the patients subsequent breathing. Two visual feedback models with different displays and cognitive loads were investigated: a bar model and a wave model. The audio instructions were ascending/descending musical tones played at inhale and exhale respectively to assist in maintaining the breathing period. Free-breathing, bar model and wave model training was performed on ten volunteers for 5 min for three repeat sessions. A total of 90 respiratory waveforms were acquired. It was found that the bar model was superior to free breathing with overall rms displacement variations of 0.10 and 0.16 cm, respectively, and rms period variations of 0.77 and 0.33 s, respectively. The wave model was superior to the bar model and free breathing for all volunteers, with an overall rms displacement of 0.08 cm and rms periods of 0.2 s. The reduction in the displacement and period variations for the bar model compared with free breathing was statistically significant (p = 0.005 and 0.002, respectively); the wave model was significantly better than the bar model (p = 0.006 and 0.005, respectively). Audiovisual biofeedback with a patient-specific guiding waveform significantly reduces variations in breathing. The wave model approach reduces cycle-to-cycle variations in displacement by greater than 50% and variations in period by over 70% compared with free breathing. The planned application of this device is anatomic and functional imaging procedures and radiation therapy delivery.

DMLC motion tracking of moving targets for intensity modulated arc therapy treatment – a feasibility study

Purpose. Intensity modulated arc therapy offers great advantages with the capability of delivering a fast and highly conformal treatment. However, moving targets represent a major challenge. By monitoring a moving target it is possible to make the beam follow the motion, shaped by a Dynamic MLC (DMLC). The aim of this work was to evaluate the dose delivered to moving targets using the RapidArcTM (Varian Medical Systems, Inc.) technology with and without a DMLC tracking algorithm. Material and methods. A Varian Clinac iX was equipped with a preclinical RapidArcTM and a 3D DMLC tracking application. A motion platform was placed on the couch, with the detectors on top: a PTW seven29 and a Scandidos Delta4. One lung plan and one prostate plan were delivered. Motion was monitored using a Real-time Position Management (RPM) system. Reference measurements were performed for both plans with both detectors at state (0) “static, no tracking”. Comparing measurements were made at state (1) “motion, no tracking” and state (2) “motion, tracking”. Results. Gamma analysis showed a significant improvement from measurements of state (1) to measurements of state (2) compared to the state (0) measurements: Lung plan; from 87 to 97% pass. Prostate plan; from 81 to 88% pass. Sub-beam information gave a much reduced pattern of periodically spatial deviating dose points for state (2) than for state (1). Iso-dose curve comparisons showed a slightly better agreement between state (0) and state (2) than between state (0) and state (1). Conclusions. DMLC tracking together with RapidArcTM make a feasible combination and is capable of improving the dose distribution delivered to a moving target. It seems to be of importance to minimize noise influencing the tracking, to gain the full benefit from the application.

INTEGRATION OF REAL-TIME INTERNAL ELECTROMAGNETIC POSITION MONITORING COUPLED WITH DYNAMIC MULTILEAF COLLIMATOR TRACKING: AN INTENSITY-MODULATED RADIATION THERAPY FEASIBILITY STUDY

PURPOSE Continuous tumor position measurement coupled with a tumor tracking system would result in a highly accurate radiation therapy system. Previous internal position monitoring systems have been limited by fluoroscopic radiation dose and low delivery efficiency. We aimed to incorporate a continuous, electromagnetic, three-dimensional position tracking system (Calypso 4D Localization System) with a dynamic multileaf collimator (DMLC)-based dose delivery system. METHODS AND MATERIALS A research version of the Calypso System provided real-time position of three Beacon transponders. These real-time three-dimensional positions were sent to research MLC controller with a motion-tracking algorithm that changed the planned leaf sequence. Electromagnetic transponders were embedded in a solid water film phantom that moved with patient lung trajectories while being irradiated with two different plans: a step-and-shoot intensity-modulated radiation therapy (S-IMRT) field and a dynamic IMRT (D-IMRT) field. Dosimetric results were recorded under three conditions: no intervention, DMLC tracking, and a spatial gating system. RESULTS Dosimetric accuracy was comparable for gating and DMLC tracking. Failure rates for gating/DMLC tracking are as follows: +/-3 cGy 10.9/ 7.5% for S-IMRT, 3.3/7.2% for D-IMRT; gamma (3mm/3%) 0.2/1.2% for S-IMRT, 0.2/0.2% for D-IMRT. DMLC tracking proved to be as efficient as standard delivery, with a two- to fivefold efficiency increase over gating. CONCLUSIONS Real-time target position information was successfully integrated into a DMLC effector system to modify dose delivery. Experimental results show both comparable dosimetric accuracy as well as improved efficiency compared with spatial gating.

Four-dimensional IMRT treatment planning using a DMLC motion-tracking algorithm

The purpose of this study is to develop a four-dimensional (4D) intensity-modulated radiation therapy (IMRT) treatment-planning method by modifying and applying a dynamic multileaf collimator (DMLC) motion-tracking algorithm. The 4D radiotherapy treatment scenario investigated is to obtain a 4D treatment plan based on a 4D computed tomography (CT) planning scan and to have the delivery flexible enough to account for changes in tumor position during treatment delivery. For each of 4D CT planning scans from 12 lung cancer patients, a reference phase plan was created; with its MLC leaf positions and three-dimensional (3D) tumor motion, the DMLC motion-tracking algorithm generated MLC leaf sequences for the plans of other respiratory phases. Then, a deformable dose-summed 4D plan was created by merging the leaf sequences of individual phase plans. Individual phase plans, as well as the deformable dose-summed 4D plan, are similar for each patient, indicating that this method is dosimetrically robust to the variations of fractional time spent in respiratory phases on a given 4D CT planning scan. The 4D IMRT treatment-planning method utilizing the DMLC motion-tracking algorithm explicitly accounts for 3D tumor motion and thus hysteresis and nonlinear motion, and is deliverable on a linear accelerator.

TH-D-BRC-07: Impact of Respiratory Biofeedback On Adaptively Sampled 4D-CBCT Image Quality: Initial Experiences

Purpose: Linac‐mounted 4D‐Cone Beam CT(CBCT)imaging is an important tool for IGRT. Our protocol uses an in‐house built audio‐video (AV) biofeedback device to regularize patient respiration during 4D‐CBCT data acquisition. In this study, the impact of the respiratory stabilization on phase‐correlated 4D‐CBCT image quality is evaluated in our full field‐of‐view (FOV) 4D‐CBCT procedure using the Varian OBI and RPM respiratory monitoring system. Materials and methods: We have implemented a 4D‐CBCT imaging procedure with a 450.0 mm diameter FOV using adaptive acquisition time and projection sampling frequency. An AV respiratory biofeedback device consisting of a LCD visual and audio feedback channels was used. It provides the patient with a real‐time visual and auditory cues whenever the RPM deviates from the selected reference breathing trace. During the 8∼10 minute data acquisition, the patient was instructed to duplicate the reference trace as closely as possible both in terms of displacement and time. The acquired projections were sorted into 10 groups by the associated phase recorded by the RPM system. The effects of the AV biofeedback on the respiratory regularity were studied in terms of average cycle length, baseline variation and displacement. Subsequently, the corresponding 4D‐CBCT images were compared with different acquisition AV modes, such as free breathing, audio only and AV regularization. Results: The patient respiratory track presents larger variation in the free‐breathing data acquisition. The auditory instruction to the patient exhibits control of the displacement but a noticeable baseline drift through the time. The AV biofeedback device improves the reproducibility of respiratory pattern. Conclusion: Our results demonstrated that the AV biofeedback device improved the reproducibility of patient respiratory pattern in our 4D‐CBCT data acquisition. The image quality benefits from the improved projection consistency within an extended time span in every phase bin. Supported by PPG NIH P01 116602.

TU‐EE‐A3‐05: Simultaneous Tracking and Four‐Dimensional Radiotherapy Delivery: Accounting for Spatial and Morphological Tumor Changes

Purpose: The aim of this work was to develop the formalism for, and experimentally verify, radiotherapy delivery to tumors undergoing spatial and morphological changes induced by respiration.Method and Materials: To determine the leaf sequence to be delivered at a given time based on a 4D plan, LPlan (M,θ), in which the leaf sequence varies not only with monitor units, M, but also respiratory phase θ can be summarized by L Del (t) = L Plan (M,θ) + R[T Del (x,y,z) − T Plan (x,y,z,θ)] where T is the 3D target position and L are leaf positions. A 4D treatment plan of a translating, rotating, deforming tumor exhibiting hysteresis with values well above those typically observed clinically was created. The theory derived was coded into a prototype 4D MLC controller. The treatment plan was loaded onto the controller, and delivered on a linear accelerator. The motion was detected by the monitoring of the marker block by the RPM system. The RPM signal was output in real time to the MLC controller that reshaped the beam according to the position and phase of the incoming signal. An EPID, operating in cine mode was used as the detector.Results: The treatment plan from phase T0–T9 matched the real‐time EPIDimages. The EPIDimages demonstrate the ability of the MLC leaves, driven based on the theory derived above, to conform to spatial and morphological changes. Conclusion: A theory has been developed to deliver 4D plans in which the leaf sequences can vary as a function of phase to account for the spatial and morphological tumor and normal tissue changes with respiration. This theory has been integrated into a MLC controller. A 4D radiotherapy treatment plan that includes translation, rotation, hysteresis and deformation was delivered. The method allows for variable respiratory patterns during treatment delivery. Conflict of Interest: Research supported by Varian Medical Systems.

TU‐FF‐A3‐04: Empirical Investigation of 3D Intrafraction Motion Management Using a Generalized Methodology for Tracking Translating, Rotating and Deforming Targets

Purpose: Real‐time tracking of tumor motion is a highly promising approach for intrafraction motion management in thoracic and abdominal cancerradiotherapy. We investigate the geometric and dosimetric impact of a generalized methodology for conformal and IMRT‐based radiation delivery to translating, rotating and deforming targets. Materials and Methods: The methodology is based on the concept of “relocation vectors”, which correlate instantaneous target position to each point on the treatment aperture(s), throughout the entire respiratory cycle. These vectors are determined during 4D treatment planning by combining instantaneous target position information, obtained from a real‐time position‐monitoring (RPM) system, with concurrently acquired 4DCT. The resulting plan comprises multiple “control points”. For each point, a set of MLC leaf positions is defined so as to conform to the instantaneous shape of the target and deliver the desired fluence. The methodology is, therefore, independent of the nature of target motion. During dose delivery, real‐time RPM data are used to update the relocation vectors so as to dynamically account for changes target shape/position relative to the shape/position observed in the planning stage. Initial studies were performed to demonstrate tracking of linear and elliptical motion. A laboratory system was designed and optical measurements of tracking accuracy and system latency were obtained. The methodology was subsequently tested on a moving lung phantom placed under a clinical linac, and dosimetric measurements were performed using a 2D ion‐chamber array. Results: Tracking accuracy (without using any predictive algorithm) was observed to be ∼1.25 mm for motion parallel to MLC leaf travel. For motion perpendicular to leaf travel, accuracy was significantly lower. Dosimetric measurements indicate that tracking achieves efficient dose delivery to the target and, simultaneously, significant dose reduction in surrounding regions. Conclusions: We have developed a robust, universal tracking methodology to manage 3D intrafraction motion. This work was partially supported by Varian.