Herbert Cattell

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.

ELECTROMAGNETIC-GUIDED DYNAMIC MULTILEAF COLLIMATOR TRACKING ENABLES MOTION MANAGEMENT FOR INTENSITY-MODULATED ARC THERAPY

PURPOSE Intensity-modulated arc therapy (IMAT) is attractive because of high-dose conformality and efficient delivery. However, managing intrafraction motion is challenging for IMAT. The purpose of this research was to develop and investigate electromagnetically guided dynamic multileaf collimator (DMLC) tracking as an enabling technology to treat moving targets during IMAT. METHODS AND MATERIALS A real-time three-dimensional DMLC-based target tracking system was developed and integrated with a linear accelerator. The DMLC tracking software inputs a real-time electromagnetically measured target position and the IMAT plan, and dynamically creates new leaf positions directed at the moving target. Low- and high-modulation IMAT plans were created for lung and prostate cancer cases. The IMAT plans were delivered to a three-axis motion platform programmed with measured patient motion. Dosimetric measurements were acquired by placing an ion chamber array on the moving platform. Measurements were acquired with tracking, without tracking (current clinical practice), and with the phantom in a static position (reference). Analysis of dose distribution differences from the static reference used a γ-test. RESULTS On average, 1.6% of dose points for the lung plans and 1.2% of points for the prostate plans failed the 3-mm/3% γ-test with tracking; without tracking, 34% and 14% (respectively) of points failed the γ-test. The delivery time was the same with and without tracking. CONCLUSIONS Electromagnetic-guided DMLC target tracking with IMAT has been investigated for the first time. Dose distributions to moving targets with DMLC tracking were significantly superior to those without tracking. There was no loss of treatment efficiency with DMLC tracking.

Real-time dynamic MLC tracking for inversely optimized arc radiotherapy

BACKGROUND AND PURPOSE Motion compensation with MLC tracking was tested for inversely optimized arc radiotherapy with special attention to the impact of the size of the target displacements and the angle of the leaf trajectory. MATERIALS AND METHODS An MLC-tracking algorithm was used to adjust the MLC positions according to the target movements using information from an optical real-time positioning management system. Two plans with collimator angles of 45 degrees and 90 degrees , respectively, were delivered and measured using the Delta(4)(R) dosimetric device moving in the superior-inferior direction with peak-to-peak displacements of 5, 10, 15, 20 and 25 mm and a cycle time of 6s. RESULTS Gamma index evaluation for plan delivery with MLC tracking gave a pass rate higher than 98% for criteria 3% and 3 mm for both plans and for all sizes of the target displacement. With no motion compensation, the average pass rate was 75% for plan 1 and 70% for plan 2 for 25 mm peak-to-peak displacement. CONCLUSION MLC tracking improves the accuracy of inversely optimized arc delivery for the cases studied. With MLC tracking, the dosimetric accuracy was independent of the magnitude of the peak-to-peak displacement of the target and not significantly affected by the angle between the leaf trajectory and the target movements.

Electromagnetic Detection and Real-Time DMLC Adaptation to Target Rotation During Radiotherapy

PURPOSE Intrafraction rotation of more than 45° and 25° has been observed for lung and prostate tumors, respectively. Such rotation is not routinely adapted to during current radiotherapy, which may compromise tumor dose coverage. The aim of the study was to investigate the geometric and dosimetric performance of an electromagnetically guided real-time dynamic multileaf collimator (DMLC) tracking system to adapt to intrafractional tumor rotation. MATERIALS/METHODS Target rotation was provided by changing the treatment couch angle. The target rotation was measured by a research Calypso system integrated with a real-time DMLC tracking system employed on a Varian linac. The geometric beam-target rotational alignment difference was measured using electronic portal images. The dosimetric accuracy was quantified using a two-dimensional ion chamber array. For each beam, the following five delivery modes were tested: 1) nonrotated target (reference); 2) fixed rotated target with tracking; 3) fixed rotated target without tracking; 4) actively rotating target with tracking; and 5) actively rotating target without tracking. Dosimetric performance of the latter four modes was measured and compared to the reference dose distribution using a 3 mm/3% γ-test. RESULTS Geometrically, the beam-target rotational alignment difference was 0.3° ± 0.6° for fixed rotation and 0.3° ± 1.3° for active rotation. Dosimetrically, the average failure rate for the γ-test for a fixed rotated target was 11% with tracking and 36% without tracking. The average failure rate for an actively rotating target was 9% with tracking and 35% without tracking. CONCLUSIONS For the first time, real-time target rotation has been accurately detected and adapted to during radiation delivery via DMLC tracking. The beam-target rotational alignment difference was mostly within 1°. Dose distributions to fixed and actively rotating targets with DMLC tracking were significantly superior to those without tracking.

Tumor-tracking radiotherapy of moving targets; verification using 3D polymer gel, 2D ion-chamber array and biplanar diode array

The aim of this study was to carry out a dosimetric verification of a dynamic multileaf collimator (DMLC)-based tumor-tracking delivery during respiratory-like motion. The advantage of tumor-tracking radiation delivery is the ability to allow a tighter margin around the target by continuously following and adapting the dose delivery to its motion. However, there are geometric and dosimetric uncertainties associated with beam delivery system constraints and output variations, and several investigations have to be accomplished before a clinical integration of this tracking technique. Two types of delivery were investigated in this study I) a single beam perpendicular to a target with a one dimensional motion parallel to the MLC moving direction, and II) an intensity modulated arc delivery (RapidArc®) with a target motion diagonal to the MLC moving direction. The feasibility study (I) was made using an 2D ionisation chamber array and a true 3D polymer gel. The arc delivery (II) was verified using polymer gel and a biplanar diode array. Good agreement in absorbed dose was found between delivery to a static target and to a moving target with DMLC tracking using all three detector systems. However, due to the limited spatial resolution of the 2D array a detailed comparison was not possible. The RapidArc® plan delivery was successfully verified using the biplanar diode array and true 3D polymer gel, and both detector systems could verify that the DMLC-based tumor-tracking delivery system has a very good ability to account for respiratory target motion.

SU‐GG‐J‐19: Electromagnetic Detection and Real‐Time DMLC Correction of Rotation during Radiotherapy

Purpose: A novel system has been developed to detect and correct tumor rotation in real‐time during radiotherapy delivery. The aim of this study was to quantify the geometric accuracy of the system. Method and Materials: A research Calypso system was integrated with a real‐time DMLC tracking system, employed on a Varian IXlinac. Three 3mm diameter tungsten balls (markers) were embedded in the phantom, along with implanted electromagnetic transponders. EPIDimages were acquired for an elliptical beam, from which the beam aperture and tungsten balls were synchronously observed. For static rotation, the couch was used; for dynamic rotation, the couch was manually rotated continuously from 90 deg to 180 deg through the console. The dynamic rotation experiment was also performed by overlying the EM embedded phantom on a motion platform, rotating with a period of approximately 8 seconds. For all measurements, the beam‐target rotational alignment was determined as the difference between the major axis of the ellipse best fitting the rotated beam aperture and the marker orientation of the rotated target. The beam‐target translation alignment was determined by the different between the beam aperture center and the geometric center of the markers. Results: For static rotation, the beam‐target rotational alignment error was 0.9±1.7°. For dynamic rotation the alignment error was 0.3±1.4°. Both tests demonstrate sub‐degree accuracy for the tracking system. The beam‐target translational alignment error was 0.2±0.2mm. Conclusion: For the first time, real‐time target rotation has been accurately detected and corrected during treatment via MLC adaptation. The beam‐target rotational alignment accuracy is <1 degree; translational alignment is <1mm. The results show promise for improving radiotherapy targeting accuracy for a variety of tumor sites and motion types. Conflict of Interest: Supported by Calypso, NIH R0193626 and Varian.

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.