Ryan L. Smith

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

PURPOSEnWe 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).nnnMETHODS AND MATERIALSnIn 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.nnnRESULTSnSystem 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.nnnCONCLUSIONnWe 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.

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

PURPOSEnContinuous 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.nnnMETHODS AND MATERIALSnA 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.nnnRESULTSnDosimetric 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.nnnCONCLUSIONSnReal-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.

Evaluation of linear accelerator gating with real-time electromagnetic tracking.

PURPOSEnIntrafraction organ motion can produce dosimetric errors in radiotherapy. Commonly, the linear accelerator is gated using real-time breathing phase obtained by way of external sensors. However, the external anatomy does not always correlate well with the internal position. We examined a beam gating technique using signals from implanted wireless transponders that provided real-time feedback on the tumor location without an imaging dose to the patient.nnnMETHODS AND MATERIALSnAn interface was developed between Calypso Medicals four-dimensional electromagnetic tracking system and a Varian Trilogy linear accelerator. A film phantom was mounted on a motion platform programmed with lung motion trajectories. Deliveries were performed when the beam was gated according to the signal from the wireless transponders. The dosimetric advantages of beam gating and the system latencies were quantified.nnnRESULTSnBeam gating using on internal position monitoring provided up to a twofold increase in the dose gradients. The percentage of points failing to be within +/-10 cGy of the planned dose (maximal dose, approximately 200 cGy) was 3.4% for gating and 32.1% for no intervention in the presence of motion. The mean latencies between the transponder position and linear accelerator modulation were 75.0 +/-12.7 ms for beam on and 65.1 +/- 12.9 ms for beam off.nnnCONCLUSIONnWe have presented the results from a novel method for gating the linear accelerator using trackable wireless internal fiducial markers without the use of ionizing radiation for imaging. The latencies observed were suitable for gating using electromagnetic fiducial markers, which results in dosimetric improvements for irradiation in the presence of motion.

The correlation of tissue motion within the lung: implications on fiducial based treatments

In radiation therapy many motion management and alignment techniques rely on the accuracy of an internal fiducial acting as a surrogate for target motion within the lung. Although fiducials are routinely used as surrogates for tumor motion, the extent to which varying spatial locations in the lung move similarly to other locations has yet to be quantitatively analyzed. In an attempt to analyze the motion correlation throughout the lung, ten primary lung cancer patients underwent IRB-approved 4DCT scans in the supine position. Deformable registration produced motion vectors for each voxel between exhalation and inhalation. Modeling was performed for each vector and all surrounding vectors within the lung in order to determine the mean 3D Euclidean distance necessary for an implanted fiducial to correlate with surrounding tissue motion to within 3 mm (left lower: 1.7 cm, left upper: 2.1 cm, right lower 1.6 cm, and right upper 2.9 cm). No general implantation rule of where to position a fiducial with respect to the tumor was found as the motion is highly patient and lobe specific. Correlation maps are presented showcasing spatial anisotropy of the motion of tissue surrounding the tumor.

Tumor correlated CT: a new paradigm for motion compensated CT for image-guided therapy

Respiratory motion has significant effects on abdominal and lung tumor position, and incorporation of this uncertainty increases volumes for focal cancer treatments. Respiratory correlated CT, obtained by oversampling images throughout the respiratory cycle based on an external surrogate, is increasingly being used for radiation therapy planning. Respiratory correlated CT is dependant on a fixed relationship between the external surrogate and the tumor, which may change based on weight loss, breathing pattern changes or non-respiratory motion. Moreover, the process decouples localization of the tumor (which is the goal of tumor directed therapy) with respiratory motion management. Recently, implantable passive transponders (Calypso Medical Technologies) have been developed which can be tracked via an external electromagnetic array in real-time and without ionizing radiation. We aimed to integrate wireless electromagnetic tracking with multislice CT, and create volumetric datasets that are correlated to tumor position, as opposed to an external surrogate. We call this process tumor correlated CT (TCCT). Use of these images for treatment planning will allow localization of the tumor to predict the position of other organs during treatment delivery. We show the preliminary work in the integration of electromagnetic tracking and CT imaging.

Incorporating electromagnetic tracking into respiratory correlated imaging for high precision radiation therapy

It is well established that respiratory motion has significant effects on lung tumor position, and incorporation of this uncertainty increases the normal lung tissue irradiated. Respiratory correlated CT, which provides three dimensional image sets for different phases of the breathing cycle, is increasingly being used for radiation therapy planning. Cone beam CT is being used to obtain cross sectional imaging at the time of therapy for accurate patient set-up. However, it is not possible to obtain cross sectional respiratory correlated imaging throughout the course of radiation, leaving residual uncertainties. Recently, implantable passive transponders (Calypso Medical Technologies) have been developed which are currently FDA-cleared for prostate use only and can be tracked via an external electromagnetic array in real-time, without the use of ionizing radiation. A visualization system needs to be developed to quickly and efficiently utilize both the dynamic real-time point measurements with the previously acquired volumetric data. We have created such a visualization system by incorporating the respiratory correlated imaging and the individual transponder locations into the Image Guided Surgery Toolkit (IGSTK.org). The tool already allows quick, qualitative verification of the differences between the measured transponder position and the imaged position at planning and will support quantitative measurements displaying uncertainty in positioning.