Accuracy of a wireless localization system for radiotherapy.

PURPOSE: A system has been developed for patient positioning based on real-time localization of implanted electromagnetic transponders (beacons). This study demonstrated the accuracy of the system before clinical trials. METHODS AND MATERIALS: We describe the overall system. The localization component consists of beacons and a source array. A rigid phantom was constructed to place the beacons at known offsets from a localization array. Tests were performed at distances of 80 and 270 mm from the array and at positions in the array plane of up to 8 cm offset. Tests were performed in air and saline to assess the effect of tissue conductivity and with multiple transponders to evaluate crosstalk. Tracking was tested using a dynamic phantom creating a circular path at varying speeds. RESULTS: Submillimeter accuracy was maintained throughout all experiments. Precision was greater proximal to the source plane (sigmax = 0.006 mm, sigmay = 0.01 mm, sigmaz = 0.006 mm), but continued to be submillimeter at the end of the designed tracking range at 270 mm from the array (sigmax = 0.27 mm, sigmay = 0.36 mm, sigmaz = 0.48 mm). The introduction of saline and the use of multiple beacons did not affect accuracy. Submillimeter accuracy was maintained using the dynamic phantom at speeds of up to 3 cm/s. CONCLUSION: This system has demonstrated the accuracy needed for localization and monitoring of position during treatment.

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.

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.

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.

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.