title: Toward submillimeter accuracy in the management of intrafraction motion: the integration of real-time internal position monitoring and multileaf collimator target tracking.
abstract: OBJECTIVE
  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
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Stanford Cancer Center, CA, USA. asawant@stanford.edu
authors.forename: Amit
authors.initials: A
authors.lastname: Sawant
authors.forename: Ryan L
authors.initials: RL
authors.lastname: Smith
authors.forename: Raghu B
authors.initials: RB
authors.lastname: Venkat
authors.forename: Lakshmi
authors.initials: L
authors.lastname: Santanam
authors.forename: Byungchul
authors.initials: B
authors.lastname: Cho
authors.forename: Per
authors.initials: P
authors.lastname: Poulsen
authors.forename: Herbert
authors.initials: H
authors.lastname: Cattell
authors.forename: Laurence J
authors.initials: LJ
authors.lastname: Newell
authors.forename: Parag
authors.initials: P
authors.lastname: Parikh
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Keall