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.

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Purpose-Intensity modulated arc therapy (IMAT) is attractive due to high dose conformality and efficient delivery. However, managing intrafraction motion is challenging for IMAT. The purpose of this research was to develop and investigate electromagnetic-guided DMLC tracking as an enabling technology to treat moving targets during IMAT.Materials/Methods-A real-time 3D DMLC-based target tracking system was developed and integrated with a linac. 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 3-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 3mm/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. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Conflict of interest statement:This work was supported by Calypso Medical, the US National Institutes of Health and Varian Medical Systems. This support is acknowledged in the manuscript. There are also co-authors from both Calypso and Varian that have contributed scientifically to this manuscript. NIH Public Access Author ManuscriptInt J Radiat Oncol Biol Phys. Author manuscript; available in PMC 2012 January 1. Conclusions-For the first time electromagnetic-guided DMLC target tracking with IMAT has been investigated. 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.

SummaryCurrent-day radiotherapy systems do not account for tumor rotation, and dosimetric errors may result. This study reports a system that integrates a prototype electromagnetic tracking system to detect tumor translation and rotation with a dynamic multileaf collimator system that in real-time adapts the radiation beam to the translation and rotating tumor. Results show a rotation accuracy correction error of less than 1 degree. Dosimetric studies showed a three-fold improvement in target dose accuracy compared to current-day clinically available technology.