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Title: WE-G-17A-04: Measurements of the Electron-Return-Effect in a Commercial Magnetic Resonance Image Guided Radiation Therapy (MR-IGRT) System

Purpose: The aim of this work was to measure the influences of the Lorentz force (electron return effect) on secondary electron transport in the presence of a low magnetic field produced in a commercial MR-IGRT system using a custom heterogeneous phantom. Methods: A commercial MR-IGRT system has been commissioned in our department. The system combines real-time imaging using a split-bore 0.35-T MRI and three Co-60 heads, each collimated with a doubly-focused MLC. The integrated treatment planning system uses a Monte-Carlo algorithm that models the magnetic field effects on scattered electrons. During commissioning, a custom heterogeneity phantom was designed to acquire ionization chamber and film measurements. The 30 cm cubic phantom consists of two waterfilled annuli, each containing a central region that simulates a 6 cm cubic lung tumor using polystyrene embedded in cork. Film may be placed inbetween the halves, and small-volume ionization chambers may be placed in different positions to measure dose to the tumor and near interfaces where the electron return effect is expected. The treatment planning system was used to create open-field and IMRT treatment plans on a CT scan of the phantom. Plans were delivered to the phantom, and radiographic film and ionization chamber measurements weremore » obtained. Results: The mean ionization chamber measured dose ratio for 27 measurements for 5 plans was 0.993 ± 0.027. Lateral profile film measurements confirm that the electron-return-effect is observable, producing local dose variations of less than 10% over 5 mm with 0.35 T magnetic field for single field treatment plans. The effect becomes negligible for opposing-field and IMRT treatments. Conclusion: A heterogeneous phantom for measurement of the electron-return-effect has been designed. Ionization chamber and film measurements made with the phantom indicate that the dosimetric effect is minimal, and the treatment planning system predicts dose reasonably well for complex heterogeneous scenarios.« less
Authors:
 [1] ; ; ;  [2] ;  [3]
  1. Washington University School of Medicine, St Louis, MO (United States)
  2. Washington University School of Medicine, St. Louis, MO (United States)
  3. Washington University School of Medicine, Saint Louis, MO (United States)
Publication Date:
OSTI Identifier:
22409761
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 41; Journal Issue: 6; Other Information: (c) 2014 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; CAT SCANNING; COBALT 60; IONIZATION CHAMBERS; LUNGS; MONTE CARLO METHOD; NEOPLASMS; NMR IMAGING; PHANTOMS; PLANNING; RADIATION DOSES; RADIOTHERAPY