gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Quantitative Assessment of the Visibility of Fiducial Makers in Prostate by Monte Carlo Simulation of On-Board kV Imaging System

Meeting Abstract

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  • Y. Chen - Radiation Oncology Service, Henry M. Jackson Foundation for the Advancement of Military Medicine and Walter Reed Army Medical Center, Washington, DC, USA
  • J. O'Connell - Radiation Oncology Service, Walter Reed Army Medical Center, Washington, DC, USA
  • Chr. Ko - Radiation Oncology Service, Walter Reed Army Medical Center, Washington, DC, USA
  • J. McDonough - Department of Radiation Oncology, University of Pennsylvania Medical Center, Philadelphia, PA, USA

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog041

doi: 10.3205/09ptcog041, urn:nbn:de:0183-09ptcog0413

Published: September 24, 2009

© 2009 Chen et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



Background: Accurate target positioning and motion tracking of the prostate gland is critical for precise delivery of radiation treatment for prostate cancer patients. Gold seeds have been used for successful real-time target tracking using simultaneous kV-MV imaging in radiation therapy. Implanted gold markers, however, can cause a metal effect on CT imaging and change target density, which is a crucial effect in proton therapy. We investigate possible smaller, less dense fiducial markers that can produce comparable and acceptable visibility in a kV-kV imaging system on proton gantry.

Material and methods: The GATE (Geant4 Application for Emission Tomography and Radiotherapy) Monte Carlo simulation package was used to simulate the imaging system. Normalized x-ray spectra for a tungsten target x-ray tube were generated by the TASMIP program for different kVp. The voxelized XCAT human phantom was used for realistic human anatomy simulation. Several commercially available fiducial markers: a solid carbon marker of Φ1x3 mm, a solid gold marker of Φ1.2x3 mm, and Visicoil gold linear markers of Φ0.35x10 mm with a wire diameter of 0.05 mm, Φ0.75x10 mm (0.25 mm wire), and Φ1.10x10 mm (0.50 mm wire) were positioned in both lateral sides (comparable to future phantom experiments). They were shadowed by the prostate and other bone or soft tissues when incident cone-beam x-rays were from the lateral direction. A number of photons were generated corresponding to 8, 2, 1, 0.5, and 0.25 mAs for 50, 75, 100, 125, and 150 kVp spectra, respectively. All penetrating photons were recorded and their energies were summed to form an image in order to compute incident contrast noise ratio (CNR) for any succeeding imager. The CNR was calculated for each marker and used as a quantitative measure of its visibility.

Results: All carbon markers are invisible with CNRs under 1.0, and all gold markers shadowed by the dense bone have CNRs around 1.0. The smallest coil marker with the soft tissue has CNRs of about 2.0 for the range of 50-150 kVp. Its observed CNRs with a Varian PaxScan 4030E flat panel detector (DQE ~ 30%, MTF ~ 45%) are estimated to drop to ~ 1.0, which may be considered barely visible. The medium coil marker with the soft tissue has produced CNRs of 7.5, 10.1, 11.0, 11.6, and 11.9 for 50, 75, 100, 125, and 150 kVp, comparing to those of the solid marker of 7.4, 11.8, 13.7, 14.8, and 15.5. For 1, 2, 4, and 8 mAs under 50 kVp, CNRs are 5.1, 6.2, 6.9, and 7.5 for the medium coil marker and 2.6, 4.8, 6.4, and 7.4 for the solid marker.

Conclusion: The carbon marker is not suitable for the purpose of real-time kV target imaging. Other markers are difficult to see if they are shadowed by dense bone. The long linear coil marker may be easier to extend beyond the dense bone region. The Visicoil linear marker of Φ0.75x10 mm with a wire diameter of 0.25 mm is able to produce comparable visibility as the conventional solid gold marker, with the advantages of imaging under low energy and low fluence, providing a possible measure of fast, low dose real-time target imaging.

This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-04-2-0022. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the US Army.