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

Measurements of Dose Distribution Using a MOSFET Detector for Therapeutic Proton Beam

Meeting Abstract

  • R. Kohno - National Cancer Center Hospital East, Chiba, Japan
  • K. Hotta - University of Tsukuba, Ibaraki, Japan
  • S. Kameoka - National Cancer Center Hospital East, Chiba, Japan
  • T. Matsuura - National Cancer Center Hospital East, Chiba, Japan
  • T. Nishio - National Cancer Center Hospital East, Chiba, Japan
  • M. Kawashima - National Cancer Center Hospital East, Chiba, Japan
  • T. Ogino - National Cancer Center Hospital East, Chiba, Japan

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog115

DOI: 10.3205/09ptcog115, URN: urn:nbn:de:0183-09ptcog1155

Published: September 24, 2009

© 2009 Kohno et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.en). You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.


Outline

Text

Background: Proton dose distributions were measured using a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detector. The MOSFET response depends strongly on the degree of linear energy transfer (LET) of the proton beam. Therefore, in order to more accurately measure proton dose distributions using the TN-252RD MOSFET detector, a response correction method was developed.

Material and Methods: The depth-dose response for the TN-252RD MOSFET detector was evaluated experimentally using a 190 MeV therapeutic proton beam. Depth-dose curves measured using the MOSFET detectors for a mono-energetic proton beam were compared with results obtained using an ionization chamber (IC). In order to more accurately measure proton dose distributions using the TN-252RD, we developed a response correction method. A Bragg curve measured using the IC detector was employed to determine the correction factors (IC/TN-252RD) as a function of proton penetration depth, or residual range. The residual proton range was calculated using the pencil beam dose calculation algorithm at an arbitrary point. We prepared a bolus with an L-shaped horizontal cross section made of polyethylene and a water-equivalent length ratio of 1.02 (Figure 1 [Fig. 1]). The bolus was 50 mm thick at points where x>0 and 10 mm thick when x=0. For the 190 MeV mono-energetic proton beam, the Bragg peak position was 110 mm for protons passing through the thicker section and 150 mm for protons passing through the thinner section. Proton dose distributions produced by an L-shaped bolus were measured using the IC and the TN-252RD MOSFET detector, and the TN-252RD doses were compensated using the correction factors.

Results: The depth-dose distribution measured by the TN-252RD detector was compared to a reference depth-dose distribution measured using the IC detector for the 190 MeV proton beam (Figure 2 [Fig. 2]). The TN-252RD exhibited a good response at the Bragg peak (0.7 relative to the IC detector). Figure 3 [Fig. 3] compares the lateral dose distribution obtained using the IC and the TN-252RD MOSFET detector for the L-shaped bolus experiment at PE thicknesses of 110 mm. Because the depth at x<0 is the Bragg peak position, the response of the TN-252RD was reduced. Since edge scattering causes the lateral-dose distribution near x=0 to be determined by protons with a distribution of energies, we expected that the change in the TN-252RD response would be complicated. However, the corrected output of the TN-252RD detector agreed well with the IC results, indicating MOSFET detectors are suitable for proton dosimetry when the response is corrected.

Conclusions: For LET dependence of the MOSFET detector, we developed a new method for correcting the MOSFET response to proton beams. In dose distributions resulting from protons passing through an L-shaped bolus, the corrected TN-252RD dose agreed well with the IC results. Proton dosimetry can be successfully performed using MOSFET detectors.