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

Application of a bi-material range compensator to a RTOG phantom for proton therapy

Meeting Abstract

  • Y. Takada - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • K. Hotta - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • Y. Hara - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • H. Akasaka - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • T. Kimura - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • R. Tansho - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • T. Nihei - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • T. Himukai - National Institute of Radiological Sciences, Chiba, Japan
  • T. Kanai - Heavy Ion Medical Center, Gunma University, Maebashi, 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. Doc09ptcog198

DOI: 10.3205/09ptcog198, URN: urn:nbn:de:0183-09ptcog1984

Published: September 24, 2009

© 2009 Takada et al.
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Outline

Text

Background: We often find distortion of dose distribution in proton therapy when we insert a range compensator (abbr. as RC hereafter) with a complex structure. It will induce a possible dose error if we determine a monitor unit of a transmission ionization chamber using a single-point dose calibration method. Then we have developed a bi-material RC to improve the dose distribution in the PTV instead of the regular single-material RC [1]. We applied this method to a number of geometrical phantoms and obtained the good results [1]. Now we applied this method to a RTOG phantom simulating a clinical situation o verify the effectiveness.

Material and methods: Since the dose non-uniformity by insertion of the regular RC is induced by mismatch of lateral penumbrae of dose distributions formed by groups of protons passing through the different thicknesses of adjacent RC portions, control of the lateral penumbrae is essential to obtain a uniform dose distribution. Then we added another high-Z material to the RC to control the scattering strength in the RC and adjust the thickness of the low-Z RC material to keep the range loss. We used a brass as the high-Z material and a Cycowood (the main component is the ABS-resin) as the low-Z material. We equalized the lateral penumbrae of dose distributions formed by groups of protons passing through the different parts of the RC at the depth in water (simulating a human body) corresponding to the middle point of an overlapping region in depth of the deepest part of the SOBP (Spread-Out Bragg Peak) and the shallowest part of the SOBP [1]. We applied this method to a RTOG phantom simulating a tumor surrounding a spinal cord. We designed and manufactured both a single-material RTCOG phantom and a bi-material RTCOG phantom based on the dose calculation model we have developed. The details can be found in the reference (1). We used a 160-MeV proton beam at HIMAC to measure the dose distributions in water. We placed 96-channel parallel-plate ionization chambers arrayed in a horizontal direction (x) with a pitch of 2 mm in a water phantom. The sensitive volume of the individual chamber is 1.7 mm(H)x1.7 mm(V)x5 mm(V) . They can be moved in depth direction (z) to measure an iso-dose distribution in a horizontal plane.

Results: We found noticeable dose non-uniformity in the measured dose distribution for the single-material RC. More uniform distribution has been observed for the bi-material RC. Both measurement results have been reproduced well by calculations by our developed dose calculation model [1].

Conclusion: We applied the bi-material RC to a RTOG phantom simulating a clinical situation and attained clear improvement of dose uniformity over the case for the regular single-material RC


References

1.
Takada Y, et al. Basic study of bi-material range compensator for improving dose uniformity for proton therapy. Phys Med Biol. 2008;53: 5555-69.