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

Improvement of the dose distribution formed by carbon-ion beam with a bi-material range compensator

Meeting Abstract

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  • Y. Hara - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • Y. Takada - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • K. Hotta - Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
  • T. Himukai - Promotion of carbon therapy section, National Institute of Radiological Sciences, 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. Doc09ptcog081

DOI: 10.3205/09ptcog081, URN: urn:nbn:de:0183-09ptcog0819

Published: September 24, 2009

© 2009 Hara et al.
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Outline

Text

Background: It is important for carbon-ion radiotherapy to form a uniform dose distribution in a target volume. However, when shape of the target volume is complicated, use of a single-material range compensator (abbr. as RC hereafter) may induce the dose inhomogeneity in the target region. The purpose of this study is to improve the dose distribution in the target volume using a bi-material RC made from a low-Z material (chemical wood) and a high-Z material (brass) instead of a single-material RC.

Material and methods: We designed and manufactured a 4 x 4 matrix-shaped single-material RC and a bi-material RC with the same range losses at corresponding parts of the RC. Since the bi-material RC can control the scattering strength at various parts of the RC in addition to the range loss, it equalizes the lateral penumbrae of dose distributions at a same water-equivalent depth formed by carbons traversing different parts of the RC to improve the dose distribution. We measured dose distributions in water formed by a range-modulated 290 MeV /u carbon-ion beam with a SOBP (Spread-Out Bragg Peak) width of 60 mm traversing the single-material RC or the bi-material RC, using the HIMAC biology beam port. We used for measurements an array of 96 channel parallel-plate ionization chambers with a horizontal pitch of 2 mm immersed in water and moved in the depth direction. Measurement results are compared with calculations based on the analytical dose-calculation model we have developed [1]. We converted the physical dose distributions to the biological ones using the biological model developed by the HIMAC group [2].

Results and discussion: Whereas clear improvement of biological dose distributions has been observed for the bi-material RC compared with the single-material RC, there remains a certain amount of dose non-uniformity in the target volume. We found that the cause of the non-uniformity arises from non-uniformity of the SOBP. It turns out that the non-uniformity of the SOBP comes from the incomplete design model of the ridge filter (RF) in which the weights of the RF components are derived under the assumption that the pristine Bragg curve formed by carbons passing through a RF component has the same shape with that formed by carbons with the maximum energy. However, such an assumption does not hold precisely due to the influence of nuclear fragmentation reactions.

Conclusion: We have attained significant improvement of the dose non-uniformity induced by a conventional RC by using the bi-material RC. The overall shape of the dose distribution has been reproduced well by a dose calculation model. Yet, there remains some non-uniformity due to the incomplete SOBP. We need a more precise design model of the RF to improve the situation further.


References

1.
Takada Y, Himukai T, Takizawa K, et al. The basic study of a bi-material range compensator for improving dose uniformity for proton therapy. Phys Med Biol. 2008;53:5555-6.
2.
Kanai T, Endo M, Miyahara N, et al. Biophysical characteristics of HIMAC clinical irradiation system for heavy-ion radiation therapy. Int J Radiat Oncol Biol Phys. 1999;44:201-10.