Evaluation of absolute doses for irradiation fields of diameter of 10 cm and 5 cm in SOBP carbon ion beams

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  • SAKAMA Makoto
    Department of Electrical and Electronic Engineering, College of Industrial Technology, Nihon University
  • KANAI Tatsuaki
    Heavy Ion Medical Research Center, Gunma University
  • FUKUMURA Akifumi
    Research Center for charged particle therapy, National Institute of Radiological Sciences
  • HIMUKAI Takeshi
    Accelerator Engineering Corporation

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Other Title
  • SOBP炭素線における10cmと5cm照射野サイズに対する絶対線量の評価

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In charged particle therapy with passive method, Spread-Out Bragg Peaks (SOBP) with a ridge or wheel filter are used to achieve uniform biological responses in the irradiated tumor. Therefore, there are primary and fragment particles with variable energies from the filter in the SOBP. Currently, the dosimetry in SOBP beams using the ionization chambers has been performed with the beam quality correction factors for monoenergetic beams due to unknown physical quantities within the SOBP. Calorimetry is a most direct method to measure the absolute dose. We determined precisely the absorbed dose in the SOBP using the graphite calorimeter at the center of SOBP, 60 mm for irradiation fields of diameter of 10 cm and 5 cm in 290 MeV/n carbon ion beams. Under the same geometry and measurement position, the dosimetry was performed using the parallel and cylindrical ionization chambers. Next, the ionization chamber dosimetry was done in a water phantom at the center of SOBP. We compared and verified the dose using the ionization chambers with that from the calorimetry. At the center of SOBP in 290 MeV/n carbon ion beams, the comparisons to our calorimeter measurements revealed that the absorbed dose using the ionization chambers and beam quality correction factors from IAEA TRS 398 comes out low by about 3 %. For irradiation fields of diameter of 10 cm and 5 cm in 290 MeV/n carbon ion beams, the detailed information on dosimetry using the calorimeter and beam quality correction factors are reported.

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