title: Correction method for the physical dose calculated using Clarkson integration at the center of the spread-out Bragg peak for asymmetric field in carbon-ion radiotherapy.
abstract: OBJECTIVE
  We previously proposed a calculation method using Clarkson integration to obtain the physical dose at the center of the spread-out Bragg peak (SOBP) for a treatment beam, the measurement point of which agrees with the isocenter [Tajiri et al. Med. Phys. 2013; 40: 071733-1-5]. However, at the measurement point which does not agree with the isocenter, the physical dose calculated by this method might have a large error. For this error, we propose a correction method.
  
  
  METHODS
  To confirm whether the error can be corrected using in-air off axis ratio (OAR), we measured the physical dose at the center of an asymmetric square field and a symmetric square field and in-air OAR. For beams of which the measurement point does not agree with the isocenter, as applied to prostate cancer patients, the physical dose calculated using Clarkson integration was corrected with in-air OAR.
  
  
  RESULTS
  The maximum difference between the physical dose measured at the center of an asymmetric square field and the product of in-air OAR and the physical dose at the center of a symmetric square field was - 0.12%. For beams as applied to prostate cancer patients, the differences between the measured physical doses and the physical doses corrected using in-air OAR were -0.17 ± 0.23%.
  
  
  CONCLUSIONS
  The physical dose at the measurement point which does not agree with the isocenter, can be obtained from in-air OAR at the isocenter plane and the physical dose at the center of the SOBP on the beam axis.
authors.affiliation: Department of Hospital, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan. Electronic address: m_tajiri@nirs.go.jp.
authors.forename: Minoru
authors.initials: M
authors.lastname: Tajiri
authors.affiliation: Department of Hospital, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan.
authors.forename: Takamasa
authors.initials: T
authors.lastname: Maeda
authors.affiliation: Department of Hospital, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan.
authors.forename: Yoshiharu
authors.initials: Y
authors.lastname: Isobe
authors.affiliation: Department of Hospital, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan.
authors.forename: Katsuyuki
authors.initials: K
authors.lastname: Tanimoto
authors.affiliation: Department of Hospital, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan.
authors.forename: Koichi
authors.initials: K
authors.lastname: Shibayama
mesh_terms: D000465:Algorithms; D002244:Carbon / Q000737:chemistry*; D020450:Heavy Ions; D006801:Humans; D007477:Ions; D008297:Male; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D011878:Radiotherapy / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012984:Software; D014867:Water / Q000737:chemistry
chemical_list: D007477:Ions; D014867:Water; D002244:Carbon
keywords: Asymmetric field; Clarkson integration; Heavy-ion radiotherapy