Field-size effect of physical doses in carbon-ion scanning using range shifter plates.
A field-size effect of physical doses was studied in scanning irradiation with carbon ions. For the target volumes of 60 x 60 x 80, 40 x 40 x 80, and 20 x 20 x 80 mm3, the doses along the beam axis within the spread-out Bragg peaks reduced to 99.4%, 98.2%, and 96.0% of the dose for the target of 80 x 80 x 80 mm3, respectively. The present study revealed that the observed reductions can be compensated for by adopting the three-Gaussian form of lateral dose distributions for the pencil beam model used in the treatment planning system. The parameters describing the form were determined through the irradiation experiments making flat concentric squared frames with a scanned carbon beam. Since utilizing the three-Gaussian model in the dose optimization loop is at present time consuming, the correction for the field-size effect should be implemented as a "predicted-dose scaling factor." The validity of this correction method was confirmed through the irradiation of a target of 20 x 20 x 80 mm3.
Inaniwa
Taku
T
Medical Physics Research Group, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba 263-8555, Japan. taku@nirs.go.jp
Furukawa
Takuji
T
Nagano
Ai
A
Sato
Shinji
S
Saotome
Naoya
N
Noda
Koji
K
Kanai
Tatsuaki
T
D000465:Algorithms; D002244:Carbon; D006801:Humans; D008962:Models, Theoretical; D009010:Monte Carlo Method; D016011:Normal Distribution; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D013997:Time Factors; D014867:Water / Q000737:chemistry
D014867:Water; D002244:Carbon