title: [Accuracy of absorbed dose calculation and measurement of scatter factors with different depth of mini-phantoms using a pinpoint ionization chamber].
abstract: The output factor of high-energy X-ray machines varies with collimation. According to Khan's theory, collimator and phantom scatter factors contribute to total scatter factor. For precise X-ray irradiation, the two factors need to be taken into consideration. To obtain proper factors, we made two original polystyrene cylindrical mini-phantoms. These phantoms are both 4 cm in diameter and have a pinpoint ion chamber placed at a depth of 5 cm and 10 cm, respectively. Using a 6 MV X-ray machine, collimator scatter factors were calculated for various field arrangements (i.e., field sizes ranging from 4 cm x 4 cm to 40 cm x 40 cm at isocenter). To determine if calculated values were appropriate, we measured point doses of 20 X-ray irradiation patterns using a Farmer-type ion chamber with a water equivalent phantom at depths of 5 cm and 10 cm, respectively. Two hundred MUs were irradiated to the above-mentioned depths for each field. Based on the measured doses, variations were obtained for four calculation methods. Accounting for 1) secondary collimator (jaw) setting, 2) blocked field (multi-leaf collimator) setting, 3) Khan's theory using a 5 cm mini-phantom, and 4) Khan's theory using a 10 cm mini-phantom. Dose variations in each method of calculation were as follows: 1) +0.3 to +10.2% (mean, +2.0 to +3.2%) , 2) -2.3 to 0.0% (mean, -0.8 to -0.6%), 3) 0.0 to +1.5% (mean, +0.1 to +0.3%), 4) 0.0 to +1.4% (mean, -0.1 to +0.1%).
authors.affiliation: Department of Radiology, Hokkaido University Hospital.
authors.forename: Masataka
authors.initials: M
authors.lastname: Oita
authors.forename: Yoshiharu
authors.initials: Y
authors.lastname: Watanabe
authors.forename: Katsuhisa
authors.initials: K
authors.lastname: Fujita
authors.forename: Teruo
authors.initials: T
authors.lastname: Furuya
authors.forename: Junya
authors.initials: J
authors.lastname: Nankumo
authors.forename: Makoto
authors.initials: M
authors.lastname: Oda
mesh_terms: D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity