[Comments on evaluation method for equivalent square fields to the collimator scatter factors of rectangular fields].
In the measurement of 4 MV and 10 MV X-ray collimator scatter factors (S(c)), the method of using an acrylic mini-phantom showed no significant differences between cases in which the chamber axis was either parallel or perpendicular to the beam axis. Chamber readings with an aluminum or acrylic build-up cap were not reflected by contaminant electrons when the chamber axis was parallel to the beam axis. On the basis of the data on 4 MV and 10 MV X-ray collimator Sc measured using an acrylic mini-phantom, we examined three methods of obtaining square fields equivalent to rectangular fields, and reached the following conclusions: (1) The A/P method was not accurate because it did not take into account the structure of the radiation head. (2) Regarding the geometrical weight factor (k) used in the field-mapping method, more accurate k values were obtained when using the geometrical places of the flattening filter (or the second source, taken from the concept of extra-focal radiation), the upper and lower collimators, and the chamber, rather than when using the geometrical places of the source, the upper and lower collimators, and the chamber. (3) The most accurate k values could, in general, be obtained when determined on the basis of measured S(c) data.
Nara
Tetsuzo
T
Department of Radiology, Aomori Prefectural Center Hospital, Japan.
Iwasaki
Akira
A
Sato
Kazuhiko
K
Horanai
Yoshiaki
Y
Komai
Fumio
F
Fukushi
Hideto
H
D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D012542:Scattering, Radiation