title: [Estimation of collimator scatter factor, S(c), of small field sizes using long-SCD method and two saturation models].
abstract: To estimate the collimator scatter factor, S(c) of small field sizes in which a mini-phantom cannot be fully included at the nominal treatment distance (NTD=100 cm), we measured the in-air output of 4 MV and 10 MV X-rays of a Varian's Clinac 2100 C/D using a mini-phantom at NTD and at a long source-to-chamber distance (SCD=200 cm) with field-size defined at the isocenter down to 4.6 x 4.6 cm(2) and 2.3 x 2.3 cm(2), respectively. We then compared the fitted curve to the NTD dataset by a cumulative exponential distribution model with that by a cumulative Gaussian distribution (error function) model containing a zero-field extrapolated term derived from the long SCD dataset. The results showed that the zero-field extensions of two fitted curves coincided for a 4 MV X-ray, but a large discrepancy was seen between them for a 10 MV X-ray. Therefore, the S(c) of small field sizes not measurable using a mini-phantom at the NTD can be well estimated by applying the cumulative exponential model to the NTD dataset in the case of a 4 MV X-ray beam filtrated with a cone-shaped flattener. However, to estimate the S(c) of such small field sizes in the case of a 10 MV X-ray beam filtrated with a bell-shaped flattener, we consider it preferable to also measure in-air output at a long SCD and to apply the cumulative Gaussian model as described here.
authors.affiliation: Department of Medical Radiation Technology, School of Health Sciences, Niigata University (Currently, Department of Radiology, Komaki Citizen's Hospital).
authors.forename: Hiromi
authors.initials: H
authors.lastname: Adachi
authors.forename: Hideki
authors.initials: H
authors.lastname: Inakoshi
authors.forename: Takahide
authors.initials: T
authors.lastname: Hayakawa
authors.forename: Tomio
authors.initials: T
authors.lastname: Inoue
authors.forename: Toshifumi
authors.initials: T
authors.lastname: Kasahara
authors.forename: Satoshi
authors.initials: S
authors.lastname: Igarashi
authors.forename: Hideki
authors.initials: H
authors.lastname: Hayakawa
authors.forename: Satoshi
authors.initials: S
authors.lastname: Tanabe
mesh_terms: D015233:Models, Statistical; D016011:Normal Distribution; D019047:Phantoms, Imaging; D012542:Scattering, Radiation