On the measurement of dose in-air for small radiation fields: choice of mini-phantom material. (26th February 2015)

• Record Type: Journal Article
• Authors: Hug, Benjamin; Warrener, Kirbie; Liu, Paul; Ralston, Anna; Suchowerska, Natalka; McKenzie, David; Ebert, Martin A
• Is Part Of: Physics in medicine & biology. Volume 60:Number 6(2015:Mar.)
• Page Start: 2391
• Page End: 2402
• Publication Date: 2015-02-26
• Subjects: Monte Carlo -- in-air output factor -- small field dosimetry -- scintillation detector -- stereotactic radiotherapy; Biophysics -- Periodicals; Medical physics -- Periodicals; 610.153
• DOI: 10.1088/0031-9155/60/6/2391
• Languages: English
• ISSNs: 0031-9155

Abstract

Abstract: The purpose of this work was to determine the effect of choice of mini-phantom material on the measurement and calculation of in-air output factors ( S c ) in small fields. Monte Carlo simulations in conjunction with a theoretical determination of S c were used to validate previously reported measurements. Options for alternative mini-phantom materials were compared. A 6 MV beam from a Varian Novalis linear accelerator operating in stereotactic (SRS) mode was modelled. Phase-space data were used to determine the theoretical value of S c . To validate previously reported S c measurements the data were used to model the fibre-optic detector and brass mini-phantom. The impact of mini-phantom material was investigated by comparing the energy spectra of electrons entering the detector volume as a function of field size, and comparing the simulated S c -measurement to the theoretical calculation. In order to determine factors leading to changes in S c with field size, the origins of particles in the beam as incident on the mini-phantom were determined. S c values derived from simulated measurements using a brass mini-phantom on a fibre-optic detector agreed with the measured S c to within 0.7%. For simulation of measurement for all other mini-phantom materials, S c values agreed with the theoretically calculated values to within 0.6%. The dominant processes responsible for a decrease in S c with field size is occlusion of the focal and primary collimator contributions, while the secondary scatter, from the flattening filter and cone collimators, has minimal effect. The secondary electron spectrum is affected by the choice of mini-phantom material, but is almost independent of field size. For cone-collimated small fields in the Novalis beam (<30 mm), the decrease in S c with field size is primarily due to collimation of the focal radiation beam and scatter from the primary collimator. A fibre optic detector with either a brass, gold or lead mini-phantom with at least d max equivalent height is suited to measure S c for small SRS fields. The use of materials with higher electron/physical density can be used to reduce the size of the mini-phantom and reduce spatial averaging.