Dependence of the tray transmission factor on collimator setting and source-surface distance

Herman Van Kleffens*, Jack Venselaar, Stan Heukelom, Niek Jager, Ben Mijnheer, Rob Van Der Laarse, Hans Van Gasteren, Carel Westermann

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review


When blocks are placed on a tray in megavoltage x-ray beams, generally a single correction factor for the attenuation by the tray is applied for each photon beam quality. In this approach, the tray transmission factor is assumed to be independent of field size and source-surface distance (SSD). Analysis of a set of measurements performed in beams of 13 different linear accelerators demonstrates that there is, however, a slight variation of the tray transmission factor with field size and SSD. The tray factor changes about 1.5% for collimator settings varying between 4 x 4 cm and 40 x 40 cm for a 1 cm thick PMMA tray and approximately 3% for a 2 cm thick PMMA tray. The variation with field size is smaller if the source-surface distance is increased. The dependence on the collimator setting is not different, within the experimental uncertainty of about 0.5% (1 s.d.), for the nominal accelerating potentials and accelerator types applied in this study. It is shown that the variation of the tray transmission factor with field size and source-surface distance can easily be taken into account in the dose calculation by considering the volume of the irradiated tray material and the position of the tray in the beam. A relation is presented which can be used to calculate the numerical value of the tray transmission factor directly. These calculated values can be checked with only a few measurements using a cylindrical beam coaxial miniphantom. (C) 2000 American Association of Physicists in Medicine.

Original languageEnglish
Pages (from-to)2117-2123
Number of pages7
JournalMedical Physics
Issue number9
Publication statusPublished - 1 Jan 2000

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