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

When blocks are placed on a tray in megavoltage x-ray beams, generally a si ngle correction factor for the attenuation by the tray is applied for each photon beam quality. In this approach, the tray transmission factor is assu med to be independent of field size and source-surface distance (SSD), Anal ysis of a set of measurements performed in beams of 13 different linear acc elerators demonstrates that there is, however, a slight variation of the tr ay transmission factor with field size and SSD, The tray factor changes abo ut 1.5% for collimator settings varying between 4x4 cm and 40x40 cm for a 1 cm thick PMMA tray and approximately 3% for a 2 cm thick PMMA tray. The va riation with field size is smaller if the source-surface distance is increa sed. The dependence on the collimator setting is not different, within the experimental uncertainty of about 0.5% (1 s.d.), for the nominal accelerati ng potentials and accelerator types applied in this study. It is shown that the variation of the tray transmission factor with field size and source-s urface 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 calcula te the numerical value of the tray transmission factor directly. These calc ulated values can be checked with only a few measurements using a cylindric al beam coaxial miniphantom. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)00209-1].