Changes in incident photon fluence of 6 and 18 MV x rays caused by blocks and block trays.

When a block and tray are placed in a x-ray beam the dose to a point in a phantom is changed by the following factors: (1) attenuation of photon and electron fluence from the head of the accelerator by the tray and the block, (2) decrease in the scatter in the phantom by a reduction in the phantom volume that receives radiation, and (3) generation of scatter off the tray and block. This third factor is generally ignored in dosimetry calculation but has been measured in this work. Measurements of incident photon fluence for 6 and 18 MV x rays were made with a columnar miniphantom of 10 cm depth. The tray factor for a 9 mm thick Lexan tray is found to be variable and to increase by 1.8% due to scatter off the tray when the field size is increased from a 3cm x 3 cm to 40cm x 40 cm field. Also, it was found that scatter off a block could increase the incident photon fluence by as much as 2%. The magnitude of this block scatter depends on the length of the inner edge of the opening in the block and on amount of block that is being irradiated, the overlap of the block by the radiation field. The total block-tray factor can be as much as 3% larger than the single-value tray factor measured with a 10cm x 10cm field that is traditionally used. An analytical equation is developed that accurately models the block-tray factor.