Abstract
It is common practice to attach plastic shadow trays to the collimating heads of megavoltage teletherapy units for holding lead shielding blocks. The presence of the trays leads to some electron contamination in the beam and to consequent increase in the skin dose. Using lithium fluoride and thin sheets of Lucite, the authors have measured actual doses at the surface and at depths of up to 25 mm in a Lucite phantom for 6 MV X-rays, both with and without a shadow tray. As expected, the increase in dose due to the tray depends on the field size. For example, for a field of 25*25 cm at an SSD of 100 cm, the tray (at a distance of 30 cm from the surface) increases the surface dose from 37 to 53% of the maximum dose, and the dose at a depth of 1 mm from 50 to 75%. The depth for an 80% dose changes from 2.6 to 1.4 mm. For a field of 5*5 cm, the changes are negligible. That the changes are due to electrons is very simply demonstrated by introducing a thin lead sheet under the shadow tray, when the doses return to their pre-tray levels. These results and their implications for skin-sparing effects were fully discussed.