Effect of block trays on skin dose in radiotherapy.

Percentage dose at the surface and at 1 mm depth for megavoltage photon beams are increased through the influence of block trays. This represents a decrease in skin sparing properties for both the epidermal and dermal layers. The increase in percentage dose varies with type and thickness of block tray material. At 6MVp, 20cm x 20cm field size, the percentage surface dose is 26%, 26.5%, 33% and 35% for open, steel honeycomb tray, 6mm perspex and 10 mm perspex block trays respectively. At 1 mm depth these values are 52%, 52.5%, 61%, 60% respectively. A similar effect is seen at higher energies. Results show that care should be taken when selecting an appropriate block tray if skin sparing is of importance.