Extended field treatment flatness filter for 4 MV linear accelerators

An auxiliary flatness filter for large field treatment on 4 MV linear accelerator provides an isodose distribution which does not vary more than 6% over the central 80% of the axis of a 32 x 32-cm field at 80 cm in air, and at a 2.5, 6, and 12-cm depth in water. This lead disc filter does not affect the central axis depth dose values, and results in only a 16% reduction in dose rate on the central field axis.