Multi-layer energy filter for realizing conformal irradiation in charged particle therapy

A new type of filter for charged particle radiotherapy is developed to redu ce unwanted dose transfer to the normal tissues around a tumor. The new fil ter can make a static irradiation field where the width of the spread-out B ragg peak (SOBP) is two-dimensionally adjusted. That makes the field confor mal to the tumor three-dimensionally. The Alter is made of many layers prod uced by using stereolithography. The layer has a miniaturized structure tha t has geometrical similarity to the conventional ridge filter. Shapes of co ne and pyramid are also usable for the unit-cell constructing the layer. Th e spread of the field in the depth direction is decided by the thickness of the filter, or by the number of layers. The experimental result of the irr adiation using the ridge-type construction shows a good agreement with an e stimate by the Monte Carlo calculation. By combining this technique with in tensity modulation that has lateral position dependence, the conformal irra diation can be achieved by a simple procedure. (C) 2000 American Associatio n of Physicists in Medicine. [S0094-2405(00)00902-0].