Development of the new type ridge filter to modulate width of SOBP in each lateral position

In a conventional method of particle beam therapy (2D irradiation) treatment, beams are laterally broadened by a pair of wobbler magnets and a scatter, and the sharp Bragg peaks are longitudinally broadened by a ridge filter. The thickness of SOBP is fixed in the whole radiation field. The distal edge of the irradiation volume is shaped to target by using range compensator. The constant SOBP over the field results in undesirable dose to the normal tissue in front of the target. Usually in the peripheral part of the target, 100% dose level will be administered to the normal tissue in the 2D irradiation. To reduce this extra dose, we propose and develop the new type ridge filter, which forms various width of SOBP corresponding to the target thickness in each position. However we tested the energy filters of multi layer type up to now, by the problem of accuracy of setting position, it was difficult to form the ideal SOBP. Next, we develop the Cone shaped energy filter. And now, we have some experiment with this new filter. (author)