Use of a dynamic rangeshifter for modifying the depth-dose distributions of negative pions

In order to use negative pions for the treatment of large deep-seated tumors in radiotherapy, it is necessary to produce depth-dose distributions tailored to specific shapes. We present here a method of beam shaping which utilizes a fluid-filled piston having a programmable, computer-controlled, time-dependent thickness. The fluid alters the residual range of the pions such that predetermined depth-dose distributions can be obtained. Changing from one distribution to another can be accomplished simply and rapidly without access to the treatment room. Depth-dose distributions which are flat over a range in depth up to 10 cm have been produced. Distributions tailored to produce flat "effective dose" versus depth have also been obtained.