Determination of beam orientation in radiotherapy planning.

A computer-assisted method for selecting beam orientation in a one-step procedure is presented. Inverse and adjoint techniques are developed to obtain the best beam directions. Both methods rely on determining the "path of least resistance" to radiation from the tumor location to the surface of the section. The effectiveness of beam directions is then determined by monitoring the dose distribution along the section boundary. The inverse and adjoint calculations are performed for three tumor cases using a two-dimensional discrete ordinates transport code. The proposed treatment plans from these calculations are verified against typical treatment plans. The new techniques improved the dose distribution in the treated section. The inverse calculations are useful in sections involving low-density tissues. The adjoint technique can effectively deal with multiple target volumes and/or sections with complex geometry. The proposed method is potentially useful in selecting beam orientations for three-dimensional planning systems and in determining beam intensities in rotational and conformal therapy.