Compensation in three-dimensional non-coplanar treatment planning

This paper presents a technique for producing uniform dose distributions within the target volume with noncoplanar field arrangements. The method is based upon the principle used for producing homogeneous dose distributions for a pair of fields in two dimensions, namely, that if the isodose distributions for the fields are made parallel to one another, the combined dose distribution will be uniform. For the three-dimensional non-coplanar case, homogeneous target dose distributions are obtained by designing field modifiers which produce a uniform dose distribution on the perpendicular bisector plane for each pair of fields. It is demonstrated that for three noncoplanar fields irradiating a spherical phantom with spherical target volumes, the target dose distribution will be homogeneous for any arbitrary non-coplanar field arrangement. Furthermore, this technique can be extended to any number of fields. Therefore, target dose distributions for non-coplanar plans can be as homogeneous as the coplanar case. An example of the application of the method to the treatment of rectal carcinoma with non-coplanar fields is given. Furthermore, it is demonstrated that the therapeutic ratio is improved over traditional methods for this clinical example.