A digital method for computing target margins in radiotherapy

Computer methods for determining the planning target volume from the gross tumor volume for both conventional and conformal radiotherapy are presented. Production of a two-dimensional (2D) treatment plan is assisted by projecting outlines of the gross tumor onto a single transverse plane, so that the total extent of the tumor can be easily visualized. A 2D margin can then be added to the resulting outline, so as to account for microscopic tumor spread, organ motion, and setup uncertainty. The margin may be anisotropic to account for the known differences in setup accuracy in the anterior, posterior, left and right directions. For three-dimensional (3D) treatment planning, it is necessary to add a 3D margin to the gross tumor volume to define the planning target volume, the anisotropy of the margin now being allowed to extend to the superior and inferior directions also. Robust methods for automatically calculating these regions are described, and illustrated for the case of a prostate tumor. It is demonstrated that while a slicewise 2D margin is adequate for 2D planning, a fully 3D margin must be used for 3D conformal planning to avoid underdosing the superior and inferior extremities of the clinical target volume. (C) 1998 American Association of Physicists in Medicine.