A series of contiguous treatment fields is frequently used in radiotherapy, as with irradiation of the lymphatic chain for testicular tumors and lymphomas. With the desire to deliver a homogeneous dose to target volumes in depth, it is necessary to place adjacent treatment portals on the skin surface in such a manner that "hot" spots in normal tissue areas and "cold" spots in tumor volumes are avoided.A method of separating fields is presented where isodose curve measurements or geometric calculations are modified by experiment, using lithium fluoride and Adlux film in phantoms. It is shown that these separated fields, while giving homogeneous doses in depth, avoid high doses to overlying regions. Examples are described to illustrate the clinical importance of these considerations.