A generalized film technique for the verification of vertex fields used in the treatment of brain tumors.

With the availability of commercial three-dimensional (3D)-treatment planning systems, more and more treatment plans call for the use of noncoplanar conformal beams for the treatment of brain tumors. However, techniques for the verification of many noncoplaner beams, such as vertex fields which involve any combination of gantry, collimator, and table angles, do not exist. The purpose of this work is to report on the results of an algorithm and a technique that have been developed for the verification of noncoplanar vertex fields used in the treatment of brain tumors. This technique is applicable to any geometric orientation of the beam, i.e., a beam orientation that consists of any combination of gantry, table, and collimator rotations. The method consists of superimposing a central plane image of a correctly magnified vertex field on a lateral or oblique field port film. To achieve this, the 3D coordinates of the projection of the isocenter onto the film for lateral (or oblique) as well as the vertex fields are determined and then appropriately matched. Coordinate transformation equations have been developed that enable this matching precisely. A film holder has been designed such that a film cassette can be secured rigidly along the side rails of the treatment table. The technique for taking a patient treatment setup verification film consists of two steps. In the first step, the gantry, table, and collimator angles for the lateral (or oblique) field are set and the usual double exposures are made; the first exposure corresponds to that of the treatment portal with the isocenter clearly identified and the second one a larger radiation field so that the peripheral anatomy is visible on the film. In the next step, the gantry, table, and collimator angles are positioned for the vertex field and the table is moved laterally and vertically and the film longitudinally to a position that will enable precise matching of the isocenter on the film. A third exposure is then taken with the vertex portal. What is seen on the film is a superposition of a central plane image of the vertex field onto the image of the lateral or oblique field. This technique has been used on 60 patients treated with noncoplanar fields for brain tumors. In all of these cases, the coincidence of the projection of the isocenter for the lateral (or oblique) and the vertex fields was found to be within 3 mm.