A Rapid Optical Method for Preparation of Complex Field Shapes

A technic is presented for the accurate preparation and positioning of lead shadow blocks for shielding vital organs in irregularly shaped, anterior and posterior opposing megavoltage treatment fields. The essence of the technic is the employment of the Polaroid MP-3 Land camera for recording as a transparency the shaped field as outlined by the radiotherapist on the patient's localization films. The transparency is projected, using a simple geometric formula to obtain a tracing of the true size and shape of the lead blocks at the appropriate distance. It was developed particularly for use with the Varian Clinac, but its principles are more widely useful. A search of the literature fails to reveal a comparable technic, and, therefore, the technic de. scribed in this communication is given in detail so that others may modify and use it in their own particular facilities. Materials and Methods Figure 1 is of the simulator which exactly duplicates the geometry of the Varian Clinac. The treatment beam is replaced by a diagnostic x-ray unit. Thus, a film which includes the treatment field is geometrically identical to a Clinac port film. As with the Clinac treatment couch, the simulator couch moves in all three axes, and the simulator head rotates 3600 about the patient. The position of the head is marked in degrees on an easily visible scale as in the Clinac and is therefore quite reproducible. X-ray film is located in a cassette-holder 5.0 cm beneath the couch top. The simulator table resting on the couch and positioned over the patient (Fig. 2) is similar to that used to support, 5 cm lead blocks during treatment. Since the top of the simulator table should correspond to the top of the lead blocks, it is 5 cm higher than the treatment table. Thus, the simulator table is 45.4 em and the treatment table is 40.4 cm above the couch top (Fig. 4). The simulator table has wires stretched beneath its Plexiglas top in a grid pattern forming 5.1 × 5.1-cm squares. These are projected by the simulator light field onto the patient's surface in positions identical to those projected by the diagnostic tube to the film beneath him. The center of the treatment field is determined on one surface of the patient by physical examination, radiographs or prior convention. The simulator head is used as a back-pointer to produce a corresponding center on the patient's opposite surface by positioning the initial center over the simulator cross-hairs with the simulator beneath the patient. The corresponding center is marked at the cross-hair shadow when the machine is rotated to the vertical position. The table is then positioned over the patient so that the central grid wires correspond to the cross-hairs in the simulator light, which indicate the center of the field.