3D thin lead sheet compensating system.
The compensating filter system presented consists of a simulator mounted camera and light beam device. The light beam device produces a flash light beam 3-mm wide perpendicular to the x-ray axis. The distance between flashes can be varied from 0.5 to 2.5 cm in steps of 0.25 mm. An exposure of the patient in treatment position subject to a series of light flashes will produce a three-dimensional representation of the patient surface. This map is then properly enlarged. Lead sheets are cut and fixed to a Perspex plate which fits into a slot in the head of the accelerator. Regardless of energy, all compensators are made from 0.5-mm-thick lead sheets. To determine the proper distance between light flashes, measurements were performed in a water phantom for each x-ray energy to establish the equivalence of increasing numbers of layers of 0.5-mm lead. These measurements were compared to theoretical calculating using tissue maximum ratios, and finally checked against a wedged water phantom of 25 degrees as well as against a head and neck treatment field for the Alderson phantom. This way, a beam flattening of +/- 2% compared to the standard isodose lines was achieved.