An Empirical Model for Describing the Small Field Penumbra in Radiation Therapy

Figure 1

The custom-made ball phantom for the second film exposure. The , y, and z axes of the film are defined on this figure. The second exposure with a 6 × 4 cm² field was given to the film that was previously irradiated with a 4 × 2 cm² field. The film was embedded in the equator plane with the longitudinal direction parallel to the y axis at different gantry angles for in-field penumbra perturbation study. Coplanar irradiation denotes that all the irradiations were delivered along the axis with the same isocenter.