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

Figure 4

The dose profile of the smaller 4 × 2 cm² field irradiated at a gantry angle of 0° at the flat polystyrene solid phantom, followed by a second exposure in the ball phantom with a larger field of 6 × 4 cm² at different gantry angles. The dose profile changes after second exposures at a gantry angle different from the first exposure. It also shows the full-width half maximum (the width at 50% dose of the −x axis and the +x axis) and the penumbra width from 80% to 20% of the central axis.