Factors affecting penumbral shape and 3D dose distributions in stereotactic radiotherapy

Linear accelerator based stereotactic radiotherapy or radiosurgery is usually performed with small collimated circular beams. The penumbral shape of these beams is caused by geometric penumbra and radiation penumbra; the former is affected by collimator position and focal spot size, the latter by energy and field size. This note reports measurements of penumbral width as collimator position, size and energy are varied. Radiation penumbra is a much greater effect than geometric penumbra at the distances used. The profile can also be changed by adding flattening filters to the external collimator. Adding a shaped filter to increase the primary radiation at the edges of the beam improves the shape of the profiles. What is of clinical importance is not the penumbral shape itself, but the effect it has on the three-dimensional dosage distributions. Cumulative dose-volume histograms (DVHS) have been used to compare treatment plans produced by beams with different penumbral shapes. None of the aforementioned factors changes the DVH as much as a change in the field radius of a few mm. Ensuring that sufficient collimators are made, and reducing the interval between successive field sizes, is the single most important thing that can be done to avoid irradiating unnecessarily large volumes of tissue.