The relationship between the number of shots and the quality of Gamma Knife radiosurgeries

Radiosurgery is a non-invasive alternative to brain surgery that uses a single focused application of high radiation to destroy intracerebral target tissues. A Gamma Knife delivers such treatments by using 201 cylindrically collimated cobalt-60 sources that are arranged in a hemi-spherical pattern and aimed to a common focal point. The accumulation of radiation at the focal point, called a "shot" due to the spherical nature of the dose distribution, is used to ablate (or destroy) target tissue in the brain. If the target is small and spherical, it is easily treated by choosing one of four available collimators (4, 8, 14, or 18 mm). For large, irregular targets multiple shots are typically required to treat the entire lesion, and the process of determining the optimal arrangement and number of shots is complex. In this research, fast simulated annealing and a novel objective function are used to investigate the relationship between the number of shots and the quality of the resulting treatment. Sets of 5, 10, 25, 50, and an unrestricted number of shots are studied for an arteriovenous malformation (AVM). As the shot limit increases the following improvements in plan quality are observed: the conformity of the prescription isodose line increases, the lesion dose becomes more homogeneous, and an increase use of smaller collimators to deposit dose. Large improvements in plan quality are realized by increasing the number of shots from 5 to 50, and to achieve a similar magnitude of improvement past 50 requires an increase over 1500 shots for the complex lesion investigated. This observation suggests that it is clinically valuable to improve the Gamma Knife's delivery capabilities so that 50 shot treatments are possible.