Optically guided linac radiosurgery with a Leksell head frame as an adjunct to Gamma Knife treatment

Because of geometrical limitations in the helmet of the Leksell Gamma Knife(Elekta Corp., Atlanta, GA, USA) certain regions within the cranium cannot be targeted for treatment. We describe a method by which lesions in these regions can be treated with the Varian-Zmed stereotactic radiosurgery system utilizing an infrared optical positioning system attached to a Leksell head frame. We have measured the accuracy of the optical tracking system using a phantom attached to a Leksell frame and have determined that the system can target a linear accelerator radiosurgery beam to an accuracy of within 1 millimeter.