"In 1947 Leksell visited Wycis in Philadelphia and then developed and described his instrument in a publication in 1949. This was the first example of a stereotactic system based on the principle of ‘‘center-of-arc’’.^[8] In contrast to the Cartesian coordinate system of the Spiegel-Wycis device, Leksell’s frame utilized three polar coordinates (angle, depth and anterior–posterior location). This ‘‘arc-quadrant’’ device provided maximum flexibility in choosing probe entry point and trajectory, and was therefore much easier to use. The frame has been modified over the ensuing years, but remains remarkably similar in function and appearance to the original 1949 device.^[9] The use of a movable semi-arc with an electrode carrier implies that the tip of a probe can reach the target regardless of the position of the carrier or the angling of the arc relative to the skull fixation device, a frame or base plate with bars for bone fixation screws. This construction permits also transphenoidal, straight lateral and suboccipital probe approaches. Leksell was in many respects a perfectionist and for the rest of his life he continued to change and revise the design of virtually every small part of his instrument though the basic semicircular frame was retained. He focused not only on upgrading the function of the instrument but also on its aesthetic appearance. An important feature was that ‘‘the apparatus should be easy to handle and practical in routine clinical work’’ and ‘‘a high degree of exactitude is necessary.’’ An oft-cited quotation is ‘‘Tools used by the surgeon must be adapted to the task and where the human brain is concerned, no tool can be too refined.’’^[10] (Wikipedia article on Lars Leksell, accessed 3-2020).

Leksell Gamma Knife. "Over the subsequent 50 years Gamma Knife surgery has evolved to cover much of what is done in neurosurgery and there are more than 330 Gamma Knife centers all over the world. By the end of 2017 more than 1.2 million patients had undergone Gamma Knife surgery" (Niranjan, Lunsford, Kano (eds),"The origins and development of radiosurgery and the Leksell Gamma Knife," Leksell radiosurgery. Prog. Neurol. Surg. 34 (2019) 1-8.