Anomalous primary radiation from the Leksell Gamma Knife Unit

The Leksell Gamma Knife Unit, Type U, utilizes 201 separate 60Co sources intersecting at a common focus for radiosurgical treatment of the brain. It has been generally understood that all primary unattenuated radiation beams are confined within the protective housing during all phases of Gamma Knife operation. We have found that when the unit's shielding door is lowered for treatment, twelve primary beams exit the unit's door opening. Furthermore, in the original Gamma Knife unit design, two of the twelve primary beams failed to be attenuated by either the unit base or the unit wings. This has implications for the design of structural shielding and the protection of personnel during emergency procedures. Engineering modifications to eliminate the problem are discussed.