IEC 60601-2-11:2013 applies to the basic safety and essential performance of gamma beam therapy equipment, including multi-source stereotactic radiotherapy equipment. This particular standard of the 60601 series, establishes requirements to be complied with by manufacturers in the design and construction of gamma beam therapy equipment. It states tolerance limits beyond which interlocks must prevent, interrupt or terminate irradiation in order to avoid an unsafe condition. Type tests which are performed by the manufacturer, or site tests, which are not necessarily performed by the manufacturer, are specified for each requirement. It does not attempt to define the optimum performance requirements for a gamma beam therapy equipment for use in radiotherapy. Its purpose is to identify those features of design which are regarded at the present time as essential for the safe operation of such equipment. It places limits on the degradation of equipment performance at which it can be presumed that a fault condition applies, e.g. a component failure, and where an interlock then operates to prevent continued operation. This third edition cancels and replaces the second edition of IEC 60601-2-11 published in 1997 and its Amendment 1:2004. This edition constitutes a technical revision which brings this standard in line with the third edition of IEC 60601-1 and its collateral standards.