This report describes a computer control system for a radiation therapy machine with an isocentric gantry and a multileaf collimator. It discusses the motivation and rationale for some of the features and development activities, and reports several measures of development effort, performance, and quality, determined after two years of operating experience.

Notable features of the control system include construction based on standard (vendor-independent) hardware and software components connected by a network, a simple and efficient user interface, close integration with the treatment planning system, automated record keeping for patient quality assurance and machine maintenance, and ease of maintenance and upgrading. Separation of control functions from data management functions results in a control program which is small, fast, and feasible to analyze thoroughly. Choice of features and internal design were informed by fifteen years experience using and maintaining computer controlled therapy machines.

The development method was chosen to ensure exceptional safety, reliability, and clinical acceptability. Much of the detailed specification was expressed in formal (mathematical) notation. This formal specification (not the prose description) served as the basis for most coding, test planning, and safety analysis. Testing and reviews were supplemented by new automated analyses methods including theorem proving and model checking.

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