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Session: Advances in Safety [Return to Session]

Using System Theoretic Process Analysis to Design a Commissioning Procedure for MLC Tracking

J Hindmarsh1*, S Dieterich2, J Booth3, P Keall1, (1) ACRF Image X Institute, Faculty of Medicine and Health, University of Sydney, Eveleigh, NSW, AU, (2) Department of Radiation Oncology, UC Davis Medical Center, Sacramento, CA, (3) Northern Sydney Cancer Centre, Royal North Shore Hospital, St Leonards, NSW, AU


TU-H-BRA-6 (Tuesday, 7/12/2022) 2:45 PM - 3:45 PM [Eastern Time (GMT-4)]

Ballroom A

Purpose: To design commissioning guidelines for multi-leaf collimator (MLC) tracking based on the results of System Theoretic Process Analysis (STPA).

Methods: STPA is a deductive hazard analysis technique intended for use on complex systems. It is based on system theory and was developed in the 2000’s by Leveson. MLC tracking, a real time adaptive treatment technique, is a potentially hazardous complex system. STPA was applied to MLC tracking by first creating a system diagram covering plan approval through to treatment. This diagram maps the flow of control and feedback between the sub-systems included in the overall system, including human, mechanical and software elements. Using key phrases, such as “not provided” and “provided too late”, the ways the included control actions could become unsafe were generated. Then, using the system diagram as a reference, scenarios with the potential to lead to unsafe control actions were produced. These scenarios were used to identify where and how to prevent the control actions becoming unsafe by the implementation of countermeasures.

Results: The analysis of MLC tracking using STPA resulted in 46 unsafe control actions. Many scenarios were identified, and the resulting countermeasures were grouped and combined to produce MLC tracking commissioning guidelines.The commissioning guidelines include 11 questions, 2 actions to take and 3 statements to check. The 11 tests are used to inform further testing and the development of procedures and processes, such as ‘what is the impact of losing connection during treatment’. The 2 actions assist in ongoing QA. The 3 statements to check, ‘treatment can’t start without…’, ‘beam hold is activated if…’, and ‘is user notified if…’ are intended to have binary answers that can help determine potential weaknesses in the system’s safety.

Conclusion: STPA was applied to MLC tracking and has facilitated the generation of commissioning guidelines

Funding Support, Disclosures, and Conflict of Interest: This research was supported by a Cancer Institute NSW Translational Program Grant


Quality Assurance, MLC, Commissioning


IM/TH- Formal Quality Management Tools: Failure modes and effects analysis

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