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Session: Quality and Safety in Radiotherapy I [Return to Session]

System Modeling for Safety Analysis: Leveraging Process Maps to Create Control Structures

L Wong*, T Pawlicki, UC San Diego, La Jolla, CA


SU-E-BRA-6 (Sunday, 7/10/2022) 1:00 PM - 2:00 PM [Eastern Time (GMT-4)]

Ballroom A

Purpose: A model provides a basis for any safety analysis. A process map is the model integral to analyses such as Failure Mode and Effect Analysis. A control structure is the counterpart for Systems-Theoretic Process Analysis (STPA). Creating a control structure can be challenging due to its novelty in healthcare applications. The purpose of this work is to provide a method to create a control structure from an existing process map.

Methods: A safety control structure is a hierarchical collection of engineering control loops where connected levels impose constraints on the activities of one another, and accidents are viewed as the consequence of inadequate control of safety constraints. Information that is used in control structure creation is extracted from the process map by: 1) identifying the decision makers (controllers) for each process map step, 2) identifying the process being controlled, 3) identifying the tasks and activities in the process map steps, 4) categorizing the tasks and activities into control actions, feedback (by relabeling, as applicable), process inputs, and process outputs.

Results: The method was applied to a published process map modeling the production of digitally reconstructed radiographs (Ford et al, 2009). The source process map has 18 steps involving three actors and four decisions. The product control structure has three controllers, five control actions, and two feedback items.

Conclusion: While some information and the underlying modeling philosophy are different between process maps and control structures, some similarities exist that can be used in the development of control structures. This could facilitate a wider application of safety improvement using a systems approach.

Funding Support, Disclosures, and Conflict of Interest: UC San Diego receives research funding from Varian Medical Systems.


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