1. IAEA E-learning Program
Safety and Quality in Radiotherapy

2. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Sections:
9.1 Purpose and principles
9.2 Treatment planning
9.3 Information transfer
9.4 Calibration
We will first look at the purpose and principles of Fault Tree Analysis and then perform demonstration Fault tree Analyses for treatment planning, Information transfer and calibration failures.

3. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Sections:
9.1 Purpose and principles
9.2 Treatment planning
9.3 Information transfer
9.4 Calibration
In the previous section, 9.1 we had a general discussion of Fault Tree Analysis. In the section we’ll explore the development of a Fault Tree around an error that might happen during the treatment planning process.

4. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
OBJECTIVES
To develop a fault tree describing a potential error in the calculation of monitor units due to distractions.
To suggest a few systematic, organizational issues which, if addressed, could minimize the probability of such errors.
To reinforce the need for quality control as a component of a critical process.
The main objectives for section 2 are, to develop a fault tree describing a potential error in the calculation of monitor units due to distractions, and to suggest a few systematic, organizational issues which, if addressed, could minimize the probability of such errors. 

5. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
A High Level Process Map1
So, here we are in the treatment planning step of the overall radiotherapy process.

6. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Failure Modes and Effects Analysis: Treatment Planning
Here’s our Failure Modes and Effects Analysis that we developed in Section 8.2. As we saw in Section 9.1 we use the results of a Failure Modes and Effects Analysis to select those Failure Modes and Failure Pathways to concentrate on.

7. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
A Fault Tree
Event/situation/circumstance
AND OR
We choose one potential Failure Mode and one Failure Pathway from the FMEA on the previous slide and use those to generate a Fault Tree of the type displayed here.

8. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Event/situation/circumstance
AND OR
Failure Mode
Potential Failure Mode
We can enter the potential Failure Mode in the left most box

9. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Event/situation/circumstance
AND OR
Failure Pathway
Basic Cause
Latent Condition
Potential Failure Mode
In the right most boxes we speculate on the causes, situations or circumstances that could lead to such a Failure Mode. As we discussed during our Root Cause Analysis, Module 6, the items in the right most boxes should be actionable. In other words, once we have suggested the causes of the potential Failure Mode we need to develop strategies to minimise the probability of these causes actually leading to a failure.

10. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Calgary Basic Cause Table2
We can take Basic Causes from this taxonomy to give it some structure.

11. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Failure Pathway: Distraction
Event/situation/circumstance
AND OR
Wrong MU sent to machine
Our potential Failure Mode is that the wrong Monitor Units are sent to the machine for treatment and the Failure Pathway that we’ll explore is “distractions”. Distractions are common in a busy clinic and they can easily lead to errors. We saw in Module 6.2 that distractions can lead to slips in skill based tasks. As we go through this analysis, we might need to revise our “AND” and “OR” gates in this diagram.

12. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Failure Pathway: Distraction
Event/situation/circumstance
AND OR
Error in plan
Wrong MU sent to machine
Now, we can ask what could happen (or not happen) along the pathway to the potential Failure Mode. We are looking at the situation where an error was made in the plan and, let’s suppose, the plan is not checked before being sent to the machine. Both of these conditions would have to be met (assuming that a perfect checking procedure could be developed) for the Failure Mode to happen. Hence, these two boxes are joined by an “AND” gate.
No checks done

13. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Failure Pathway: Distraction
Event/situation/circumstance
AND OR
Distraction
Error in plan
Critical phase in planning
Wrong MU sent to machine
Why might there be an error made in the plan? A likely answer is that the planner was distracted or interrupted during a critical part of the plan generation. These circumstances are joined by an “AND” gate.
No checks done

14. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Failure Pathway: Distraction
3.1 Inadequate hazard assessment
Event/situation/circumstance
AND OR
Distraction
Error in plan
4.1 Inadequate work planning
Critical phase in planning
Wrong MU sent to machine
Now we have to postulate what causes or other factors increase the likelihood of distractions occurring. Here, we are suggesting 2 from the basic causes table. Perhaps the potentially serious consequences of distractions are not appreciated. Distractions are just accepted as part of the normal working day. We can describe this situation as “Inadequate hazard assessment” Another possibility is that the layout of the work area is such that there is no quiet space in which to fully concentrate on the work at hand. Since  either of these possible systemic weaknesses predispose the planning process to an error we join them with an “OR” gate. We have noted before and we note again that what we call basic or root causes do not make the Failure inevitable. The causes we postulate here do, however, make a Failure more likely. Note also that both the causes identified here are actionable. Work areas can generally be re-arranged. A strategy for minimising interruptions is discussed in Module 10.
No checks done

15. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
Failure Pathway: Distraction
3.1 Inadequate hazard assessment
Event/situation/circumstance
AND OR
Distraction
Error in plan
4.1 Inadequate work planning
Critical phase in planning
Wrong MU sent to machine
Turning to the lower branch which addresses the quality control aspects of the planning process, we can speculate on the reasons for no checks being done. It’s possible that there is no requirement in place to ensure plans are checked before being sent to the machine. It is also possible that there is a policy but, because of staff shortages, the policy is not being followed . With these causes now identified it is possible to consider the changes necessary to minimize the probability of this particular potential Failure Mode from being realised in practice.
1.1 No policy developed
No checks done
4.6 Personnel availability

16. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
A few final notes
Basic causes identified in the right most boxes of a fault tree do not, in general, make failure inevitable.
To be useful, basic causes should be actionable.
Addressing the basic causes identified with a fault tree should reduce the occurrence value in the accompanying failure modes and effects analysis.
Incorporating quality control into a fault tree should reduce the detectability value in the accompanying failure modes and effects analysis.
Always be aware that changing processes, including QC, has the potential to introduce other sources of error.
We saw this in Section 9.1 but it’s worth repeating.
Basic Causes identified in the right most boxes of a Fault Tree do not, in general, make Failure inevitable.
To be useful, Basic Causes should be actionable.
Addressing the Basic Causes identified with a Fault Tree should reduce the Occurrence, O, value in the accompanying Failure Modes and Effects Analysis.
Incorporating Quality Control into a Fault Tree should reduce the (un)Detectablity, D, value in the accompanying Failure Modes and Effects Analysis.
Always be aware that changing processes, including QC, has the potential to introduce other sources of error.

17. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
summary
We have:
Developed a fault tree describing a potential error in the calculation of monitor units due to distractions.
Suggested a few systematic, organizational issues which, if addressed, could minimize the probability of such errors.
Reinforced the need for quality control as a component of a critical process.
To summarize, we have
Developed a Fault Tree describing a potential error in the calculation of Monitor Units due to distractions.
Suggested a few systematic, organizational issues which, if addressed, could minimize the probability of such errors.
Reinforced the need for quality control as a component of a critical process.

18. Safety and Quality in Radiotherapy
MODULE 9: fault tree analysis
Safety and Quality in Radiotherapy
Section 2: treatment planning
References and additional resources
E Ford, L Fong de los Santos, T Pawlicki, et al.. Consensus recommendations for incident learning database structures in radiation oncology. Medical Physics 39 (2012)
Cooke DL, Dubetz M, Rahim H, et al