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An alternative approach to setting Action
Levels
AAPM TG 218 (2018)
•
Preferable to use ‘universal’ action limits (correlated to
outcome)
•
If not available then use statistical process control to
determine action limits
•
∆ =
2
+ ( ҧ − )
2
where
∆
is the difference between
upper and lower action limits, is a factor derived from
process capability and is set at 6.0 at present,
2
and
ҧ
are the
process variance and mean.
•
Idea is to calculate mean and variance over eg 20 IMRT QA
measurements when process is in control then use these to
determine action levels.
➢
So it reflects what the system is capable of.