196 MI-42-0001 Rev. 19
QUALITY TESTING: Exposure Testing
Sources of
Variability
1. Variability associated with ion chamber. Generally, properly calibrated ion chambers are
expected to perform within an error margin of well under 5%.
2. Variability associated with entrance exposure measurement at 12 o'clock. Since the radiation
beam may not turn on and off exactly at the 12 o'clock position during a single spin, some
variability may be observed in Entrance mR measurements. If the error in step 7 is found to
exceed 20%, it is recommended that the average of measurements at the 3, 6, 9, and 12
o’clock positions is used to calculate Entrance mR, to rule out variability associated with the ion
chamber at this location.
3. Operator error. If errors in step 7 exceeds 20% despite ruling out the variability at the 12 o'clock
position, precise positioning of the dosimetry phantoms at isocenter, as well as stationary
positioning of the Ring with respect to the table during the exposure should be verified.
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