Answer to Question #10824 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Equipment

The following question was answered by an expert in the appropriate field:

Q

I would like to know the way to measure the “ON” position leakage measurement in 60Co teletherapy machines. I used a pocket dosimeter as a detector but I am hesitant to accept its accuracy. When I tried with a Farmer® ionization chamber I do not get any charges so I converted the detector into pocket dosimeter. Please suggest the correct way to measure the “ON” position leakage in the patient plane and in other than the patient plane.

A

This is a difficult task. As you mentioned, typical Farmer® chambers are not sensitive enough to make this measurement. Two common methods are:

  1. If you have an integrating survey meter, setting that at measurement points and integrating over exposures for which the allowed exposure would put you about mid-scale.
  2. If you have a good television camera, setting it to view an exposure-rate-reading survey meter.

You had mentioned using a pocket dosimeter for the measurements but being concerned about the accuracy. I assume this is an integrating pocket dosimeter. If the detector is NOT a low-energy model and has been calibrated using a 137Cs beam, the energy response difference with 60Co should not be enough to invalidate it for these measurements. These measurements are for protection and regulatory compliance, not to determine patient treatment doses, so the accuracy of the pocket chamber should be adequate. If that pocket dosimeter is the equipment you have available to make this measurement and satisfies the two requirements above (not a low-energy model and calibrated with 137Cs), you should be fine using it.

Bruce Thomadsen, CHP

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