Answer to Question #10905 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Could you please state the frequency of beam output calibration of a teletherapy machine and frequency of calibration for the ion chamber used for dosimetry of these machines? Could you please mention references of the U.S. Nuclear Regulatory Commission (NRC) and the International Atomic Energy Agency (IAEA)?
A. The NRC requires output calibration of a 60Co teletherapy unit annually [10 CFR 35.632 (a)(3)], with checks of the output once in each calendar month [10 CFR 35.642(a)]. The dosimetry system, that is the ionization chamber, must have a calibration as described in 10 CFR 35.630(a):
"(1) The system must have been calibrated using a system or source traceable to the National Institute of Standards and Technology (NIST) and published protocols accepted by nationally recognized bodies; or by a calibration laboratory accredited by the American Association of Physicists in Medicine (AAPM). The calibration must have been performed within the previous 2 years and after any servicing that may have affected system calibration; or
"(2) The system must have been calibrated within the previous 4 years. Eighteen to thirty months after that calibration, the system must have been intercompared with another dosimetry system that was calibrated within the past 24 months by NIST or by a calibration laboratory accredited by the AAPM. The results of the intercomparison must indicate that the calibration factor of the licensee's system had not changed by more than 2 percent. The licensee may not use the intercomparison result to change the calibration factor. When intercomparing dosimetry systems to be used for calibrating sealed sources for therapeutic units, the licensee shall use a comparable unit with beam attenuators or collimators, as applicable, and sources of the same radionuclide as the source used at the licensee's facility."
The most usual practice is to have the chambers calibrated every two years.
B. The IAEA, in Technical Report Series (TRS) 398, recommends calibration of ionization chambers at a secondary standards dosimetry laboratory (SSDL) at intervals not exceeding two or three years (Paragraph 4.3). If calibrations for various energies are being obtained, no energy's calibration should exceed six years. It seems that the current protocol, TSR 398, does not recommend a frequency with which to calibrate the teletherapy unit.
C. Therapy units need to have periodic checks to ensure that the output has not changed. I have worked with 60Co units that have had sudden changes in output due to a shift in source position. Linear accelerators can change their output for many reasons. Daily checks with ionization chambers are a simple and reliable method to assure that the output is as expected. Almost any phantom that can hold the chamber reproducibly can be used for this check.
D. There is little evidence that ionization chambers need calibration as often as required by the NRC or even as recommended by the IAEA. In fact, normal Farmer-type chambers seldom change calibration significantly without trauma to the chamber because of repair. However, unusual sudden changes can happen. A check for the chamber should be established. This can be using a 60Co unit. As long as the reading of the chamber follows the expected decay, it is most likely that the unit and the chamber are working properly. Comparing two chambers is a good quality check. Following the consistency of more than one linac also can be a consistency check until a discrepancy occurs.
Parallel-plate ionization chambers and small-volume chambers often do not show the same consistency as Farmer-type chambers. As recommended in TRS 398, any chamber should be followed for its consistency for five years before assuming it will be behaved.
E. In all cases, practitioners must be familiar with their local regulations regarding frequency of calibration of units and ionization chambers.
F. TRS 398 can be downloaded free from http://www-naweb.iaea.org/nahu/DMRP/documents/CoP_V12_2006-06-05.pdf.
Thank you to Larry DeWerd for clarifying conversations on this topic.
Bruce R. Thomadsen, CHP