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Timer error: the possibility of confusion arising from two different sign conventions

¹ School of Applied Physics, Universiti Kebangsaan Malaysia 43600 UKM Bangi, Selangor, Malaysia
² School of Health Science, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
³ SSDL Malaysia, Malaysian Institute for Nuclear Technology Research 43000 Kajang, Selangor, Malaysia

(The Editors do not hold themselves responsible for opinions expressed by correspondents)

The Editor—Sir,

Despite the fact that ⁶⁰Co units are being phased out and are being replaced by medical linear accelerators (linacs), presently the number of ⁶⁰Co units is still comparable with the number of linacs. Recent statistics on the availability of teletherapy units worldwide [1] show that there are 2740 ⁶⁰Co units compared with 5033 linacs. In addition, most Secondary Standard Dosimetry Laboratories (SSDLs) are likely to continue using a ⁶⁰Co unit as the main calibration unit for medical radiation dosimetry purposes [2, 3].

To achieve accuracy in measurements with a ⁶⁰Co unit, quality control (QC) tests must first be carried out in addition to QC tests of any associated measuring equipment. The International Atomic Energy Agency (IAEA) [4] has outlined a series of ⁶⁰Co QC tests that must be undertaken. One of them is timer error [4–6], on which the IAEA has set a tolerance value of 1%. Methods for timer error determination have been well documented [7–14], and both positive timer error [7, 11, 12] and negative timer error [7, 13] have been reported. Recommendations [11–16] on the use of timer error for determining the effective irradiation time in order that an accurate prescribed dose is obtained have also been reported. Upon examining the recommendations however, we find that inconsistency has occurred. We believe this is due to the sign convention of the timer error, which results in authors making confusing or ambiguous statements. The purpose of this letter is to raise a question as to whether such inconsistency should be of concern.

The timer can be considered either (a) as a measuring instrument, where the actual time elapsed is the "real" quantity and the timer indication is an approximation, or (b) as a setting instrument, where the indication on the timer is the "real" quantity and the time elapsed is an approximation. In case (a) [11, 12], the error is counted as positive when the timer reading is greater than the elapsed time, and therefore the prescribed dose should be obtained when the timer setting is increased above the calculated value. In case (b) [11, 13–15], the opposite convention is used, and when the error is positive the timer setting must be set to a smaller value to achieve the correct dose.

For example, using the graphical method, measured dose is plotted against timer setting. The resulting straight line will normally have a negative intercept on the dose axis (indicating a dose deficit) and therefore a positive intercept on the timer axis. It is natural to quote this positive intercept as a positive error, which is in line with convention (a), i.e. the timer is used as a measuring instrument. Other techniques will more naturally lead to convention (b).

Normally, confusion should not occur unless someone uses one convention when measuring error and the opposite convention when applying it. It is also important to use the correct algebraic treatment of signs. For example, if the rule is to "subtract the timer error", then when the error is negative, the result will be to change the timer setting in the positive direction. Some authors have attempted to avoid ambiguity here by saying "if the error is negative, then it should be added to the timer setting", when they should have said "its modulus should be added...". If their advice is taken literally, it could result in the timer error being doubled rather than being eliminated. This possible source of confusion appears to be present in several studies [11, 13, 14].

It is therefore recommended that workers in this field should review their written protocol for the treatment of timer error and should ensure that there is no possibility of the correction being applied with the wrong sign.

Received for publication December 18, 2000. Accepted for publication January 24, 2001.

References

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