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A Quality Assurance Procedure for Measurement of an LDR Prostate Implant Isotope that does not yet have a NIST traceable calibration point.


Use of LDR brachytherapy isotopes for low dose rate brachytherapy treatment of prostate is increasing. Several new isotopes are expected to be available in 1998. Although there is no NIST traceable calibration for some of these new isotopes, the increasing demand for prostate treatment is presenting physicists with the need to perform quality assurance measurements of these isotopes to verify their relative activity to the manufacturer’s stated value. Since the average energy of these isotopes are close to that of iodine seeds which have NIST traceable calibrations, an additional quality assurance check can be established for these seeds using a calibration factor for iodine.

A quality assurance procedure for LDR brachytherapy isotopes can be established using the HDR 1000 Plus Brachytherapy Well Chamber as outlined below. We suggest that the institution establish this procedure at the start of their implant procedures. The equipment needed is:



QA Measurements

Response as a ratio to 125I A relative response factor for a new isotope, Rnew, can be obtained taking the nominal activity of the new isotope as stated by the manufacturer and dividing it by the current measured with the HDR 1000 Plus Brachytherapy Well Chamber, after an air density correction has been applied using the standard equation using temperature and pressure. This relative response factor, Rnew , can then be compared to the ADCL calibration factor obtained for 125I, NI .

The consistency of your well chamber should be monitored in some manner (see the instruction manual).

This procedure written in equation form is:

Rnew = A c/new/ Mc/new * CT,P * CEF

and: Rnew / NI = a constant


Ac/new is the relative activity of the new isotope seed, stated by the manufacturer
Mc/new is the reading for the seed from the HDR 1000 Plus Well Chamber
CT/P is the standard correction for temperature and pressure
CEF is the correction for the electrometer scale
NI is the calibration factor for 125I

This should be done for a number of seeds and an average value of Rnew should be established. In addition, the average of the ratio of this response factor to the 125I calibration factor should be established. This ratio should remain constant in time. Once the response factor Rnew is established by the institution, a quality assurance procedure can be set up such that a measurement of a new batch of seeds can be compared to previous batches. The following equation would be used.

Am/new = Mm/new * CT/P * CEF * Rnew


Am/new is the calculated (or measured) activity of the new isotope seed
Mm/new is the reading from the HDR 1000 Plus Well Chamber
CT/P is the standard correction for temperature and pressure
CEF is the correction for the electrometer scale
Rnew A c/new/ Mc/new* CT,P * CEF is the previously established factor for the isotope in use

The ratio NI can always be checked if there are any questions regarding consistency. Also, values of Rnew can be checked periodically.


Quality Assurance for new LDR brachytherapy seeds for prostate implant procedures can easily be done using the SeeDOS HDR 1000 Plus Brachytherapy Well Chamber. The method given here can be used to establish the consistency of the stated activity of the seeds.

NEW (February 2003) New Brachytherapy source measuring and handling systems pdf


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Please contact Colin Walters at cwalters@seedos.com if you would like further  information or you have questions

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SeeDOS Ltd, 26, The Maltings, Leighton Buzzard, Bedfordshire LU7 4BS, United Kingdom
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