Comments on the calibration of pre-cut iridium-192 wires for low dose rate interstitial brachytherapy using a farmer-type ionization chamber.

Abstract

CorrespondenceComments on the calibration of pre-cut iridium-192 wires for low dose rate interstitial brachytherapy using a Farmer-type ionization chamberJ H PlaneJ H PlaneRegional Medical Physics Department, South Cleveland Hospital , Middlesbrough TS7 8RZ, UKSearch for more papers by this authorPublished Online:28 Jan 2014https://doi.org/10.1259/bjr.74.879.740291SectionsView EPUBFull Text ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail About"Comments on the calibration of pre-cut iridium-192 wires for low dose rate interstitial brachytherapy using a Farmer-type ionization chamber." The British Journal of Radiology, 74(879), pp. 291–293References1 Vollans SE, Wilkinson JM. Calibration of pre-cut iridium-192 wires for low dose rate interstitial brachytherapy using a Farmer-type ionization chamber. Br J Radiol 2000;73:201–5. Link ISI, Google Scholar2 Aird EGA, Jones CH, Joslin CAF, Klevenhagen SC, Rossiter MJ, Welsh AD, et al. Recommendations for brachytherapy dosimetry. Report of a joint BIR/IPSM Working Party. London: British Institute of Radiology, 1993. Google ScholarbjradioBr. J. Radiol.The British Journal of RadiologyBJR 0007-1285 1748-880XBritish Institute of RadiologyAuthor's replyJ M WilkinsonNorth Western Medical Physics, Christie Hospital, Manchester M20 4BX, UK032001The Editor—Sir,The letter by Dr Plane raises two concerns that call for further comment. First, one must distinguish between measurements that are done to calibrate a source, that is to determine the magnitude of the source specification quantity, and measurements that are done to verify the outcome of a dose computation. For many years now there have been strong recommendations that brachytherapy sources should be specified in terms of an emission quantity [1–4], and the joint BIR/IPSM Working Party [5] endorsed the use of reference air kerma rate (RAKR) for this purpose. It is, therefore, somewhat disappointing to find that the source supplier for the Varisource machine continues to use a content quantity, namely activity. It is also disappointing to find that some treatment planning systems, including it would seem Cadplan BT, require sources to be specified in terms of activity. There are two arguments against using activity for source specification. First, there is potential for confusion between the true activity and the apparent (or equivalent) activity, where the latter is the activity of a hypothetical unfiltered point source of the radionuclide that would give the same emission rate at some reference point. Ironically, both the true activity and the apparent activity of a brachytherapy source will be determined by working backwards from an emission quantity measurement, but derivation of the former would involve making an additional correction for the source size and the net effect of attenuation and scattering in the sheathing material. The second argument against using activity is that there seems to be no universally accepted value for the iridium-192 air kerma rate constant. For example, Dutreix et al [6] quote 0.1157 µGy h−1 MBq−1 m2, whereas Godden [7] quotes 0.111 µGy h−1 MBq−1 m2. Clearly, if a source manufacturer uses one of these values in going from an air kerma rate measurement to a source activity specification and a computer software package uses the other in the reverse process for clinical dosimetry, then a systematic error in excess of 4% is introduced. These uncertainties and potential errors are avoided by using RAKR as the specification quantity.The second concern raised by Dr Plane's letter relates to the use of a Farmer-type chamber in high fluence gradients such as those that exist in the close vicinity of a brachytherapy source. To a good approximation, the effect is to produce a reading that is indicative of the average emission rate over the volume occupied by the detector rather than of the emission rate at a precisely defined point (e.g. the geometric centre of the chamber). If the requirement is to determine the air kerma rate (or dose rate) at a point, then a geometry correction has to be applied. For a small (approaching a point) source, a Farmer-type chamber (e.g. 24 mm long by 6 mm internal diameter) and a source-to-detector centre distance of 20 mm, the correction factor for a measurement in air is approximately 1.10 using the Kondo and Randolph data [8] or 1.12 using the non-isotropic data from Bielajew [9]. These are not insignificant adjustments, and it is not obvious by how much they would change, or in which direction even, for measurements that are done in water. An appropriate factor for use with measurements done in a water tank would have to be determined by a Monte Carlo calculation. The problem is not reduced by irradiating the detector from different directions. The size of the required correction reduces with increasing distance but it is still about 2%, and therefore still significant, for accurate dosimetry, at a source-to-detector distance of 50 mm. Anyone making measurements with a Farmer-type chamber in these conditions must, therefore, exercise extreme caution.