A large air-filled parallel-plate extrapolation chamber with thin graphite front and back electrodes is used as a primary standard measuring device for low-energy interstitial brachytherapy sources from which the unit of air-kerma strength or reference air-kerma rate can be derived. The chamber is suitable for low-energy photons with energies up to 40 keV. The underlying principle is that the air-kerma rate at a given point is proportional to the increment of ionization per increment of chamber volume at chamber depths greater than the range of secondary electrons originating from the electrode. The fundamentals for evaluating the extrapolation curves are presented as well as a detailed description of the present set-up of the PTB large-volume extrapolation chamber (GROVEX). Comparisons between the GROVEX and other primary standards for air-kerma and reference air-kerma rate are presented.

A new quantity termed the "practical peak voltage" is proposed. This quantity is derived by equating the low level contrast in an exposure made with an X-ray tube connected to a generator delivering any arbitrary wave form, to the contrast produced by the same X-ray tube connected to a constant potential generator. Out of the great number of possible contrast configurations one is selected as being suitable for diagnostic radiology. By means of an eigenvalue problem a direct link is established between the electrical quantity X-ray tube voltage and the practical peak voltage which was initially defined through the properties of the X-ray field. It is shown that the spread in total X-ray tube filtration as encountered in medical diagnostic radiology can influence the result of a measurement of the practical peak voltage only marginally.

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A phantom study for dosimetry in the urethra using alanine/ESR during (192)Ir HDR brachytherapy of prostate cancer is presented. The measurement method of the secondary standard of the Physikalisch-Technische Bundesanstalt had to be slightly modified in order to be able to measure inside a Foley catheter. The absorbed dose to water response of the alanine dosimetry system to (192)Ir was determined with a reproducibility of 1.8% relative to (60)Co. The resulting uncertainty for measurements inside the urethra was estimated to be 3.6%, excluding the uncertainty of the dose rate constant Lambda. The applied dose calculated by a treatment planning system is compared to the measured dose for a small series of (192)Ir HDR irradiations in a gel phantom. The differences between the measured and applied dose are well within the limits of uncertainty. Therefore, the method is considered to be suitable for measurements in vivo.

In diagnostic radiology the practical peak voltage was initially derived by postulating that, for a given combination of X-ray tube and contrast geometry, a constant X-ray tube voltage should produce the same low level contrast as an arbitrarily pulsating X-ray tube voltage. It has been shown previously that the practical peak voltage can be properly defined as a weighted average of the X-ray tube voltage. Up to now the concept of practical peak voltage was based entirely on the results of calculations. The present paper describes the experimental investigations for measuring and comparing the contrast-equivalent X-ray tube voltage and practical peak voltage derived from an invasive measurement of the time-dependent X-ray tube voltage. Within the experimental uncertainties, the results demonstrate the mutual equivalence of the practical peak voltage and the contrast-equivalent X-ray tube voltage.