Determination of absorbed dose to water with ionisation chambers calibrated in free air for medium-energy X-rays

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Abstract
In order to determine absorbed dose to water from an in-phantom measurement by an ionisation chamber, three corrections need to be applied: (a) the ratio of water and air mass energy absorption coefficients, (b) a quality factor which corrects for the effect on the chamber response of the different photon spectra in-phantom and in-air, and (c) a perturbation factor which corrects for the water displaced by the chamber. Values of these corrections have been derived theoretically for a 0.6 cm3 cylindrical chamber for a range of medium-energy X-rays. The perturbation corrections are found to be less than 1%; these values, which are verified experimentally, agree with ICRU report 23 and not with the recent (1987) IAEA code of practice. This conclusion is expected to hold also for other small ionisation chambers. Tissue-air ratios have been derived from corrected ionisation chamber measurements and demonstrate that, for accurate dosimetry, medium-energy X-ray beams should be specified by peak tube potential as well as by half-value layer.