The measurement of k(ch) factors for application with the IPEMB very low energy dosimetry protocol

In 1996, the IPEMB issued a new code of practice detailing the procedures b y which the output of therapeutic kilovoltage x-ray devices are to be deter mined. For x-ray beam qualities in the range 0.035-1.0 mim Al half-value th ickness (HVL), the equation for converting the instrument reading into abso rbed dose to water contained a factor named k(eh). This was included to acc ount for the change in response of the parallel plate chamber from its cali bration conditions free in air to those in the user's measurement phantom. As no data were available with which to quantify this factor at very low en ergies. the code advised that the user take a value of unity until sufficie nt data became available upon which to base an addendum. In this work. k(eh) values have been determined for four chambers: two PTW 23344 large volume soft x-ray chambers, one PTW 23342 small volume soft x-r ay chamber and a Markus electron chamber. Variations in the value of k(eh) were investigated for changes in FSD, applicator size and beam quality. The water equivalence of the Mix-D phantom used for these measurements was als o verified. A comparison of the results for the two PTW 23344 chambers show ed no significant differences for any experimental situation, indicating th at k(eh) is a factor of chamber design rather than variations in constructi on. No variation in k(eh) was identified with changes in FSD. A small depen dence on applicator size was identified for larger applicators, and this wa s found to be dependent upon chamber design. The measured values of k(eh) w ere found to increase with energy and again differences were seen between c hamber designs. Overall. the values of k(eh) recorded during these measurements ranged from 1.01 to 1.08. These results highlight a significant underestimation of dos es calculated using the very low energy code of practice. This supports the need for further work to confirm these results, and the production of an a ddendum to the code in its present form.