The measurement of kappach factors for application with the IPEMB very low energy dosimetry protocol.

Abstract

In 1996. the IPEMB issued a new code of practice detailing the procedures by which the output of therapeutic kilovoltage x-ray devices are to be determined. For x-ray beam qualities in the range 0.035-1.0 mm Al half-value thickness (HVL), the equation for converting the instrument reading into absorbed dose to water contained a factor named kch. This was included to account for the change in response of the parallel plate chamber from its calibration 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 energies, the code advised that the user take a value of unity until sufficient data became available upon which to base an addendum. In this work, kch values have been determined for four chambers: two PTW 23344 large volume soft x-ray chambers, one PTW 23342 small volume soft x-ray chamber and a Markus electron chamber. Variations in the value of kch were investigated for changes in FSD, applicator size and beam quality. The water equivalence of the Mix-D phantom used for these measurements was also verified. A comparison of the results for the two PTW 23344 chambers showed no significant differences for any experimental situation, indicating that kch is a factor of chamber design rather than variations in construction. No variation in kch was identified with changes in FSD. A small dependence on applicator size was identified for larger applicators, and this was found to be dependent upon chamber design. The measured values of kch were found to increase with energy and again differences were seen between chamber designs. Overall, the values of kch recorded during these measurements ranged from 1.01 to 1.08. These results highlight a significant underestimation of doses 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 addendum to the code in its present form.