Chamber-dependent wall correction factors in dosimetry

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Abstract
The calculation of the response of ionisation chambers to different photon beam qualities used in radiotherapy requires electron stopping-power data. These data have recently been revised and are used here to derive a consistent set of correction factors for ionisation chambers of different wall and build-up cap composition. Theoretically derived parameters km and katt which relate the exposure calibration of an ionisation chamber to the absorbed dose to the air of the cavity, are compared with the experimentally derived product kmkatt showing generally good agreement but also significant discrepancies for plastic-walled chambers with inner graphite coatings. A table of km values is given for a large number of commercial ionisation chambers. The new stopping-power data are also used to evaluate the wall-dependent correction factor (pwall) that enters into the determination of the absorbed dose to water in photon beams, results being given as a function of the quality of the beam. The authors' theoretical calculations of pwall are consistent with existing experimental data.