Exposure Dose Measurements in Megavoltage Therapy

Arguments are presented to show that an ionization chamber in a scattering medium measures the exposure dose at its own centre provided it is at a depth sufficient to be on the exponential part of the depth dose curve. The minimum depth for the establishment of electron equilibrium is defined. This is not in accord with the usual assumption that the equilibrium is established at the depth dose curve maximum, and it is shown that the difference in exposure dose measured according to the two definitions can be as much as 5% for 20 MV radiations. At 2 MV the difference is negligibly small. Use of the N.P.L. roentgen calibration factor leads to a statement about the dose at the centre of the hole made in the phantom to receive the chamber. The correction factors to be applied to yield the exposure dose at the centre of the filled hole are discussed. For 2 MV radiation the measured factor, which is independent of field size and chamber depth, is 0·991.