On the measurement of the effective linear attenuation coefficient mueff in 60Co wedge fields

It is difficult to obtain a correct measurement of mueff. Moreover, wedge calculations using a single mueff value are not correct. We present a program for the analysis of wedge measurements.

The program employs parameters of an open field to correct the dosimetry of a wedge field. For this the ASCII export files of an MP3 (PTW) system are automatically imported into our program. The principle is illustrated for ionization measurements from a Chisobalt 2B75 cobalt unit. The wedge field dose profiles are corrected by the off-centre ratio of the open field and mueff is analysed as well as its dependents on geometry.

We found that the dose attenuation by a wedge can be described by the product of wedge thickness and the effective linear attenuation coefficient mueff. However, mueff values for a given field size decrease with depth. This is interpreted as an effect of beam hardening. For the usual fields in radiotherapy this effect is independent of field size and can be described as an exponential function of wedge thickness.

The effective linear attenuation coefficient mu(eff) is depth-dependent. A correction could be added to planning programmes, i.e.: exp. (delta mu.tw.[d-dmax]).