unity. A values between 1.4 and 2.0 have been reported [Vadash and Bjärngard 1993, Yu and

Sloboda 1995, Georg and Dutreix 1997, Kim et al 1997]. A can be determined experimentally for

each accelerator. For that purpose a set of output ratio values should be measured keeping one

collimator setting fixed and varying the other one. Yu and Sloboda [1995] pointed out that the

relative weigth (A) of the upper and lower jaws is strongly dependent on their distance from the x-

ray source and can be estimated using the following empirical formula:

A = (SL / SU)2 (4.2)

where SL and SU are, for conventional collimators, the distances from the x-ray source to the top of

the lower and the upper collimator, respectively (see Fig. 3.3). In case of an MLC replacing the

lower or upper pair of jaws, SL or SU are the distances from the source to the midpoint of the MLC

leaves. The top edge of the conventional collimators is always closest to the central beam axis,

while for the MLC (single focused), the midpoint is always closest. The edge closest to the central

beam axis defines the volume of the flattening filter that can be seen from the point of measurement.

The authors found, in agreement with the results of other groups, that the energy of the beam and

the presence of a wedge has a small influence on the value of A.

Kim et al [1997] obtained the same equivalent square formula that accounts for the

collimator exchange effect as proposed by Vadash and Bjärngard [1993]. The formula was derived

by a method called the field mapping method, in which a field, that is defined in the source plane by

back-projection from the point of measurement (i.e. the detector’s eye view), is mapped back into

the detector plane by an equivalent field relationship. According to their method, A in formula (4.1)

can be given by:

A = (SL / SL1) / (SU / SU1) (4.3)

where SL1 and SU1 are the distances from the isocentre to the top of the lower and the upper jaws,

respectively.

In the following paragraphs results of output factor measurements in a large water phantom

and in a mini-phantom are reported for the five different treatment units, including the CEE data.

The reproducibility of these measurements is estimated to be better than or equal to 0.5%.