ABSTRACT
Whatever the method employed for a particular type of equipment, there is a need to
convert an elapsed time or a reading on a dose meter into the absolute dose delivered. For
modern x-ray treatment machines, it is usual to have an ionisation chamber situated in the
beam close to the x-ray source (see Section 11.7.3.1). This produces a reading proportional to
the dose delivered by the beam. By its nature, the reading will generally have a relationship to
the dose delivered that is dependent on various beam parameters. For this reason, the term
Monitor Unit (MU) has been introduced. In principle, it is not necessary that there is a one-
to-one correspondence between monitor units and dose, but it is usual to calibrate the machine
so that a MU corresponds to 1 cGy at a specified point in a phantom and with a specified
field size.
