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.