ABSTRACT
This chapter is concerned with the accurate determination of the absorbed dose in gray
at the reference depth in a water phantom. This is sometimes known as calibrating the beam and
is expressed in terms of a number of monitor units per gray for a given standard field size,
Source to Surface Distance (SSD) and beam quality. For a better understanding of some of the
concepts discussed in this chapter, the reader should consult Part E on the general charac-
teristics of photon and electrons beams.
