ABSTRACT
The accurate determination of absorbed dose is crucial to the success of radiotherapy
because of the relatively steep sigmoidal dose-response curves for both tumour control and
normal-tissue damage. A difference of only a few percent in the dose (to the tumour) may
separate uncomplicated tumour eradication from either failure to control the tumour through
underdosage or serious normal tissue damage through overdosage (see Section 37.4.2). There
are many different steps involved in the determination of the absorbed dose distribution in the
patient. One of the most important of these involves measurements with a detector (often
termed a dosimeter) in a phantom (often water, sometimes water-like plastic) placed in the
radiation field. Such measurements include determining the absolute dose at a reference depth
in a reference-size field (Chapter 18), relative doses at many positions in the phantom in order
to map out a complete dose distribution (Chapter 19), and so-called in-vivo doses on the
patient’s skin during treatment (Section 40.2). In all cases the detector will, via a calibration
factor, yield the dose D
det
to its own sensitive material from a quantity of charge, light, film
blackening, etc. Generally, the dose is required at a position r in the medium in the absence of
the detector, D
med
(r). The conversion of D
det
to D
med
is, therefore, a fundamental step and
requires a knowledge of the theoretical aspects of radiation dosimetry. The same is true of the
calculation of the dose distribution inside the complicated inhomogeneous geometry of the
patient (see Part F).
