BIOLOGY MATTERS - Radiation Oncology: A Physicist's-Eye View - page 87

few months after Roentgen's discovery of X-rays. And, “modern”

radiotherapy featuring supervoltage ( 1 MeV effective energy)

radiation equipment and so-called three-dimensional conformal
radiation therapy (3DCRT) - which features the use of multiple
uniform intensity beams directed and shaped according to the
patient's tumor and anatomy - has been in use for decades. This has
given rise to a large body of clinical experience for a certain set of
fairly well-accepted practices. I term these established experience .
There are a number of established treatment regimes which, for
example, deliver the entire dose in only a few, or even one, fractions,

≥

or which deliver more than one fraction per day. However,

established experience, as I refer to it in this topic, involves the
delivery of radiation in multiple (say, 30 to 40) daily fractions over
several (5 to 8) weeks with a small (1.8 to 2 Gy) dose per fraction.
The radiation is often delivered in two courses: the first covering
suspected extensions of disease into regional lymphatics as well as the
gross tumor volume, and the second concentrated on just the gross
tumor volume. Beams are designed and shaped to deliver as uniform
a dose as possible to the tumor and to irradiate as little uninvolved
tissue as possible, and beam directions are chosen to spare sensitive
uninvolved adjacent tissues, where possible.

Therapeutic ratio
Whenever one departs from established experience, both the tumor
and normal tissue responses will be altered. One is little interested in
either of these independently, rather one wishes to know whether,
with the new regime, one does relatively better as regards its impact
on the tumor as compared with its impact on normal tissues. For
example, one hopes that better tumor control will be obtained for no
change in morbidity or, conversely, that less morbidity is experienced

for the same tumor control - or something in between. When this

obtains, one says that one has improved the therapeutic ratio . In some

situations, one can give a quantitative meaning to the therapeutic
ratio, but, for the most part, the term is used qualitatively.

Types of models
There are several factors affecting radiation therapy, which one might
wish to model. Among these are:
Overall time One would like to model the effect of varying the
overall time of treatment. In a very general way, shorter times tend