BIOLOGY MATTERS - Radiation Oncology: A Physicist's-Eye View - page 107

irradiation of the lateral lung alone, with that of the heart plus lateral
lung. The result is shown in Figure 5.11. It is clear that irradiation of
the heart markedly reduced the tolerance of the lung to radiation.
Such a result is by no means predicted by models of parallel
architecture - although one can think of physiologic reasons rather
than inter-cell communication to account for these observations.
Rectal damage The geometry of “long” cylindrical and
tubular organs affects their radiation response. It seems that it is the
irradiated fraction of the cross-section of a cylindrical organ, or the
irradiated fraction of the circumference of a tubular organ, which

largely governs radiation response (see Figure 5 .12). The rectum

is an example of a

tubular structure - and,

due to the prevalence
of prostate cancer and
the success of high
dose radiation therapy
in its treatment, it is
amongst the most
studied examples. The
rectum is very close to
the prostate and so,
when a high dose is given to the prostate, it is inevitable that a
similarly high dose will be delivered to the anterior rectal wall. Benk
et al. (1993) first showed, and others have since confirmed, that it is
the fraction of the circumference of the rectum that is correlated with
rectal complications, in this case rectal bleeding. It is now generally
accepted that not more than 40% of the anterior half of the rectal wall
should receive more than 70 Gy.

Figure 5.12. Schematic drawing of the

partial-cross section irradiation of (a) a solid
cylindrical, and (b) a tubular organ.

There is another suggestive finding (Jackson, 2001) that is interest-

ing in the present context. In treating prostatic carcinoma, while

delivering a very high dose to the anterior rectal wall, some patients
received a higher dose to the posterior rectal wall than others. It was
observed that the rate of complications was higher in the group whose
posterior rectal wall received in the range of 40 to 50 Gy than in the

group receiving a lower dose. That is, the radiation experience of

the environment around the high dose region significantly affected
the complication probability.

Paired organs The parallel models don't quite know how to
deal with paired organs such as the lungs or kidneys. They can
assume that the two organs act as one big organ in terms of the critical