Physics and Radiobiology of Nuclear Medicine

In the indirect action, the radiation hits the water molecules, the major
constituent of the cell, and other organic molecules in the cell, whereby free
radicals such as perhydroxyl (HO 2 •) and alkoxy (RO 2 •) are produced. A
variety of reactions that can occur after radiation interacts with water
molecules is shown below.

236 15. Radiation Biology

T A

G C

A T

T A

T A

A

A T

C G

C G

C G

G C

DIRECT ACTION

INDIRECT ACTION

H

H

OH • O

G

Fig. 15.10. Illustration of direct and indirect action of radiation on the DNA mole-
cule. In direct action, radiations hit the DNA structure directly, whereas in indirect
action, radiations produce free radicals in the cytoplasm, which react adversely with
the DNA molecule to cause structural damage.