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8 Ionizing Radiation and Life
8.11 Radiotherapy
Radiotherapy is the use of implanted or ingested radioactive substances in treatment
of disease. (In distinct contrast, radiation therapy uses external beams of ionizing
radiation focused on diseased tissue or organs.) When the radiation source is placed
near or within an intended target, the treatment is called ‘brachytherapy’.
As an example of radiotherapy, consider cancer treatment of the thyroid with
radio-iodine. Iodine-131 is a beta-particle emitter, and therefore damaging to cells,
particularly those dividing. High ingested doses have been used to treat thyroid
cancer. 131
53 I has a physical half-life of τ 1/2 = 8.0198 d, decaying via beta and
gamma emission according to
131
53 I → β(0.3338 MeV) +
131
54 Xe
∗ (7/2+, 0.636989 MeV) 7.27%
and
131
53 I → β(0.6063MeV) +
131
54 Xe
∗ (5/2+, 0.364489 MeV) 89.9%.
The excited xeon subsequently decays into
131
54 Xe(3/2+, 0 MeV)
by the release of a γ . The biological half-life of ingested iodine is patient dependent,
but 66–80 days is typical if a functioning thyroid is present. Thus, the physical halflife of 8 days means that physical decay predominates the loss of the isotope. During
the first few weeks after ingestion, caution must be taken in the handling of urine
and excretion.
Other examples of radiotherapy are shown in Table 8.4.
Table 8.4 Radioisotopes used in radiotherapy
Product Emits Half-life Max E
Application
125 I
125 T e
γ
59.4 d
35 keV
Ocular implant a
131 I
131 Xe
β, γ
8.02 d
606 keV
Cancer of thyroid
192 I r
192 P t
β, γ
73.8 d
1.46 MeV
Brain, breast implant
90 Y
90 Zr
β, γ
64.1 h
2.28 MeV
Liver via μ-spheres b
177 Lu 177 Hf
β, γ
6.73 d
497 keV
Imaging, targeted therapy c
153 Sm 153 Eu
β, γ
46.3 h
807 keV
Metastatic bone disease
32 P
32 S
β
14.3 d
1.709 MeV
Blood disorders
223 Ra 219 Rn
α
11.4 d
5.78 MeV
Targeted alpha therapy
a Diagnostic imaging
b Delivery of radioisotopes via impregnated microspheres called radioembolization c Radioisotopes
attached to specific chemicals target specific cells
8 Ionizing Radiation and Life
8.11 Radiotherapy
Radiotherapy is the use of implanted or ingested radioactive substances in treatment
of disease. (In distinct contrast, radiation therapy uses external beams of ionizing
radiation focused on diseased tissue or organs.) When the radiation source is placed
near or within an intended target, the treatment is called ‘brachytherapy’.
As an example of radiotherapy, consider cancer treatment of the thyroid with
radio-iodine. Iodine-131 is a beta-particle emitter, and therefore damaging to cells,
particularly those dividing. High ingested doses have been used to treat thyroid
cancer. 131
53 I has a physical half-life of τ 1/2 = 8.0198 d, decaying via beta and
gamma emission according to
131
53 I → β(0.3338 MeV) +
131
54 Xe
∗ (7/2+, 0.636989 MeV) 7.27%
and
131
53 I → β(0.6063MeV) +
131
54 Xe
∗ (5/2+, 0.364489 MeV) 89.9%.
The excited xeon subsequently decays into
131
54 Xe(3/2+, 0 MeV)
by the release of a γ . The biological half-life of ingested iodine is patient dependent,
but 66–80 days is typical if a functioning thyroid is present. Thus, the physical halflife of 8 days means that physical decay predominates the loss of the isotope. During
the first few weeks after ingestion, caution must be taken in the handling of urine
and excretion.
Other examples of radiotherapy are shown in Table 8.4.
Table 8.4 Radioisotopes used in radiotherapy
Product Emits Half-life Max E
Application
125 I
125 T e
γ
59.4 d
35 keV
Ocular implant a
131 I
131 Xe
β, γ
8.02 d
606 keV
Cancer of thyroid
192 I r
192 P t
β, γ
73.8 d
1.46 MeV
Brain, breast implant
90 Y
90 Zr
β, γ
64.1 h
2.28 MeV
Liver via μ-spheres b
177 Lu 177 Hf
β, γ
6.73 d
497 keV
Imaging, targeted therapy c
153 Sm 153 Eu
β, γ
46.3 h
807 keV
Metastatic bone disease
32 P
32 S
β
14.3 d
1.709 MeV
Blood disorders
223 Ra 219 Rn
α
11.4 d
5.78 MeV
Targeted alpha therapy
a Diagnostic imaging
b Delivery of radioisotopes via impregnated microspheres called radioembolization c Radioisotopes
attached to specific chemicals target specific cells