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Radiopharmaceuticals

many hundreds of individual doses can be pre-

pared for individual patient use.

Technetium-99m pertechnetate (

99m

TcO

4–

) is a

relatively stable form (VII) of technetium. Alone

it will usually not bind to other compounds and

so it is most often reduced by reducing agents,

such as stannous chloride, to more positively

charged technetium species (III to VI) much more

amenable to chemical reactions with a host of

other compounds. An exception to this important

rule is

99m

Tc sulphur colloid, which seems to have

the Tc(VII) oxidation state. Since technetium

sulphide is relatively insoluble, it allows

99m

Tc t o

remain in the VII state of oxidation. Stabilising

agents such as gelatin and albumin are often

used.

Radiopharmaceuticals for Lymph Node Detection

and Lymphoscintigraphy

For the purposes of lymph node detection and

lymphoscintigraphy, labelled colloids seem to be

most appropriate. Indeed, many different labelled

radiopharmaceuticals are available for lympho-

scintigraphy, and a brief summary of these has

already been provided in Chap. 1.

To some extent the choice of colloid will

depend on the purpose of the study and there are

clearly a number of issues which merit considera-

tion. Ultimately, however, in this increasingly

regulatory environment only nationally registered

radiopharmaceuticals can be used routinely in

patients. This is already limiting the choices and

possibilities. Registration of new radiopharma-

ceuticals is very costly and is hampering the

development of the field as a whole. Most of

the present compounds were manufactured for

the purpose of imaging liver, spleen and bone

A radiopharmaceutical is a specific compound

which has been labelled with a small amount of a

radionuclide in order to allow this product to be

detected externally once given parenterally to a

patient. Both the substance to be labelled and

the radioactive nuclide are usually used in such

small amounts that no pharmacological effect

ensues upon administration to the patient. Often

the designations “tracer” and “trace amounts”

are employed since the chemical quantities

which are labelled are in the order of a millionth

of a gram. External detection of the signal

emitted by the radiopharmaceutical allows the

tracer to be recorded either by a probe (such as

for the peroperative detection of the sentinel

node) or by an imaging device, such as the

Anger gamma camera. The latter is then used

to obtain an image of the distribution of the

tracer in a specific region of the body. In the case

of visualisation of the lymphatic pathways, the

word “lymphoscintigraphy” is most appropriately

used.

In conventional and routine nuclear medicine

applications the radionuclide most often used for

labelling purposes is technetium-99m (

99m

Tc).

It is now universally available for daily use at a

very economic price.

99m

Tc is pro duced indirectly

either by the neutron irradiation of molybdenum-

98 or as a fission product of uranium-235.

99m

Tc

generators are supplied by a variety of com-

mercial manufacturers, usually on a once a week

basis. The generator consists of molybdenum-99

adsorbed on an alumina column. Through fission,

carrier-free molybdenum-99 is obtained, such

that large quantities of the nuclide can be adsorb-

ed onto a rather small alumina column. The gen-

erator can be eluted daily with saline, and sterile

and pyrogen-free

99m

Tc in the form of pertechne-

tate is obtained. In the larger radiopharmacies

chapter

2