Choice of radionuclide

1.2.2.2 Choice of radionuclide

The ultimate radionuclide suitable for internal radionuclide therapy of primary and
metastatic malignancies requires the following properties: 1) The radioisotope must
have an appropriate radiation spectrum for treating small to large multiple tumours.
Large tumours with a vascular periphery but a necrotic centre take up less
microspheres per volume, therefore a high energy β-emitter with a subsequently high
tissue range is needed to reach the interior of the tumour. 2) A high dose rate is
advantageous for the radiobiological effect [41,42]. Consequently, a short half-life is
preferable. 3) For external imaging of the biodistribution of the radioisotope with a
gamma camera, a γ-emitter is necessary. However, the energy should be low to
prevent unnecessary radiation burden to the patient and environment [13]. 4) The
labeling of particles has to be simple without any leakage of the isotope. 5) A large
thermal neutron cross section is needed to enable high specific activities to be
achieved within short neutron activation times [16].
Only a few radioisotopes have characteristics, which make them potentially
suitable for the treatment of tumours (see Table 3). Taking into account the
aforementioned properties three radioisotopes are the most likely candidates, yttrium-
90, rhenium-188 and holmium-166. Yttrium-90 has two major disadvantages as a
radioisotope for therapy. First, long neutron activation times (>2 weeks) are needed to
achieve therapeutic activities of yttrium because 90Y’s precursor has a small thermal
neutron cross section of 1.28 barn. Secondly, the biodistribution of microspheres
loaded with 90Y cannot be directly determined in clinical trials, since 90Y is a pure β-
emitter and does not produce imageable γ-rays.