Contributions of nuclear medicine to the therapy of malignant tumors

Abstract

Radionuclides are applied in oncology for diagnosis and therapy. The former demands gamma — emitting radionuclides for labeling specific substrates for localizing malignant tissue and for analyzing tumor metabolism in vivo. Here, positron emission tomography (PET) may register in vivo the metabolism, for example, of glucose, amino acids, and receptors and of potentially useful cytotoxic agents. The advantage of the positron emitting radionuclides of carbon, nitrogen and fluorine is the labeling of substrates without changing substrate specificity within the metabolic reaction chain; also, substrate concentration in situ may be quantified. With regard to therapy radionuclides that emit β-and α-particles or decay by electron capture with the Auger effect, are administered in ionic form or with tumor seeking substrates. Examples are radioiodine for treating thyroid malignancy and radiophosphorus for myelopoliferative diseases. Organically bound radionuclides are given as labeled ligands for specific receptors, such as meta-iodo-benzylguanidine (MIBG) for treating the catecholamine producing tumors phaeochromocytoma and neuroblastoma and labeled monoclonal antibodies for tumors specific receptors. Highly localized energy depositions come from Auger emitters such as¹²⁵I and by the neutron capture therapy, where boron-10 in the tumor cell is exposed to thermal neutrons for initiating the B¹⁰ (n; α) Li⁷ reaction, especially for treating neuro- and glioblastoma and melanoma. Endogenous radiotherapy with radionuclides rely on the success of delivering a proper amount of energy into individual tumor cells with optimal protection of normal tissue. The inevitable heterogeneity of energy deposition events from such approaches demands careful dosimetric assessment for which the classical methods of dosimetry for percutaneous radiotherapy are not applicable.