The current status of targeted radiotherapy in clinical practice.

Biologically targeted radiotherapy in clinical practice requires a molecule which has a relative specificity for tumour tissue--the missile--coupled to a radionuclide with appropriate physical characteristics--the warhead. When administered to a patient this combination should result in selective irradiation of the target tumour cells with relative sparing of normal tissues. Simple ions and small molecules which follow physiological pathways as either the natural substrates or analogues form the best examples of biological targeting. Clinically valuable results are seen with, for instance, iodine uptake by normal and malignant thyroid cells, incorporation of the calci-mimetic element strontium in areas of increased bone metabolism and accumulation of the catecholamine analogue meta-iodobenzylguanidine in neuroblastoma. The use of monoclonal antibodies as targeting vehicles has not proved to be a panacea, yet some patients with lymphoma, hepatoma and ovarian carcinoma have obtained benefit. Current clinical studies in targeted radiotherapy focus on the integration of radionuclide treatment with conventional treatments, and the optimization of such combined approaches. The development of modifications to offset the limitations inherent in the use of crude antibodies also offers an opportunity for improved clinical outcomes.