Alpha Particle Emitters

Alpha particles, which are helium nuclei, look attractive for therapeutic applications because of their high linear energy transfer (LET) and relative biological efficiency (RBE). Cell killing is less dependent on oxygen for radiotherapeutic efficacy, that is, low oxygen enhancement ratio (OER) (38-40). Although their path length in tissues may exceed several cell diameters, it is not likely long enough to effectively treat larger tumors. Alpha emitters are however useful in treating micrometastases from solid tumors or in disseminated intracavitary diseases such as intraperitoneal seedling from ovarian malignancies, tumors in the meninges as well as acute leukemia. Ultra short-lived radioisotopes of bismuth,

212 213

Bi and 213Bi, are under investigation for leukemia (rapid cell proliferation) treatment (3) and intracavitary malignancies (diffuse and microscopic disease) (41). Radioisotopes with longer half-life, lead-212 (212Pb) and actinium-225

(225Ac)

may be suitable for use in systemic treatment (larger tumors). The idea of using high LET alpha emitters as targeted atomic nanogenerators is a recent proposition (42). A single atom emitting an alpha particle may kill a target cell. Investigators have targeted several cancers using novel constructs of internalizing monoclonal antibodies labeled with alpha emitters without increasing systemic toxicity (3,39). Because of the nature of their interaction with matter (short path length), no special shielding precautions are needed.

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