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MIRD Continuing Education: Bystander and Low–Dose-Rate Effects: Are These Relevant to Radionuclide Therapy?^*

¹ School of Medicine, Johns Hopkins University, Baltimore, Maryland; ² School of Medicine, Stanford University, Stanford, California; ³ Wayne State University and Karmanos Cancer Institute, Detroit, Michigan; ⁴ School of Medicine, University of Maryland, Baltimore, Maryland; and ⁵ Harvard Medical School, Boston, Massachusetts

Correspondence: For correspondence or reprints contact: George Sgouros, PhD, CRB II, 4M61, 1550 Orleans St., Baltimore, MD 21231. E-mail: gsgouros{at}jhmi.edu

Bystander and low–dose-rate effects influence the dose–response relationship in a manner not predicted by current dosimetric methodologies. Radiation-induced bystander effects refer to biologic responses in cells that are not traversed by an ionizing radiation track and, thus, not subject to direct energy deposition; that is, the responses occur in nonirradiated cells. Low–dose-rate hypersensitivity effects have been documented as a reduction in the survival of cells irradiated at dose rates of 0.1–1.0 Gy/h, with total doses ranging from 1.5 to 5 Gy. For humans undergoing external radiotherapy, evidence of bystander events has been observed in the form of abscopal effects, wherein irradiation of one portion of the anatomy affects a portion outside the radiation field, whereas low–dose-rate hypersensitivity has not been described. In this report, the historical literature is briefly reviewed, key experiments are summarized, and current understanding of the factors thought to be involved in the bystander and low–dose-rate effects is conveyed. The mechanisms associated with these events are still being investigated, and questions remain on their impact in radionuclide therapy. Although current findings do not yet sufficiently justify changing traditional dose estimates used to predict the outcomes of radionuclide therapy, it is important to appreciate the potential importance of these effects and to begin revising methods to reflect the emerging empiric and mechanistic knowledge.

Key Words: radiobiology • radionuclide therapy • radiation safety • bystander effects • dosimetry • low–dose-rate effects

^* NOTE: FOR CE CREDIT, YOU CAN ACCESS THIS ACTIVITY THROUGH THE SNM WEB SITE (http://www.snm.org/ce_online) THROUGH OCTOBER 2008.

No potential conflict of interest relevant to this article was reported.

COPYRIGHT © 2007 by the Society of Nuclear Medicine, Inc.

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