Comparison of Cesium-137 and X-ray Irradiators by Using Bone Marrow Transplant Reconstitution in C57BL/6J Mice

Studies of hematopoietic stem cell transplantation have evolved over the past 60 y.1,9 Many preclinical investigations involving cell and gene therapy and hematopoietic stem cell function are performed in mouse models, and techniques such as adoptive cell transfer and bone marrow transplant are commonly used in these studies. Such techniques often require a supralethal dose of irradiation to ensure adequate engraftment with donor cells and subsequent survival. Conventional γ-emitting irradiators (137Cs and 60Co sources) have been used to deliver myeloablative doses of radiation prior to bone marrow transplantation (BMT). After the terrorist attack of 11 September 11 2001, security measures regarding active radioactive source irradiators have been heightened. In 2005, the US Congress passed the Energy Policy Act, in which the US Nuclear Regulatory Commission was assigned to evaluate and prevent malicious misuse of radioactive materials. As a result, increased security controls were imposed on radioactive material sources and quantities of concern, including shielded active source irradiators.14 Mandated security measures now include fingerprinting and a criminal-history record check to allow persons unescorted access to various radioactive materials.15 Background checks and fingerprinting procedures can be time-consuming and present an additional expense that usually is passed on to individual investigators. These enhanced security measures have significantly increased the expense associated with use of these irradiators, and federal regulations as proposed in 10 CFR Part 37 are likely to become more stringent in coming years.16 This situation has correspondingly led to an increased interest in the use of X-ray irradiators as a substitute for γ-ray sources such as 137Cs, and many animal facilities across the country have begun to purchase these units, even though there is no unbiased comparative information regarding the effectiveness of the instruments. In addition to decreased security requirements, X-ray irradiators are substantially less expensive to purchase than are active-source irradiators. After reviewing quotes, we estimate that the initial purchase price of an X-ray irradiator is about one sixth that of a cesium source. These figures do not include the costs of shipping, installation, or disposal of old active-source machines, and thus actual starts up costs are much higher. Annual maintenance as well as annual or semiannual dosimetry assessment costs are relatively comparable between the 2 sources. X-ray irradiators offer an additional financial advantage in that they do not require the strict security measures required for active γ-source irradiators. Given the number of disadvantages for the possession and use of γ-emitting irradiators, the use of X-ray irradiators in research likely will increase in the future. Extensive review of the literature did not reveal any studies in which bone marrow transplantation (BMT) efficiencies, kinetics, or overall responses in mice were compared between 137Cs and X-ray irradiators. We hypothesized that both the 137Cs and X-ray sources would ablate the bone marrow effectively and allow for comparable donor bone marrow reconstitution, and we sought to compare any differences in cell population engraftment after the use of each source. Recipient hematologic recovery after irradiation and reconstitution with bone marrow was assessed by determining the percentages of B and T lymphocytes and myeloid cells in the peripheral blood at 90 d after engraftment. In light of previously published work, we hypothesized that using the X-ray source before BMT would require a reduced dosage of radiation compared with that for the 137Cs source.4,6,9 Historically, lethal-dose curves have been generated to calculate the dose which is lethal for 50% of the irradiated animals (mice, in this case) over a 30-d period (that is, the LD50:30); this method allows approximation of the radiation sensitivity of a cohort of experimental mice.11 A mouse in which 100% of the bone marrow has been ablated will be unable to recover hematopoietic function and will die. A priori, if the animal dies, one can assume that the minimal lethal dose has been reached or exceeded; conversely, if the mouse survives, the minimal lethal dose was not achieved. Because of the number of mice needed to calculate an accurate LD50:30, we elected to perform a broad lethal-dose curve (1100 to 500 cGy in 200-cGy increments) to determine the point at which 100% death was reached for both sources. We then used this information as the lower radiation exposure limit for a bone marrow reconstitution curve (refined into 50-cGy increments), thereby allowing us to examine the bone marrow reconstitution response after differing radiation exposures.9 There is ample support in the literature for the broad lethal-dose test range chosen in this study.4,9 Previous work with thymocyte reconstitution after bone marrow ablation has demonstrated that irradiation exposures of approximately 400 cGy are required to establish a population of donor-derived thymocytes in the recipient.5 Therefore, we used a dosage test range above 400cGy in the current study. The dose range for this study was 500 to 1100 cGy, and we expected to see morbidity and mortality primarily due to bone marrow failure between 8 and 20 d in the lethal-dose curve.12