TemporaryhairlossSkin3.0–5.0 3.5 5.0 http://ebooksmedicine.net dose levels and large exposure fields, severe lymphopenia may appear within hours of irradiation, along with shrinkage of the lymph nodes and spleen. Radiation directly destroys lymphocytes, both in the circulating blood and in tissues (nodes, spleen, thymus, gut). With sublethal doses of radiation, regeneration from viable progenitors is prompt, leading to restoration of a normal lymphocyte count. Hematopoietic precursors in the bone marrow are also quite sensitive to radiant energy, which produces a dose-dependent marrow aplasia. The acute effects of marrow irradiation on peripheral blood counts reflects the kinetics of turnover of the formed elements—the granulocytes, platelets, and red cells, which have half-lives of less than 1 day, 10 days, and 120 days, respectively. After a brief rise in the circulating neutrophil count, neutropenia appears within several days. Neutrophil counts reach their nadir, often at counts near zero, during the second week. If the patient survives, full recovery of granulocytes may require 2 to 3 months. Thrombocytopenia appears by the end of the first week, with the platelet count nadir occurring somewhat later than that of granulocytes; recovery is similarly delayed. Anemia appears after 2 to 3 weeks and may persist for months. Understandably, higher doses of radiation produce more severe cytopenias and more prolonged periods of recovery. Very high doses kill marrow stem cells and induce permanent aplasia (aplastic anemia) marked by a failure of blood count recovery, whereas with lower doses the aplasia is transient.
• Environmental exposure and cancer development. Any cell capable of division that has sustained mutations has the potential to become cancerous. Thus, an increased incidence of neoplasms may occur in any organ after exposure to ionizing radiation. The level of radiation required to increase the risk of cancer development is difficult to determine, but there is little doubt that acute or prolonged exposures that result in doses of 100 mSv cause serious consequences, including cancer. This is documented by the increased incidence of leukemias and tumors at various sites (such as thyroid, breast, and lung) in survivors of the atomic bombings of Hiroshima and Nagasaki, the increase in thyroid cancers in survivors of the Chernobyl accident, and the development of “second cancers,” such as acute myeloid leukemia, myelodysplastic syndrome, and solid tumors, in individuals who received radiation therapy for cancers such as Hodgkin lymphoma. It is believed that the risk of secondary cancers following irradiation is greatest in children. This is based in part on a large-scale epidemiologic study showing that children who receive at least two CT scans have very small but measurable increased risks for leukemia and malignant brain
Table 8.8 Effects of Whole-Body Ionizing Radiation tumors, and on older studies showing that radiation therapy to the chest is particularly likely to produce breast cancers when administered to adolescent females.
• Occupational exposure and cancer development. Radon is a ubiquitous product of the spontaneous decay of uranium. The carcinogenic agents are two radon decay byproducts (polonium-214 and polonium-218, or “radon daughters”), which emit alpha particles and have a short half-life. These particulates are deposited in the lung, and chronic exposure in uranium miners may give rise to lung carcinomas. Risks also are present in those homes in which the levels of radon are very high, comparable to those found in mines. However, there is little or no evidence to suggest that radon contributes to the risk of lung cancer in the average household.