Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

TABLE 25-4. APPROXIMATE DOSE (INAIR) THAT WILL PRODUCE VARIOUSPRODROMAL SYMPTOMS IN 50% OFPERSONS EXPOSED1.2 Gy (120 rad)1.7 Gy (170 rad)2.1 Gy (210 rad)2.4 Gy (240 rad)AnorexiaNauseaVomitingDiarrheaSource: Pizzarello DJ, Witcofski RL. Medical radiationbiology, ed. 2. Philadelphia: Lea & Febiger, 1982.The latent period ends with the onset of the clinical expression of organ systemdamage, called the manifest illness stage, which can last from 2 to 3 weeks. This stageis the most difficult to manage from a therapeutic standpoint, because of the overlyingimmunoincompetence that results from damage to the hematopoietic system.Therefore, treatment during the first 6 to 8 weeks after the exposure is essential tooptimize the chances for recovery. If the patient survives the manifest illness stage,recovery is almost ensured; however, the patient will be at higher risk for cancer andhis or her future progeny for genetic abnormalities.The most radiosensitive organ system is the bone marrow, which contains thehematopoietic stem cells. However, with the exception of the lymphocytes, theirmature counterparts in circulation are relatively radioresistant.Hematopoietic tissues are located at various anatomic sites throughout thebody; however, posterior radiation exposure maximizes damage because the majorityof the active bone marrow is located in the spine and posterior region of the ribsand pelvis. The hematopoietic syndrome is the primary acute clinical consequenceof an acute radiation dose between 0.5 to 10 Gy (50 to 1,000 rad). Healthy adultswith proper medical care almost always recover from doses lower than 2 Gy (200rad), whereas doses greater than 8 Gy (800 rad) are almost always fatal unless drastictherapy such as bone marrow transplantation is successful. During the lastdecade, growth factors such as granulocyte-macrophage colony-stimulating factorand some interleukins have shown promise in the treatment of severe stem celldepletion. Even with effective stem cell replacement therapy, however, it is unlikelythat patients will survive doses in excess of 12 Gy (1,200 rad) because of the damageto the gastrointestinal tract and the vasculature. In any case, the human LDso/6o(the dose that would be expected to kill 50% of an exposed population within 60days) is not known with any certainty. The best estimates from the limited data onhuman exposures are between 3 to 4 Gy (300 to 400 rad) to the bone marrow.The probability of recovering from a large radiation dose is reduced in patientswho are compromised by trauma or other serious concurrent illness. The severeburns and trauma received by some of the workers exposed during the Chernobylnuclear accident resulted in a lower LDso/6o than would have been predicted fromtheir ionizing radiation exposures alone. In addition, patients with certain inherited