dose Local injuries are rarely life threatening, but they are difficult to manage because they often cause a slowly progressive injury that takes months and sometimes years to fully evolve The injury develops slowly because the radiation causes progressive fibrosis of the blood vessels, which, in turn, causes tissue necrosis The ultimate extent of the injury may not be appreciated initially Healing following amputation or reconstructive surgery is poor because of deficient blood supply TABLE 90.15 APPROXIMATE ABSORBED DOSE TO PRODUCE SKIN CHANGES FROM LOCAL RADIATION INJURY Absorbed dose (Gy) Findings 3-6 Threshold for erythema 15 20 Moist desquamation Skin ulceration with slow healing >30 Gangrenous changes The hand is the most common site for localized radiation injuries The next most common sites are the thighs and buttocks because individuals are likely to put things that they find into their pockets A patient who has undergone a fluoroscopic procedure could have local radiation injury to the skin overlying the region imaged For example, the radiation source is typically positioned posterior to a patient undergoing a cardiac catheterization and therefore a radiation burn would be on the back Most industrial radiography sources deliver an extremely high radiation dose In contrast, analytical x-ray crystallography machines, which emit x-rays of much lower energy than the photons of 192Iridium, are not likely to cause deep blood vessel injury Local radiation injuries can typically be differentiated from thermal burns The effects of a thermal burn appear immediately If a patient presenting with a burnlike injury does not know the cause or time of the injury, a local radiation injury should be suspected Table 90.15 lists the dose-related findings expected after an acute local radiation exposure If erythema is seen within the first 48 hours, ulceration may occur later The erythema may come in waves that appear, disappear, and then reappear With transepidermal injury, blister formation may occur at to weeks with doses in