- A unique feature of the present cases was the appreciable differences in the latent periods observed for legions on various parts of the body. These differences cannot be expliined entirely on the basis of severity, since the severe foot lesions developed after most other lesions. However, the severity might have been in part due to the dependent position of the Loot rather than greater radiation injury. Lesions on flexor surfaces in general preceded lesions on extensor surfaces. The present data suggest that the latent period and radiation sensitivity on of various skin areas may differ. Previous work has shown that flexor surfaces with thinner epidermis are in general more sensitive than extensor surfaces with thicker epithelium." The destructive and atrophic changes of the epidermis, disturbances in keratinization, and atrophy of hair follicles are characteristic of histopathologic radiation injury of the skin.?-!3 18-3! Severeinjury to the dermis and blood vessels was not observed, The minimal dermal injury wilh severe epidermal injury is in keeping with the low energy beta component present and the marked decrease in depth dose over a distance of a few microns from the surface. Hyperpigmentation of injured areas was a consistent finding in the Marshallese, the Japanese, and (he American Negroes, Pigmented lesions were also observed tu a lesser extent in the white Americans. Pigmentation of this nature has not been described as a constant charac teristic of radiation damageto the skin. There is no satisfactory explanation for the darker dusky-gray color that appeared in some of the skin lesions as healing progressed. The color changes may be due tu alterations in local pigment production, vascular changes, or a thinning of the epidermis, rendering it more translucent with resultant darker appearance of the pigment layer. Later biopsies may explain this phenomenon, There are features of the lesions described that appear unique, e.g., the absence of visible multiphasic responses, the presence of early hyperpigmentation, the long latent periods, and the severe epidermal injury with minimal dermalinjury. It is possible that differences may in part be on a racial basis.* In addition, the marked difference in histologic response of the epidermis and dermis in the present series is in marked contrast to the usual radiation re- sponse of the skin produced by high energy X- or beta-rays. " In Table 3.3 are listed the approximate minimal surface skin doses required to produce recognizable epidermal injury in ar.mals. It is apparent from the table that beta'ray energy is - of considerable importance in determining the degree of injury. A number cluding knowledge of the beta ray spectrum from the fallout, would have to data were to be used to estimate biologically the beta dose received by the difference in dose between that required to produce threshold skin damage of assumptions, inbe made if these Marshallese. The and that for per- manent damage in pigs is 500 to 1000 rep.*° It is impossible to estimate the probability of development of radiation cancer at the site of the healed Jesions. The absence of scarring, telangiectasis, and extensive chronic vascular lesions tends to improve the prognosis since the foregoing are usually observed to precede the development of radiation cancer. A favorable prognosis is aiso suggested by the following evidence: an analysis of 1100 individuals exposed to low voltage X-ray for dermatological conditions revealed nu evidence of cancer induction 5 to 23 years after treatment. MacKee'® states that epitheliomata rarely develop after a single dose of radiation to the skin. Lastly, the incidence of skin cancer in Negroes is one sixth to one ninth the incidence in Caucasians.** If neoplasia can develop purely as the result of epidermal irradiation, the incidence of late cancerg may be enhanced since the dose of radiation and the visible gross and microscopic injury to the epidermis-greatly exceeded that to the dermis. Since many children and young adults were involved, the life expectancy of a large number of the exposed people will exceed the long induction period for development of radiation cancer observed in radiologists. Long * Reported clinical experience with radiation skin lesions is based predominantly on the response of white-skinned people, whereas the type of lesions described herein, with one clear cut exception, were observed in Japanese, Marshailese (negroid), and American Negroes. The exception was a dark brunette individual. 44