Limit for Exposure to ''Hot Particles'' on the Skin: Recommendations of the National Council on Radiation Protection and Measurements (Ncrp Report, No 106) (N C R P Report)

NCRP REPORT No. 106

LIMIT FOR EXPOSURE TO "HOT PARTICLES" ON TH.E SKIN Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION AND MEASUREMENTS

Issued December 31, 1989 National Council on Radiation Protection and Measurements 7910 WOODMONT AVENUE / Bethesda, MD 20814

LEGAL NOTICE This report was prepared by the National Council on Radiation Protection and Measurements (NCRP). The Council strives to provide accurate, complete and useful information in its reporta. However, neither the NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this report, nor any person acting on the behalf of any of these parties: (a) makes any warranty or representation, express or implied, with respect to the accuracy, completeness or usefulnees of the information contained in this report, or that the use of any information, method or procees disclosed in this report may not infringe on privately owned rights; or (b) assumes any liability with respect to the use of, or for damages resulting from the use of any information, method or process disclosed in this report, under the Civil Rights Act of 1964, Section 701 et seq. as amended 42 U.S.C. Section 2000e et seq. (Title VII) or any other statutory or common law theory governing liability.

Library of Congress Cataloging-in-PublicationData National Council on Radiation Protection and Measurements. Limits for exposure to "hot particles" on the skin: recommendations of the National Council on Radiation Protection and Measurements. p. cm.-(NCRP report ; no. 106) "Issued January 15, 1990." Includes bibliographical references. ISBN 0-929600-11-8 : $12.00 (est.) 1. Ionizing radiation-Safety measures. 2. Ionizing radiationDosageStandards. 3. Beta rays-Health aspects. 4. Skin-Effect of radiation on. I. Title. 11. Series. [DNLM: 1. Environmental Exposure. 2. Radiation Injuries. 3. Skin-radiation effects. WR 100 N277Ll RA569.N353 1990 612'.01448-dc20 DNLMDLC for Library of Congress

89-71259 CIP

Copyright 8 National Council on Radiation Protection and Measurements 1989 All rights resewed. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotation in critical articles or reviews.

Preface This report has been prepared as a result of a request to the National Council on Radiation Protedion and Measurements (NCRP) from the Nuclear Regulatory Commission (NRC). In recent years, nuclear utilities have identified the potential for a limited number of employees to come in contact with microscopic particles that are radioactive. These particles have been given the generic name, independent of their source and particular chemical and radioactive content, of "hot particles." This report addresses the potential biological effects of hot particles on the skin and reviews the presently available information on the subject. This information is presently less complete than one would wish, and more information is to be expected in the near future. In the meantime, the report makes recommendations on limits of exposure from hot particles in the work place based on avoidance of severe deterministic effects. The support of the NRC for this particular facet of the NCRP program on radiation effects on the skin is gratefully acknowledged. Serving on Scientific Committee 80-1, that prepared this report were: Thomas F. Gesell, Chairman U.S. Department of Energy Idaho Falls, Idaho Members

P. Donald Forbes Temple University Philadelphia, Pennsylvania

William C.Roesch Richland, Washington

Charles B. Meinhold Brookhaven National Laboratory Upton, New York

H. Rodney Withers University of California at Los Angeles Los Angeles, California Consultants

R. J. Michael Fry Oak Ridge National Laboratory Oak Ridge, Tennessee

Roy E. Shore New York University Medical Center New York, New York

NCRP Secretariat- William M. Beckner

The Council wishes to express its gratitude to the members and consultants of the Committee for the time and effort devoted to the preparation of this report. Bethesda, Maryland October 25, 1989

Warren K.Sinclair President, NCRP

Contents Preface ............................................................. 1. Introduction ................................................... 2. Scope of the Report ........................................... 3. Biological Effects of Irradiation of the Skin ............. 3.1 Introduction ................................................ 3.2 Nonstochastic Effects (Deterministic Effects) ........... 3.2.1 Acute Nonstochastic Effects ....................... 3.2.2 Late Nonstochastic Effects ......................... 3.2.3 Nonstochastic Effects Versus Dose for Large Area Irradiation ..................................... 3.2.4 Nonstochastic Effects From Hot Particles ........ 3.2.5 Review of Biological Effects of Hot Particle Irradiations .......................................... 3.2.5.1 Monkey Experiments ...................... 3.2.5.2 Human Experiment ........................ 3.2.5.3 Swine Experiments ........................ 3.2.5.4 Comparison of Studies ..................... 3.3 Stochastic Risk of Irradiation ............................. 3.4 Comparison of Nonstochastic Effects (Deterministic effects) and Stochastic Risk of Hot Particle Irradiation 4 Approach to Establishing a Practical Limit ............. 5. Interpretation of Experiments With Hot Particles Using the Approach of the Total Number of Beta Particles Emitted ............................................. 5.1 Monkey Studies ............................................ 5.2 Human Study ............................................... 5.3 Swine Studies with Microspheres ........................ 5.4 Swine Studies with Other Sources ....................... 5.5 Estimation of a Threshold ................................. 6 Observations on Humans Exposed Inadvertently in the Work Place ................................................ 7 Derivation of an Exposure Limit for a Hot Particle on the Skin ......................................................... 8. Recommendations on Radiation Exposure Limits for the Special Case of a Hot Particle on the Skin .......... Appendix A: Comparison of Point Dose with Dose Measured with an Extrapolation Chamber

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CONTENTS

Appendix B: Number of Beta Particles from Irradiated a96UC.Microspheres ............................. References ......................................................... The NCRP .......................................................... NCRP Publications ............................................. Index ................................................................

29 32 36 43 53

1. Introduction Small alpha-emitting radioactive particles in the lung have been previously recognized as a radiation protection issue (NCRP, 19751.' Recently, irradiation of the skin by small beta or beta-gamma emitting particles has become of concern in the nuclear reactor industry. This concern has occurred, a t least in part, as a result of the employment of more sensitive personnel monitoring equipment than was previously available. This increased sensitivity has resulted in an increase in the number of incidents involving identification of radioactive particles on the skin. These particles are known as "hot particles," 'Yeas," or "specks." The term "hot particles" will be used in this report. They most commonly contain T o or fission products. The likely source of the particles containing *Co is particles of wear-resistant alloy from valve seats, etc., containing a high percentage of stable cobalt, that enter the primary coolant and become activated in the core via the reaction 6 9 C(n, ~ Y) '%o. The source of the particles which contain fission products is fuel elements which have defects in their cladding. Hot particles usually cannot be detected by the unaided eye because they range in size from approximately one pm to several hundreds of pm. Hot particles apparently become electrically charged as a result of radioactive decay and, therefore, tend to be fairly mobile, "hopping" from one surface to another. The radioactivity of particles containing fission products ranges from 40 Bq to 400 kBq (1nCi to 10 pCi) with most particles being in the range of 400 Bq to 40 kBq (10 nCi to 1 pCi). The radioactivity of the O ' Co particles ranges from 40 Bq to 20 MBq (1nCi to 500 pCi), with most in the range from 400 Bq to 200 kBq (10 nCi to 5 pCi) (Warnock, et al., 1987). The particles are not water soluble and if embedded in clothing are difficult to remove, even by laundering. "Clean" laundry has

NCRP,1975 concluded that particulate plutonium in the lung causes no greater risk of lung cancer than the same amount of plutonium more uniformly distributed throughout the lung. However, for hot particles on the skin the effect to be protected against is the nonstochastic risk of acute ulceration of the skin (see Section 3.41, for which the concept used in NCRP, 1975 may not be applicable and therefore it is not considered further in this report.

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1. INTRODUCTION

been implicated a s the source of hot particles involved in some skin contamination events (INPO, 1987). A unique aspect of hot particles in contact with the skin is that very small amounts of tissue can be exposed to very large, highly non-uniform doses. Average doses can be calculated if the particle can be characterized by nuclide and by activity, but the result will depend strongly upon the amount of tissue included in the averaging process and the depth or depths a t which the averaging is performed. The interpretation of the resultant dose, when various forms of averaging are used, is not straightforward. Existing methods for assessing exposure of the skin are appropriate when large areas of skin, greater than a few tens of square centimeters, are irradiated. For skin irradiation of a few square centimeters, the existing limits provide more than adequate protection, and for very small areas of skin irradiation, such a s occurs with hot particles, the existing limits (NCRP, 1987) are overly restrictive. Minimizing the production and release of hot particles and prevention of skin contamination are clearly the preferable control methods, but the possibility of contamination events cannot be ignored. A consistent method for assessing the biological effect of these events is required so that reasonable radiation protection criteria can be applied to this unique situation.

2. Scope of the Report This report reviews the radiobiological effects of hot particles on the skin and recommends a limit on the product of their beta-particle emission rate and duration of e ~ p o s u r eFor . ~ the end points addressed by this report, beta particles are the radiation of concern. Relative to the beta particle dose, the gamma radiation associated with a beta-emitting hot particle on the skin does not contribute significantly to the tissue dose in the vicinity of the particle. This report does not deal with general skin contamination, skin exposure from distant sources, or inhalation or ingestion of hotparticles, and it does not deal with the special cases that might arise, for example, as a result of hot particles in the eyes or on the eardrums. For the purpose of this report, a hot particle is arbitrarily considered to be a discrete radioactive fragment that is insoluble in water and is no larger than approximately 1mm in any dimension. In addition, only hot particles directly on the skin are considered in this report.

Beta emission rate is the rate of beta particles emitted from the radionuclide(s1 making up the hot particle and not the rate of beta particles emitted from the surface of the hot particle.

3. Biological Effects of Irradiation of the Skin 3.1 Introduction The fimdamental philosophy of radiation protection includes: (1) prevention, to the extent practicable, of the occurrence of severe nonstochastic diseases (deterministic effects), (2) limitation of stochastic risks, fatal cancer and genetic effeds, to a reasonable level in comparison with non-radiation risks, and (3)maintenance of radiation exposure a t levels as low as reasonably achievable (ALARA) economic and social factors being taken into account (NCRP, 1987). The biological effects of irradiation of the skin that are of interest are acute and chronic nonstochastic effects and the stochastic risk of non-melanoma skin cancer.

3.2 3.2.1

Nonstochastic Effects ( D e t e r m h b t i c Effects)

Acute Nonstocbtic Effects

Acute, nonstochastic effects on the skin (those occurring in a few hours to a few weeks) after irradiation of 1cm2 or greater are, with increasing dose, transient erythema, more prolonged erythema, dry and then moist desquamation (after a latency of three to six weeks) and finally secondary ulceration. The latency period of these effects is not strongly dependent on dose. Dry and moist desquamation are related to the reduced reproductive capacity of target cells in the basal layer of the epithelium a t a depth of about 20 to 120 pm. If moist desquamation persists, secondary ulceration may develop with loss of dermal tissue; such ulceration heals by invasive fibrosis. 3.2.2

Late Nonstochclstic Effects

Late or chronic nonstochastic effects developing after protracted irradiation of large areas of skin, with increasing dose, and possibly

3.2 NONSTOCHASTICEFFECTS

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without acute effects, include changes in pigmentation; atrophy of the dermis, sweat glands, sebaceous glands, and hair follicles;fibrosis of the dermis: and increased susceptibility to trauma with the development of late necrosis. 3.2.3 Nonstochastic Effects Versus Dose for Large Area Irmdiation Based on experience with radiotherapy with orthovoltage x rays, Ellis (1942) and Paterson (1948) proposed safe ''tolerance" doses for irradiation of human skin. The dose of x rays or gamma rays required to produce a certain level of clinical damage (clinical tolerance level) increases as the area of irradiation decreases (Cohen, 1966; Eads, 1972; ICRP, 1984). However, the biological basis of clinical tolerance was not defined and tolerance should not be confused with isoeffect doses. The threshold d m for a visible reaction for beta particle irradiation also increases with decreasing field size (Wells et al., 1982; ICRP,1984). The slope of the curve of the mean skin reaction versus acute xray dose, for large field irradiations, required to produce the various acute nonstochastic effects for single exposures is very steep (see Figure 9 of ICRP, 1984). The acute dose required to produce faint erythema is approximately 8 to 10 Gy while the acute dose required to produce ulceration is 20 to 25 Gy. 3.2.4 Nonstochtic Effects From Hot Particles

Irradiation of the skin by hot particles does not produce acute nonstochastic effects that are comparable to those seen with large field irradiations. For example, with small areas of irradiation, migration of basal cells of the epithelium from the edge of the damaged area prevents moist desquamation. The detrimental reaction, seen after very high exposures of hot particle irradiation, is acute ulceration or necrosis. The underlying mechanism responsible for the effect is the killing of endothelial and fibroblast cells in interphase (the interval between two successive cell divisions) at a depth of about 150-300 p.m in the superficial dermis. The effect develops over a much shorter period of time (about 1to 2 weeks) than do dry and moist desquamation following large field irradiation. The rapidity of the appearance of the effect is due to the characteristics of cell death during interphase (Hopewell, 1986h3 Charles, M., Forbes, P. D., Fry, R. J. M., and Hopewell, J. W. (1989).Personal communication on the nonstochastic decta of hot particles.

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3. BIOLOGICAL EFFECTS OF IRRADIATION OF THE SKIN

3.2.5 Review of Biological Effects of Hot Particle Irradiations

Experiments, applicable to hot particles, have been performed on animals, and in one case on a human. These studies are reviewed below. 3.2.5.1 Monkey Experiments. The skin on the backs of monkeys (Macaca speciosa) was exposed to hot particles in a series of three experiments conducted a t the Los Alamos Scientific Laboratory (Biological and Medical Research Group, 1965 and Dean et d.,1970). The authors reported that histological examination of the unexposed monkey skin showed similarity to the thickness and structure of human skin, especially that on the dorsal forearm.The hot particles were 236UC2 "microspheres" which had been irradiated in a reactor. They were covered with a 25 pm layer of graphite and ranged in total diameter from 150 to 250 pm. The particles were placed directly on the skin, and exposures were "acute", taking less than 6 hours. Radiation dose was expressed by the authors as a point dose 100 pm directly beneath the particles. In the first monkey experiment, a total of 13 areas were exposed with point doses ranging from 12.7 to 96 Gy (1.27'to 9.6 krad) and, hence, only a few areas were exposed a t similar doses. The only reaction observed was a slight erythema, 1to 2 mm in diameter, a t the highest dose site. The maximum erythema occurred 48 hours post exposure suggesting that cell death during interphase was i n ~ o l v e dThere .~ was no evidence of the exposure after 26 days. The second monkey experiment involved nine areas exposed with point doses ranging from 158 to 521 Gy (15.8 to 52.1 krad). Erythema was visible a t all exposure sites a t 48 hours. There was an elevation of the stratum corneum a t the two highest dose sites. Based on current knowledge, the observed change was probably blistering due to the separation of the epidermis from the underlying damaged dennis, and therefore the initiation of a shallow ulcer.3 It was reported that a t 28 days, a shallow, dry desquamation was observed only a t the site irradiated to 521 Gy (52.1 krad). The dry desquamation had disappeared by 36 days. There was no observable ulceration or dermal necrosis reported in this series. However, from the description and current knowledge, the dry desquamation was probably a superficial ulcer covered by dry serum exudate3 By 90 days no lesions were visible or palpable. In the third monkey experiment, a total of 11sites were exposed to doses ranging from 1,570 to 6,640 Gy (157 to 664 krad). There was erythema a t all sites with a maximum diameter of 8 mm a t the highest dose, elevation of the stratum corneum, and palpable nodules

5 to 8 mm in diameter. The maximum gross observable reaction occurred a t 15 days. Ulceration was observed a t sites with point doses of 2,610 Gy (261 krad) or higher. This lesion is presumed to represent deep ~ l c e r a t i o nAll . ~ sites a t lower doses showed what was termed dry desquamation of 3 to 4 mm diameter. As suggested above, the term dry desquamation appears to have been used to describe what is now considered to be a superficial ulcer covered by dry serum e ~ u d a t eThe . ~ ulcers remained as open sores for about two weeks. By 71 days, all ulcerated sites had healed. The ulcerated sites showed a dimple about 3 mm in diameter. After 300 days the residual clinical lesions a t the higher dose sites consisted of a crater-form depression some 2.5 mm in diameter surrounded by a flattened crater lip 1.5 mm in width. The floor of the crater and the flattened lip were devoid of hair. Welldeveloped hair was present along the outer margin of the lip. Human Experiment. The inner surface of the forearm of a human volunteer was exposed to three different dose levels using 2S6UCz microspheres like those described above (Dean and Langham, 1969 and Dean et al., 1970). The point dose values were 142,400 and 540 Gy (14.2, 40 and 54 krad). The lowest dose produced a slight erythema, the intermediate dose produced an erythema and the highest dose produced a possible small dry desquamation following the erythema reaction. No ulceration was reported. However, as indicated previously, the small area of dry desquamation was probably a superficial ulcer covered with dry serum exudate3 Following the erythema response, a small freckle was visible. Two years after the experiment, the exposure sites could not be found on the skin. 3.2.56

3.2.5.3 Swine Experiments. Forbes and Mikhail (196914 exposed microspheres similar to the skin of swine to radiation from 236UCz those used for the monkey and human studies by Dean and coworkers described earlier. A total of 19 exposures were made with point doses which ranged from 2,400 to 74,000 Gy (240 to 7,400 krad). Exposures were acute with exposure times typically in the range of two to three hours. All the exposures were large enough to produce lesions described by the authors as ulcers. The diameter of the ulcers Circumstances outside the control of the authorsforced terminationof their studies before exposures sufficiently low to defme a threshold could be made. Although a report was prepared (Forbes and Mikhail, 1969) it was never published, except as an abstract,due to termination of the study. However, the basic data have been published and discussed (Wells and Charles, 1979; Charles and Wells, 1980; Wells, 1986; and Charles, 1986) and the original report was made available to the committee that drafted this report by one of the authors (Forbes).

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3. BIOLOGICAL EFFECTS OF IRRADIATION OF THE SKIN

ranged from 0.5 mm a t the lower doses to 8 mm a t the highest dose. Ulcer diameter was defined by the authors as the diameter of the denuded, oozing sore and, in turn, the diameter of the eschar (scab) which subsequently filled the denuded space. The clinical sequence of events is summarized as follows. The site of each exposure exhibited erythema by the time the particles were removed from the skin. Reddening peaked a t 24 hours and reappeared irregularly thereafter, possibly because of infection. Hair growth ceased in the erythematous area but began again after 30 days except in the central scarred area. Beginning on the third day, darkening of normally pigmented skin formed a halo around the center of the exposed spot. A narrow ring of hyperpigmentation remained permanently around the edge of the scar.The skin nearest the particle was permanently depigmented. The disappearance of early erythema coincided with the appearance of one or more very small vesicles (blisters). These gave way to a single bulla or larger blister which was always transient, usually granular in appearance and variable in contour. With the loss of this surface material, each lesion developed into a n oozing sore. The authors interpreted this sore as indicating loss of functional epidermis. The larger lesions, associated with the higher doses, developed cavitation which the authors interpreted as evidence for loss of some dermis. The authors applied the designation of ulcer to all of the lesions but suggested that the smaller lesions may not have been ulcers in the strict sense. These observations are consistent with the description given in the human and monkey studies. In those studies,the change was described as elevation of the stratum corneum, although after lower doses than those used in these studies on swine. All ofthe observations of Forbes and Mikhail on swine are consistent with interphase cell death and deep ulceration that involved damage to the dermis (Hope~e11,1986).~ The development of the lesion was more rapid a t the sites of higher dose; healing events occurred more rapidly after lower doses. An effect described as moist desquamation appeared at the sites as early as the second day after exposure. Today, such a finding would be considered to be blistering, as described earlier, since moist desquamation can not occur a t such an early time as two days after irradiat i ~ nThe . ~ area of involvement gradually spread outward to its maximum size; full development of the lesion took two to four weeks. Dry eschar (scab) formation commenced within three days after the appearance of an ulcer; six weeks after exposure all lesions were dry. Scarification was completed in most cases by 12 weeks. Residual evidence of damage included scars, flaking, hair loss, and pigment

alterations. Reappearance of inflammatory changes was noted a t irregular intervals but healing proceeded satisfactorily. A team of British researchers has performed an extensive series of measurements of the effects of beta sources of different sizes and different energies on swine skin. The results are summarized by Hopewell et al., (1986). Some of their measurements were conducted with sources too large to approximate hot particle geometry. For the larger sources, they obtained about the same threshold doses observed by others for such sources (Moritz and Henrique, 1952; George and Bustad, 1966). Hopewell et al, (1986), however, did conduct extensive studies with a range of beta-particle energies and exposures of a large number of areas of pig skin, which included experiments with 1and 2 mm "Sr sources, with 0.1,0.5 and 1mm 17Tmsources and with a 2 mm 147Pmsource. They performed multiple exposures a t numerous dose levels and constructed plots of the probability of an effect as a function of dose. This approach allowed the determination of the dose required to produce the effect 50 percent of the time and allowed estimates of the threshold and the dose required to produce the effect 100 percent of the time. (This type of analysis was possible because the number of irradiated areas was larger than was used in the other studies.) The doses reported in the British studies were not precisely the point doses reported by the studies described above but were doses measured with an extrapolation chamber with a window thickness of 16 pm and an effective collecting area of 1.1mm2. Their sources were attached to source holders making the irradiation geometries different from that involved with the 236UC2 microspheres. The endpoint used in the British studies was designated by the authors as acute necrosis which is biologically consistent with acute ulceration. The authors also explicitly stated that moist desquamation could not occur with source sizes less than 2 rnm in diameter because the mechanism of the lesion is different with small area irradiation. The time course of acute necrosis varied with the dose level. For doses which induced the effect more than 50 percent of the time, the acute necrosis was visible in about two weeks and persisted for four to five weeks. For doses that induced the effect less than 50 percent of the time, the effect was very transient, so much so that scoring the animals twice a week resulted in more positive observations than scoring them once a week. Comparison of Studies. The data h m the studies described above are plotted in Figure 3.1 using end-points and doses as reported by the authors. It can be seen that the data for monkeys, the human and the swine study of Forbes and Mikhail(1969) show a consistent 3.2.5.4

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3. BIOLOGICAL EFFECTS OF IRRADIATION OF THE SKIN - KEY TO EFFECTS A

WEPPECT

B

WLDERNEY*

C D

ERrmEYl DRYDEBOUIYITK)(I ACUTENECROBLS ULCERATON

E F

Fig. 3.1 Biological effects of small radioactive particles on the skin as a function of dose.

pattern. These studies were all conducted with irradiated 236U62 fuel particles in the 100 to 200 micrometer size range. All of these studies related effects to a point dose a t 100 pm beneath the particle. Where observed, the authors reported mild erythema, erythema, dry desquarnation and u l c e r a t i ~ n . ~ The points plotted in Figure 3.1 for the swine studies summarized by Hopewell, et al. (1986) differ in several respects from the others shown. The doses were delivered by 1mm diameter and by 0.1, 0.5 and 1mm 170Tmsources. All three sizes of the 17% sources gave essentially the same dose response and are plotted as a single point in Figure 3.1. The reported doses were measurements made with an extrapolation chamber with a window thickness of 16 pm and an effective collection area of 1.1mm2.Other measurements were made a t different depths and over different areas. The points plotted represent the dose required to produce acute necrosis 50 percent of the time. The authors did not publish data for other endpoints such as erythema. Estimated thresholds (zero percent probability of effect) could have been plotted. These thresholds are two to three times less than the dose required to produce an effect 50 percent of the time. The term dry desquamation when used to describe an effect aseociated with hot particle irradiation is assumed to be synonomous with superficial ulceration that was covered with dried serum exudate, Charles, M., Forbes. D., Fry, R. J. M. and Hopewell, J. W. (1989)personal communication.

3.3 STOCHASTIC RISKS OF IRRADLATION

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-KEY TO EFFECTS A NOERECT 0 UnDrnEYA C CRYrnaU D D R I ~ A ~ O N E *CUILtlEcROSP) F ULcEnAmc

Fig. 3.2 Biological effects of small radioactive particles on the skin. Doses have been adjusted to a common basis.

In a further comparison of the results of the various studies, the doses for the 236UCzmicrospheres were reevaluated by calculating the dose they would have produced had they been measured using the extrapolation chamber used in the British studies (see Appendix A). This calculation resulted in a reduction of the point doses for the microspheres by a factor of three (see Figure 3.2). With the doses on a common basis and current interpretation of the description of the lesions, there is reasonable agreement among the results of the studies.

3.3 Stochastic Risk of Irradiation

The principal stochastic risk associated with irradiation of the skin is non-melanoma skin cancer, i.e. basal cell and squamous cell skin cancers (NASINRC, 1980). The risk of skin cancer following irradiation of the skin by hot particles is less than when extended areas of the skin are irradiated due to the very small number of cells involved and the greater potential for cell killing from the possibly high local beta particle dose. In the past decade, a number of radiation epidemiology studies have obtained data on skin cancer incidence. Most have found an association between radiation and non-melanoma skin cancer. (The

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3. BIOLOGICAL EFFECTS OF IRRADIATION OF THE SKIN

association of radiation with malignant melanomas is equivocal.) The irradiated skin areas in the studies have ranged from about 100 cm2up to whole body exposures. Ten studies have the requisite information for deriving risk estimates: a defined irradiated study group with reasonably complete follow-up for skin cancer incidence and estimates of skin dose. Six of them provide data on skin cancer induction following irradiation of skin areas which have appreciable ultraviolet radiation (UVR)exposure (primarily head and neck) (Schneider et al., 1986; Van Daal et al., 1983 and Van Vloten et al., 1987; Hildreth et al., 1985; Ron et al., 1988; Shore et al., 1984; Sevcova et al., 1978, Sevcova et al., 1984 and Sevc 1988). Five of the studies provide data on irradiation of relatively UVR-shielded skin areas (trunk irradiation or irradiation of blacks who are UVR-shielded by melanin) (Davis et al., 1987, Hrubec et al., 1989 and Boice, 1988; Shore et al., 1986 and Hildreth and Shore, 1988; Boice et al., 1985; Hay et al., 1984; Shore et al., 1984). These studies show that radiation induction of skin cancer is greater in UVR-exposed skin than in WR-shielded skin, suggesting that UVR is a promoter of skin cancer in cells initiated by ionizing radiation. This interpretation is supported by experimental results as well (Fry et al., 1986). Composite estimates of excess skin cancer incidence have been calculated based on the studies of UVR-exposed and WR-shielded skin. The estimates were projected out to lifetime risks using lifetable methods, under the assumption that exposures were received over a working lifetime of ages 20 to 60 (Shore 1989). Although the available data tend to favor a relative risk (RR) projection model over an absolute risk (AR) model (Shore 1989), the data are too limited to be definitive, so both models were examined. Separate estimates were calculated for males and females (who differ somewhat in baseline skin cancer rates and in life-spans), but the results are similar enough that averages across sexes can be used. If the irradiation of an area of 2 mm2 per hot particle a t 100 pm depth (Dean and Langham, 1969) is assumed, then the risk of skin cancer per hot particle exposure would reflect this small irradiated area. The estimates of skin cancer induction, therefore, were scaled down in proportion to skin surface area from the approximate area of UVR-exposed (3,000 cm2)or UVR-shielded (15,000 cm2)skin to an area of 2 mm2. The risks of skin cancer induction for a 2 mm2 irradiated area are shown in Table 3.1. For UVR-exposed skin the risk estimates are