acute skin lesions due to localized "hot particle" radiation exposures

7/29/2019 ACUTE SKIN LESIONS DUE TO LOCALIZED "HOT PARTICLE" RADIATION EXPOSURES

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ACU TE SKIN LESIONS DUE TO LOCALIZED"HOT PARTICLE" RADIATION EXPOSURES*

J.W. Baum, A. L. Carsten, D.G.L. Kaurin, and C.W. SchaeferBrookhaven National Laboratory, Upton, NY 11973-5000

ABSTRACTThe purpose of these studies was to determine the incidence and severity of lesions resulting from very localizeddeposition of do se to the skin from small (< 0.5 mm ), discrete radioactive particles. Hanford mini-swine wereexposed to localized d oses from 0.2 to over 600 Gy (averaged over 1 cm2 at 70 urn depth) from isotopes havingmaxim um be ta particle energies from about 0.3-3 M eV. The incidence of erythema and scabs (indicating ulceration)were scored routinely for u p to 71 days post-irradiation.The re spo nse s followed normal probability distributions, and thus, no true threshold could be defined. Ten and

50 % incidence rates w ere deduced using probit analyses. The lowest dose which produced 10% incidence was about1 Gy for e xposu res to Yb- 175 (0.5 MeV maximum energy) beta particles. The severity of lesions was estimated byanalyzing th e results in term s of their diameters and persistence. From preliminary consideration s of the proba bilityof induction, size, and persistence of acute lesions, a special limit for hot particle exposures in the range of 5-50 Gymay be reason able, w ith an action level between about 1 Gy and the limitINTRODUCTION

Exposu re to w orke rs to small radioactive particles, typically ranging in size from one m icron to a few hundredmicrons in diame ter ha s bee n identified as a problem in the nuclear power industry. These particles have becom eknow n as "h ot pa rticles" due to their relatively high specific activity and small size. Wh en deposited o n the skin orinhaled, they can cause intense local irradiation of small areas of tissue and may cause reddening, ulceration, necrosis,and a possible incre ase in the risk of skin cancer.In D ecemb er 1989, the National Council on Radiation Protection and Measurements (NCR P) published a reportwhich reviewe d the effects of exposure to hot particles in a human volunteer, in monke ys, and pigs (1). Based on thisreview, a re comm ended limit of 10' beta particles (75 uCi h; 10 GBq s) emitted from a "point" particle or a particleof less than 1 mm in diameter w as suggested for particles in contact with the skin. This limit was intended to prevent"deep" ulcera tion bu t, adm ittedly, might allow transient effects w hich frequently d isappea r in less than a week, a sobserved by Hop ewell et al. (2). For beta particles from activated fuel, this exposure causes a do se of about 5 Svaveraged over 1 cm2 at a dep th of 70 um in tissue. The risk of skin cancer following irradiations of the skin by hotparticles wa s deem ed less than that when extended areas of the skin are irradiated due to the small number of ce llsexposed and th e gr eater po tential for cell killing from the localized beta particle dose .In 1987, an International Commission on Radiological Protection (ICRP) Task G roup reviewed much of thesame material review ed by the NC RP. However,- the Task Group based its recommen dations on preventing acutetransient ulceration and recommended restricting to 1 Sv the dose delivered within a few hours ove r an area of 1 cm2measured be tween depths o f 100-150 urn (10-15 mg/cm2) (3). The latest recommendations of the ICRP, neverthe-less, retain the limit of 500 mSv averaged over any 1 cm2, at a nominal depth of7 mg/cm 2, regardless of the areaexposed (4 ). Th e ob jectives of this work were to provide additional scientific data to use in the decisions andrecomm endations o f thes e committees and the regulatory agencies.MATERIALS AND METHODS

Hanford mini-sw ine w ere exposed to four types of particles at Brookhaven National Laboratory (BN L): 1)activated UC2 particles of 250 um diameter for comparisons with the data of Forbes and Mikhail used in NCRPReport 106 (1); 2) Tm -170 particles with dimensions of about 230-500 um for com parisons with Hopewell et al'sdata used extensively by the ICRP Task Group (3); 3) Yb-175 particles with dimensions of 280-500 urn andmaximum beta particle energ y of 0.5 MeV (to simulate Co-58 which emits a 0.47 Me V positron); and 4) Sc-46 withbeta and gamm a em issions very similar to those of Co-60. Cobalt-60, Co-58, and activated fuel are the comm onestsources of hot particles in nuclear power plants. Scandium-46 was used rather than Co-60 because its shorter half-life and low er density a llowed the production of particles that simulated 100-um -diameter C o-60 particles with muchshorter activation periods. UC2 particles were 252-um-diam eter spheres coated with 18.5 um of pyrolytic graphite.Other particles were cut from foils of thickness 258 u m (Tm), or 130 um (Yb and Sc). The largest dimension of thelatter pieces was 53 5 u m.

*This wo rk w as supported by the U.S. Nuclear Regulatory Commission. C. Schaefer was supported bythe Health Physics Fellowship Program, administered by Oak Ridge Institute for Science and Education for theU.S. Department of Energy.


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Irradiation s wer e made with in tattooed grids on the flanks of anestisized pigs. Sources were mounted o nstyrofoam blocks to m inimize backscatter. Exposures of 45 s or less were made using a hand-held cone/rod jig withthe sour ce fastened to the end of a styrofoam con e, 8 cm long, attached to a plexiglass rod 22 cm in length. Longerexposure s w ere m ade using a cloth harness with elastic straps which flexed as the animal breathed, yet held thesource in a fixed position for up to two hours.UCj and Tm-170 particles were employed both touching the skin and slightly above the skin to test thehypothesis that more beta particles might be required to induce ulcers in the latter case which minimizes particles onclothin g. Anim als rang ing in age from 3.8 to 15 months were exposed to see if age at exposure had any effect on theoutcom e. Various scoring p rocedures were explored to test the sensitivity of results to the number of observationsper w eek, the num ber of numb er of observers, and the number of days of observation post-exposure. The sensitivityof results to back grou nd co rrections also was studied. In addition, the diameter of any lesion and area of erythemawa s m easured, using an overlaid scale, and their persistence recorded.The following w ere the scoring procedures for erythema and scabs:Me thod A . wh ich detected the most lesions, is a modified Hopew ell et al. (2) method in which the mini-swinewer e scored twice per w eek for 28 days using two or sometimes three observers (Hopewell, personal comm unica-tion). To model this scoring criterion, we eliminated data from one of the three weekly observations for the first 21days pos t-irradiation. Thereafter, BNL observers made only two observations per w eek, which was similar to theHopew ell scoring sch edule. The BNL lesion-scoring criteria were as follows. If two observers scored an area as alesion on the s ame day, or if two observers scored the area as a lesion on one day and one observer scored it as alesion on the suc cessive d ay, or visa-versa, the area was scored as one-half of a lesion. If two observ ers scored the

- area as a lesion on two conse cutive days , then it was considered to be a full lesion./ Method B. or "4-of-674-of-4," gave fewer lesions than Method A. F or the first three weeks in which the mini-"," sw ine wer e score d three tim es per week, a lesion was considered to be radiation-induced if two independent- observers each sc ored an area as a lesion a minimum of twice in one week. This method was called 4-out-of-6, since! there were two obs ervers a nd three observation periods, giving six possible outcomes, and four scorings of a lesion we re required out of the six possibilities, with each observer seeing the lesion at least twice. After the first 21 days,. - the pigs w ere scored twice per week, and a lesion was considered to be radiation-induced if two observers each-"'. scored a lesion on two consecu tive observation days -w hic h was called 4-out-of-4 .'", Me thod C. or "6-of-8/5-of-6,'* was the most restrictive scoring technique used, and was designed to minimize:" any spu rious apparent back ground. A lesion'was considered radiation-induced if two observers scored an area as a" lesion three time's each out of a possible four consecutive scoring periods for the first tljree wee ks. Afler three week s,7 wh en the pigs we re only scored twice per week, and a lesion was considered radiation-induced if one observer scored- an area as a lesion a minimum of two times and another observer scored the same area as a lesion a minimum of threeji times in three consecutive scoring periods.r" RESULTSAnalysis of the results as a function of the animal's age at exposure revealed no single pattern of changes with


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Table I. Comparison of the Findings of Hopewell et a!., BNL, andEPRI*Isotope

Pm-147Co-60"Sc-46Yb-175Tm-170Sr/Y-90U (act.)

(MeV)0.220.310.360.470.971.4**

1.8 (1.1)**

E1Vf(MeV)0.0620.0960.110.120.290.55

0.71 (0.4)

HopewellED 10 (Gy)

2.7

1.93.4

ED M (Gy)3.9

4.58.4

BNLED 10 (Gy)

5.11.32.8

8.5

ED M(Gy)

125.55.9

11.4

RecceE D M (Gy )

40

18

40* Dose averaged over 1 cm2at 70 urndepth (Gy).**Average of themaximum energies emitted by the isotopes in thesource. Values inparentheses are for Reece'ssources.

Tm-170, and activated U particles were 3-4 times higher than thecorresponding results for Sc-46, Tm-170, and UC2from ourstudies and those ofHopewell et al. (5).DISCUSSION AND CONCLUSION

The lesions being scored inthis type ofstudy at lowdoses aresmall (e.g., about 1- to 2-mmdiameter),andtransient (e.g., 1-2 wee ks). These lesions are notopen festering wo unds, butrather, barely noticeable scabs. Thequestion to answer is howshould this detriment beweighted incomparison with thedebilitating effects norm allybeing avoided in setting limits for deterministic andstochastic risks.In these studies, over 1,100 individual spots were exp osed to hotparticles. About 560 of these developeddetectable scabs which were assumed toresult from a break in the integrity of the skin, and therefore, maylead toinfection. Only two of the exposed sites became infected these after exposures near 500 Gy. One was treatedtopically with ointment for a fewdays and theother wastreated topically andsystemically with anantibiotic since the' lymph nodes seemed to be infected. These large exposures leading to infection certainly are not acceptable

detriments and a limit should be set to avoid them. However, the lowincidence of infection foranimals living undertypical experimental conditions, and lack ofother serious effects, suggests that a limit set to avoid persistentulceration may be acceptable even if small transient scabs areproduced.A d ose limit could bebased on theproduct of theprobability ofscab formation, thediameter of the scabs andtheir persistence. From a preliminary evaluation of ourdata, for doses in the range of 1-200 Gy,this value is about 1-4 scab-ir.m-da/Gy. Alternately, a limit could bebased on thenumber ofdays a scab isexpected, regardless of itsdiameter. Forexample, preliminary analysis in terms of the number of scab-days for exposures between 0.1 and 50Gy indicates that an average value for allsources isabout 1.6 scab-days/Gy w ith somewhat higher values at lowerdoses andperhaps twice asmany scab-days per Gy from Tm-170 compared w ith other sources.These results together suggest that a limit in therange of5-50 Gy may be appropriate toavoid effects that aremore nearly comp arable w ith other detriments at thedose limits. Based on these results and therelatively small harmassociated with theproduction of lesions by hotparticles, it seems reasonable to set a special hotparticle limit in therange of 5-50 Gy(dose averaged over 1 cm2at70-um depth in tissue). Inaddition, anaction level for persons

receiving doses in the range 1 Gy to the limit selected could beestablished involving weekly checks for possiblelesions; anyones detected then could betreated to avoid infection.i

REFERENCES1. NCRP, Limit or Exposure to "Hot P articles" on theSkin, Report 106, NCRP, Belhesda, MD(1989).2. J.W.Hopewell, J.E.Coggle, J. Wells, et al.,Brit. J. Radio!., Suppl. 19,4 7.3. ICRP, "The Biological Basis forDose L imitation in theSkin," Pub. 59,Pergamon Press, NY(1991).4. ICRP, "1990 Recom mendations of the International Commission onRadiological Protection," Pub.60,Pergamon Press, NY (1991).5. J.W.Hopewell, Rad. Prot. Dosim., 39,1/3,11-24 (1991)6. W.D. Reece, J.W.Poston, Sr., andD.L. McFarlanc, Report TR-104781, EPRI, Palo A lto, CA(1994).


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acute skin lesions due to localized "hot particle" radiation exposures

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