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THE EVOLVING MICROLESIONCONCEPT

Paul Todd

BioprocessingandPharmaceuticalResearchCenter,3401 MarketStreet,Suite220. Philadelphia, PA 19/04, U.S.A.

ABSTRACT

The “microlesion concept , introduced by 0. Grahn in the 19711’s, was fu~-thor exploredusing published quantitative carcinogenesis data. Laboratory experimenr.s perfo’nod byT. C. H. Yang and co—workers and by R. U. 11. Fry and co—workers using the Fe ion ~eamof the Lawrence Berkeley Laboratory Bevalac have resulted in quantitative data on invitro and in vivo (respectively) carcinogenesis in mouse systems. These data sets wereinterpreted on the basis of track calculations, and it was found that th

9 action crosssection for tumor induction in cii tuned cells is approximately 0.032 ~in and that it.is about 1/1000th as great in mous’ flarderian gland cells. This great difference incarcinogenic sensitivity is a reflection of the biological ditrerences between thesetwo highly promoted systems. neither ot which nay be quantitatively applicable tohumans in space.

INTRODUCTION

After the discovery or heavy ions in the cosmic radiation, and the subsequentperformance or balloon, spaceflight, and particle accelerator eperimonts over morethan three decades, a variety of tissue effects due to single particles have b~~nireported. Microscopic counterparts of theS~ anatomical effects at heavy particles bav~been Sought in more recent years, and physical, chemical, molecul ar and cellulareffects have been characterized. Singlo partici ‘ daiia~r is clearly present inirradiated cells and tissues, and the geometrical distribution of en~rgy—loss andchemical events have been extensively characterized. All findings are consistent withthe potential for producing, as sugijestecl by 0. Grahn, MICRULESIIIMS (a linear array Utkil led and damaged cells) in marmialian tissu~ /1/. Un the basis of avei able datasets, a microlesion would he expected to consist, at a linear arraj 31 at le~sr, tourki nds of rl~aaged cell s cell s kill ad by I rrepai rahl e “di r~’ct hi ts” to :ol Illirlarionization, cells surviving but mutated as a consequence of direct hits boa to columnarionization, cells surviving but mutated as a consoquence of ott—track el:~tron laoage,and un—hit cells /2!. There is no clear evidence that columnar ionization thr~ugb th~cell cytoplasm leads to cell death or mutation. The role ot each or theso tour calltypes in the well-known induction at cancer, developmental lesions, and lenticularopacifation remains to he determined. In this oriet rport. an attempt is made rointerpret the measured carcinogenic eftects of heavy ions on mouse cells in terms orparticle tracks or rnicralesions.

REVIEW OF HEAVY—IONCARCINOGENESISDATA

Yang at al . have produced an extensive set of dose response curves tar th~ I nijction ormalignant transformation in cultured C’IH eloise 1(lTl/2 cells tV. Bombardi rig ionsincluded Fe at ~00, 400, and 300 MeV/u having LET’s of 190, :11)0, and BOO keV4in,respectively.

187

188 P. Todd

The incidence of transformed colonies par survi’ing (colony_iorcin;1 cell. , e.ppear~dto depend upon dose, D, in reds, over the ‘lose rang~ still “0, acrordi ny torelationship

I =4+80+ CD2 (I)

wh~re A, B, and C are coefficients. In th~ experinarrtal design 4=1!, end at lo.~ dosesCDg—~0. The single—hit coefficient, estimated graphically, varieS from 1.1 to 1.5 x10 transformations per cell per red in confluent ciltir’s irradiated .0th ?7~ kVp Irays and allowed to recover 24 h before resuspending end plating (epp~oxima~ing in vivaconditions). The value of B averaged over B experiments icc approximately OXi lTS’transformations per cell per red. Following Fe ion irradiation (loll ‘V/,~m( ~nd~r tb~same conditions B(Fel= 10. At low dose RIlE = B(F/Il(Xl lb. lOsing the bi~huSt valueof 8(X) observed, RBE = B, the minimum possible experimental value.

Dose-response curves for tumor induction in the Hardenian gland nt hybrid C5711l/F( xBAI_B/c, female mice hearing pituitary isografts have hewn obtained using nearly thesame irradiation conditions /4/. so that in vivo and in vitro data on highly-promotedmouse carcinogenesis systems /5/ are now avaTi~lp. The initial slope. B, of thedose—response function obtained following irradiation with 141 KoVAum Fe ions ~,as 1.03

tumor per red, and the reported ROE (vs. Co garrina radiation) was 27.

CALCULATION OF TRACK EFFECTS FROM DOSE RESPONSEDATA

The action cross section, S. for cell transformation can he estimated from rh’ data ofYang et al /3/ using the relationship

S = 16 RI. (21

where L is LET in keV/pm. From this calculation S = il.i)l2 ~ which is considerablyless than a typical cell killing cross section, about 91 qm, in this LET range /3, ii,7, P.!. The action cross section should have predictive value in assessing the effect.sof single Fe ion tracks. For emarnple. if the Harderian gland consists uf du:talepithelial cel Is with an average length x width of Tb ~o x Ill ~ni and a nuclear dierrmterof 6 urn /9!, and the gland resembles a 2 mm diameter bent cylinder /111! then t.hCaverage Fe track in a parallel he~nwill traverse (r~/4) x 2 nm ur about 1.5 mn of thegland, or about 101) cells. Of these 11)11 cells, l’(%cI /11011) I a ll)iI 10 nuclei will h~hit by the particle track. This number is confirmed when on” draws straight lin~sthrough histological sections of the gland /111/. At this LEt about (I.? of them would ~expected to die /0/, leaving about 6 cells to he transformed with a probability derivedfrom the

5atio of2the action cr~ss section to the geometric nucIu~r cross s’ctinn, or’(0.11:12 pnn’)/(R 9m 1 1.1, a ill transformations per hit surviving cell. 1nteqratin~over the 6 cells/track at risk, the transformation probability per track is ~ a illor about 151) tracks must. traverse the average ylanrl to produce a tenor. By lin~arextrapolation of this prediction 15 tracks per yl an.’) would cause a lllt tumor incidence.

Experimentally, 11) reds of Fe ion causes a 101 tumor incidence per gland /5, 9/. Thefluence, F, of Fe ions that corresponds to ill reds is calcul at”d from

F = 0 1(1.6 x 1l)~I,) (:1)

wh~re D is in reds and L is in keV/um. In this case F = 3.3 x l0~ ions/cm” or 2.6 x10 particles through a 2 x 4 mm gland. Thi s is none than 11)11 tines the nu,i=n=r ofparticles predicted on the basis of the average transformation cross section of C3H1OT1/2 cells. Presumably the probability that a damaged surviving ccl I progresses tomalignancy in the Harderian gland is 1/1001) th of that in COIl lOT)!? cells in vitro.

TRACK-BASED RISK ESTIMATES

The corresponding risk is 0.1 tu~or/2.5 x 10~particles = ~ x 111” tumor/particle pergland. At about 20 particles/on~ — day a mouse spending 16,00(1 days in free spacewould have a lOt, chance of tumor induction. On the other hand, a ‘tI x I)) x ii) cnn organwish the same tumor susc~ptihi1ity and5cellular geometry would nave a probability lOll’)cm 1(0.2 a 0.4 x 0.15 cm ) or about 11) as great, or 1.15 days in space for lOt tumorprobability. It should he kept in mind that these studies were performed in ahighly-promoted experimental carcinogenesis system /4/.

Finally, an action cross section for Harmieri an tumor induction can he estimated bycomparison with the calculated S4for C3H 10~/2 cells u~ing9the calul~ted/ohscrved liltincidence fluences: (15/2.6 x 10 1(0.1)32 um ) 2 x 10 um or 211 on’, equivalent toabout 10 base pairs in DNA.

Microlesion Concept 189

REFERENCES

1. 0. Grahn, Ed., HZE Particle Effects in Mannad Space Flight, Natl. Acad. SdU.S.A., Washington (1973).

2. P. Todd, U. C. S. Wood, U. T. Walker, and S. U. Weiss. Lethal, potentially lethal,and non lethal damage induction by heavy ions in cultured human cells, Radiat.Res. 104, S—5--S-12 (1985).

3. T. C. H. Yang, L. M. Craise, M. T. Mel, and C. A. T.obias. Neoplastic celltransformation by heavy charged particles. Radiat. Res. 104, S-177—-S—187 (1985).

4. R. U. N. Fry, A. G. Garcia, K. H. Allen, A. Sallese, E. Staffe)dt, T. N. Tahmisian,R, L. Devine, I.. S. Lombard, and E. U. Ainsworth. Effect of pituitary isografts onradiation carcinogenesis in marrenary and Harderian glands of mice. Proc. Symp.Biological and Environmental Effects of Low—Level Radiation Pertinent to Protectionof Man and His Environment. International Atomic Energy A~’ancy Report SM 202/220,Vol I Vienna 1976, pp. 213-220.

5. R. 3. M. Fry, P. Powers-Risius, E. L. Al pen, and E. U. Ainsworth. High—LETradiation carcinogenesis. Radiat. Res. 11l4, S-128—-S—195 (1985).

6. P. Todd. Heavy ion irradiation ot human and Chinese hamster cel Is in vitro.Radiat. R~s. 61, 288—297 (1975).

7. R. Katz, B. Ackerson, M. Honayoonfar, and S. C. Sharma. Inactivation of cells byheavy ion bombardment. Radiat. Res. 47, 401—405 (1971).

B, H. Wult, W. Kraft—Weyrather, H. 0. Miltenluryer, E. A, Blakely, C. A. Tohias. andU. Kraft. Heavy—ion effects on inairmalian cells: Inactivation measurements withdifferent cell lines. Radiat. Res. 104, S—12’?—-S—134 (1985).

9. R. .1. N. Fry. Abstracts, Meeting of Raliat. R~s, Soc. (1986).

10. U. II. Holland and R. U. N. Fry. Neoplasms of the integumentaty system andHarderian gland, in: The Mouse in Biomedical Research, Vol [V. ed. H. L. Foster,U. D. Sisal I, and U. U. Fox, Academic Press, NY, (1902), pp. 513—520.