INT. J . RADIAT. BIOL ., 1979, VOL . 36, NO . 2, 1 1 7-126

Dependence of the nucleoside effect on linear energy transfer

ANA FERLE-VIDOVIC, DANILO PETROVIC, JASNA SORIC,IVO SLAUS and DUBRAVKO RENDICInstitute `Ruder Boskovic', Zagreb, Yugoslavia

(Received 31 July 1978 ; accepted 24 November 1978)

L929 cells were irradiated by cyclotron-produced neutrons and by 14.8 MeVmonoenergetic neutrons . For comparison cells were also irradiated by b1Cogamma rays . Following irradiation cells were treated by an equimolar solution ofdeoxyribonucleosides, and the effect on cell survival measured . Results show thatnucleoside treatment was efficient after low-LET irradiation : gamma ray survivalcurves were altered by deoxyribbonucleosides in terms of significantly increasedextrapolation numbers only, but without D o change . Cells irradiated by neutronsfrom either of the two sources did not respond to nucleoside treatment, andconsequently their survival curves remained unaltered . These results show thatthe nucleoside effect does occur after low-LET irradiation, but apparently notfollowing high-LET irradiation . Since deoxyribonucleosides as well as other cellbreakdown products are released in irradiated and necrotic tumours due tomassive cell destruction, such a nucleoside effect could possibly enhance the cellsurvival and thus affect the result of radiotherapy. Absence of the nucleosideeffect in case of high-LET irradiation may therefore be an additional potentialgain from neutrons in radiotherapy .

1 . IntroductionThe success of a radiotherapeutic regimen largely depends on its selectivity in

killing tumour cells . The use of fast neutrons in radiotherapy is one of the attempts toincrease such selectivity . Neutrons are more efficient than low-LET irradiations incell killing in general, but for tumour cure two properties of neutrons seem to beimportant : there is less protection of tumor cells due to shortage of oxygen, and therepair of sublethal damage between dose fractions is lacking . Here we reportdifferences in response of mouse L cells in culture to treatment by deoxyribonucl-eosides after gamma and fast neutron irradiation and discuss the possible relevanceof that phenomenon to radiotherapy .

The restorative effect of nucleic acids and other cell constituents has been studiedsince the late fifties. It was found that deoxyribonucleic acid (DNA) when added toirradiated living systems can reduce certain effects of irradiation . This has beenstudied in animals, plants and at the cellular level (for review see references : Kanazir1968, Petrovic 1968) . Our research on that problem showed that not only highlypolymerized DNA, but also its breakdown products can increase the survival of Lcells irradiated by X-rays (Petrovic, Miletic, Ferle-Vidovic and Han 1966), and thedeoxyribonucleosides proved to be the actual restorative agent (Petrovic, Ferle-Vidovic, Habazin, and Vukovic 1970) . The restorative effect of the deoxyribonuc-leosides was also found in HeLa cells (Petrovic and Nias 1967) . The characteristics ofthe restorative effect of the deoxyribonucleosides in L cells were the shoulder effect(increase of shoulder of the survival curve of treated cells without alteration of theDo), and its dependence on the cell cycle (only S-phase cells respond to the

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nucleoside treatment), (Petrovic et al . 1970). It was suggested that the increase insurvival of irradiated L cells treated by deoxyribonucleosides is due to a pool effect :increased extra-cellular concentration of nucleosides may re-establish the alteredintracellular precursor pool with consequent enhancement of DNA synthesis andeventually of cell survival (Petrovic et al . 1970, Petrovic, Ferle-Vidovic and Nagy1974). Involvement of a post-replicativ :- type of repair analogous to that in bacteria(Radman, Cordone, Krsmanovic-Simic and Errera 1970, Rupp and Howard-Flanders 1968) due to enhanced DNA replication was also suggested .

In the present article we report results showing that the nucleoside effect can beachieved following low-LET irradiation, but not after fast neutrons . L cells did notrespond to treatment by the four deoxyribonucleosides if they were irradiated by fastneutrons generated by two means : (a) 14 .8 MeV neutrons obtained from aCockcroft-Walton accelerator and (b) continuous energy spectrum of neutronsproduced by the 27A1(d, n) reaction at a deuteron energy of 16 MeV .

2 . Materials and methods2.1 . Cell culture

L929 cells were grown as monolayer cultures in Eagle's Minimal EssentialMedium supplemented by 10 per cent calf serum . For clonal growth cells wereplaced in 60 mm diameter plastic Petri dishes (C . A. Greiner and Sohne,Niirtingen, Germany) in appropriate concentrations to obtain 50 to 100 colonies perdish, after 12 days of incubation at 37 °C in a humidified atmosphere of air and 3 percent of CO 2 . The colonies were then stained and counted . Based on the survival datafrom three or more experiments the survival curves were calculated and drawn bycomputer so that for each curve the means of extrapolation numbers and mean lethaldoses with their standard errors were obtained .

2 .2 . Experimental procedureBasically the same experimental procedure as described in our previous paper

(Petrovic et al. 1970) has been employed : cells were irradiated, and immediately afterirradiation an equimolar solution of the four deoxyribonucleosides was added to thegrowth medium to reach the final concentration of 50 j g deoxyribonucleosides permillilitre of growth medium . The nucleosides were present for the whole incubationtime. For comparison, irradiated but untreated samples were prepared in the sameway. In general, experiments with neutron irradiation were accompanied byexperiments with gamma irradiation . Experiments with gamma and neutronirradiations were carried out simultaneously with cells originating from the sameoriginal cell suspension and under identical conditions .

Cells for irradiation at the cyclotron were first plated in Petri dishes, incubatedovernight for attachment and then irradiated and treated with nucleosides . At theCockcroft-Walton accelerator cells were irradiated in suspension in plastic testtubes, and then diluted with growth medium containing nucleosides and plated inPetri dishes for clonal growth .

2.3 . Cell synchronizationSynchronized cells were obtained by the method of Terasima and Tolmach

(1963), by which cells in mitosis are selected from a logarithmically-growing culture .Suspensions containing mitotic cells were counted, diluted, inoculated into plastic

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Petri dishes and incubated . At different time-intervals, samples were taken forirradiation and treatment by nucleosides. After 12 days incubation colonies werecounted and survivals calculated .

The degree of synchronization was estimated for each experiment by determin-ing (a) the mitotic index at zero time, (b) the average cellular multiplicity (A .C .M .) athourly intervals and (c) the labelling index by tritiated thymidine .

2.4. DeoxyribonucleosidesDeoxyadenosine, deoxycytidine, deoxyguanosine and thymidine were purchased

from the Nutritional Biochemical Corporation, USA . Before use, the four nucl-eosides were dissolved in growth medium to obtain an equimolar solution . Theirfinal concentration in growth medium for treatment was 50µg/ml. Non-treatedcontrols received same quantities of growth medium without nucleosides .

2 .5 . IrradiationGamma irradiation was carried out at a cobalt-60 source giving 1 .25 MeV mean

energy gamma-rays with the dose rate of 0 .241 Gy per sec .Neutron irradiations : (1) The Cockcroft-Walton accelerator . A 3H(d, n) alpha

reaction at incident deuteron energy of 160 keV was used to produce 14 .8 MeVneutrons. The neutron flux was determined from the kinematical conditions of thereaction by measuring the flux of associated alpha particles . The neutron dose at theposition of irradiation, calculated from the neutron flux using Kerma factors, wasalso controlled by photodiode dosemeters (Kovacevic, Stipcic, Paic, Slaus, Eman,Pecur, and Antic 1978) . The dose rate was 0 . 15 Gy per min, at the distance of 2 . 3x 10 -2 m. The contribution of the gamma dose to the total dose was less than 5 percent. During irradiation the plastic test tube containing cells was rotating with aspeed of 45 r .p .m. around its central vertical axis .

(2) The cyclotron . An internal cyclotron beam of 16 MeV deuterons incident onthe 27Al target was used to produce a continuous energy spectrum of neutrons . Thenumber of neutrons as a function of neutron energy is given by the expression N(E)=Eexp(- E/7 ) (1-0.48/E) -1 , where T=2 .5MeV . Cells were irradiated at thedistance D=2m from the internal 27 Al-target . The neutrons were collimated to afield of 0 . 1 m x 0 . 1 m size. The neutron dose at the irradiation position was measuredby photodiode and chemical dosemeters (Kovacevic et al . 1978, Dvornik andZivadinovic 1976) . The two methods agreed to ± 10 per cent . The relative doses weremeasured at an accuracy of 5 per cent . The dose rate at 2 m was 0 .1 Gy per min . Thecontribution of the gamma dose to the total dose was less than 10 per cent .

3 . Results3 .1 . Experiments with 14 .8 Me V monoenergetic neutrons

In figure 1 survival curves of cells irradiated by gamma rays (left) and 14 .8 MeVneutrons (right), with and without deoxyribonucleosides are presented . In thegamma ray experiments deoxyribonucleosides increased the survival by means ofincreasing the extrapolation number by a factor of 1 . 6 without altering the slope (D o )of the survival curve of the treated cells . It is evident however, that de-oxyribonucleosides did not reveal any significant effect on cell survival followingneutron irradiation. In 11 experiments carried out under identical conditions byusing neutron doses up to 4 Gy, no significant difference due to nucleoside treatmenthas been observed . Additional evidence is presented in figure 3 . The r .b .e . (relative

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Figure 1 . L929 cells were irradiated in suspension in plastic tubes by gamma rays (left) and14 . 8 Mev monoenergetic neutrons (right) . After irradiation cells were diluted in growthmedium containing 50 pg per ml deoxyribonucleosides ( x) or in pure growth medium(+) and then placed in plastic Petri dishes for clonal growth . Based on survival data, thedose-effect curves were calculated and drawn by computer, and for each curve themeans of extrapolation numbers and Do with their standard errors were obtained .

biological effectiveness, defined as the rates of y-ray dose and neutron dose havingthe same effect), values at various survival levels are different, depending on whetherthe irradiated cells were treated by deoxyribonucleosides or not . Due to therestorative effect, after the gamma ray irradiation r .b .e .s in the treated samples aregreater than those in non-treated samples, giving `gain factors' that are greater athigher survival levels and decrease by the decrease of cell survival .

3.2 . Experiments with cyclotron-produced neutronsFigure 2 shows survival curves of cells irradiated by gamma rays (left) and

cyclotron-produced neutrons (right) . As in experiments with neutrons of higherenergy (see figure 1), here again the deoxyribonucleosides did not change the neutronsurvival curves. On the contrary, deoxyribonucleosides did increase the survivals ofthe cells irradiated by gamma rays, and the extrapolation number increasd from 2 . 5(untreated) to 4 . 4 (treated) without slope change . Details from figure 2 presented infigure 3 show the r.b .e. values of cells treated by deoxyribonucleosides at varioussurvival levels being higher than those observed after 14 .8 MeV neutrons, resultingin higher `gain factors' . Inversely, the r.b.e.s of non-treated cells are lower after

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Figure 2 . L929 cells were put into plastic Petri dishes, incubated overnight for attachmentand then irradiated by gamma rays (left) and cyclotron-produced neutrons (right) . Afterirradiation deoxyribonucleosides were added to reach 50 pg per ml concentration (x )while non-treated samples (+) were incubated in medium without nucleosides . Basedon survival data, the dose-effect curves were calculated and drawn by computer and foreach curve the means of extrapolation numbers and D o with their standard errors wereobtained .

cyclotron-produced neutrons . It is difficult to compare the data obtained by the twoneutron sources because of the different techniques employed . The restorative effectof the deoxyribonucleosides after gamma irradiation is higher when the cells areirradiated in Petri dishes in an attached and spread-out state, than it is when they areirradiated in suspension . This indicates a certain relevance of the cell state duringirradiation and treatment for the restorative effect . For the same reason it is also notclear whether the differences between the slopes of the survival curves of cellsirradiated by the two kinds of neutrons are due to the different energies of theneutrons, or due to the different states in which the cells were during irradiation . Thesame applies to the differences in the extrapolation numbers of the gamma raysurvival curves . As mentioned in § 2 .2, different experimental techniques wereemployed depending on the neutron source, and therefore cells were in differentstates before, during and following irradiation . Sublethal damage, potentially lethaldamage, recovery , partial sychronization and redistribution of cells in the cell cyclemight have been influenced in different ways, thus producing changes in extrapo-lation numbers .

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Figure 3 . R .b .e . values for treated and nontreated cells following irradiation by cyclotron-produced and 14 .8 MeV neutrons. Dashed parts of the curves are extrapolatedcomputer fits . `Gain factor'-ratio of r .b .e . of treated cells and r .b .e . of non-treated cellsat various survival levels .

Figure 4 shows the results of experiments with synchronized L cells . Figure 4 (A)shows the age-response of treated and non-treated cells following gamma irradiationat different times after mitosis . The pattern of the age-response of L cells as well asthe age-dependence of the restorative effect of the deoxyribonucleosides (figure4(B)) has already been published (Petrovic and Ferle-Vidovic 1968, Petrovic et al .1970) . Figure 4(C) shows the age-response of these cells irradiated by cyclotron-produced neutrons at different phases of the cell cycle . The flattened-out pattern ofthe age-response is rather common following neutron-irradiation (Masuda 1971) .Irrespective of the time of the cell-cycle when irradiation and nucleoside-treatmentwere applied, no restorative effect could have been observed . This is in strongcontrast to the response of S phase cells to the deoxyribonucleosides irradiated bygamma rays (see figure 4(B) and Petrovic and Ferle-Vicovic 1968 and Petrovic et al .1970) .

We assume that the dose-rate differences (14 .4 Gy per min of gamma rays and0. 10 and 0 . 15 Gy per min of neutrons), in our range do not influence our conclusions,because very little dose-rate effect between 0 . 1 and 15 Gy per min can be expected .According to Hall's data (Hall 1972), for low-LET irradiations there is no appreciabledose-rate effect in the range of 0 .3 to 10 Gy per min on D0 and extrapolation number .Similar conclusions emerge from Elkind's considerations of the dose-rate effectproblem in relation to sublethal damage and recovery (Elkind and Whitmore 1967) .

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Figure 4 . (A) Age-response of L929 cells irradiated by 5 Gy of gamma rays atvarious times after mitosis, After irradiation, deoxyribonucleosides (50 µg per ml) wereadded to the growth medium ( x ), while the non-treated samples (+) were incubated inpure growth medium . After incubation, colonies were stained and counted . (B) Age-dependent restorative effect of deoxyribonucleosides from (A). S 1 = survival of cellstreated with deoxyribonucleosides . S 0 = survival of non-treated cells . (C) Age-responseof L929 cells irradiated with 2 Gy of cyclotron-produced neutrons at various times aftermitosis . Treatment by deoxyribonucleosides as in (A) (x =treated, + = non-treated) .

4 . DiscussionExchange of cellular components between living cells, or uptake of parts or

molecules resulting from dead cells by living cells is a well established phenomenon .Uptake of such `foreign' material often influences the behaviour or properties of thehost cells depending on the nature of the ingested material . The properties of thecellular membrane plays here an important role, particularly in the selection of thematerial to be taken up . One of the properties which can be influenced by moleculesoriginating from other cells is the response of the cells to irradiation . Cellhomogenates (Cole, Fisher, Ellis, and Bond 1952, Congdon and Lorenz 1954, Coleand Ellis 1955, Horikawa, Sugahara, and Doida 1964), subcellular fractions(Petrovic, Miletic, and Brdar 1963, Horikawa et al . 1964), nucleic acids (Panjevac,

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Ristic, and Kanazir 1958, Taliaferro and Jaroslow 1960, Maisin, Dumont andDunjic 1960, Miletic, Petrovic, Han, and Sasel 1964), or their breakdown products(So9ka, Drasil, and Karpfel 1958, Petrovic et al . 1966, Petrovic 1970), and for reviewsee (Petrovic 1972, Kanazir 1972), increase the survival of irradiated animals andcells . This (restorative) effect has been found to be associated with increasedincorporation of DNA and RNA precursors into the cellular nucleic acids(Becarevic, Jankovic, Petrovic, Kanazir, and Jovicki 1962, Petrovic, Kanazir, andBecarevic 1962, Petrovic et al. 1970) . On the other hand evidence exists thatfragments of foreign DNA were incorporated into the host DNA and initiated repairsynthesis, but no increase in survival was noted (Ayad and Fox 1969) . Thephenomenon of DNA uptake by cells, the fate of the incorporated DNA and itspossible action on the host cells has been extensively investigated and reviewed byLedoux (1972) .

The conclusion from an analysis of data like those mentioned, is that cells ingeneral take up material originating from other cells and that, depending on variousfactors, such material may modify the effect of radiation . Regardless of the molecularmechanisms on which this is based, it can be assumed that this also occurs intumours, not only because of cell killing due to irradiation but also due to theadjacent necrotic regions . In most situations with massive cell death producing largequantities of cellular degradation products, irradiated cells have a potential chance tobe rescued from death, or from other radiation-induced damage being sub- orpotentially-lethal . Our research on 1, cells shows that subcellular fractions (Petrovicet al. 1963), DNA (Miletic et al. 1964) and increased concentrations ofdeoxyribonucleosides (Petrovic et al . 1970), significantly increase cell survival withhigh reproducibility . Such effects if present in tumours, might significantlyinfluence the success of the radiotherapy . Elimination of that restorative effect eitherby preventing cellular uptake of nucleosides and of other material of cellular origin,or by producing such damage that cannot be repaired by this means, could thereforeavoid this unwanted phenomenon .

Our results presented in this paper show that L cells do not respond to treatmentby deoxyribonucleosides following irradiation by fast neutrons . The gain factors,particularly in case of cyclotron-produced neutrons (figure 3) are not insignificant,and in those tumours where similar situations can be met, the advantage of fastneutrons over low-LET irradiations is obvious . Absence of the nucleoside effectafter high-LET irradiation may therefore be an additional potential gain fromneutrons in radiotherapy .

AcknowledgmentsThe authors express their gratitude to Drs . Branka Antolkovic, Igor Dvornik,

Guy Paic and Neda Stipcic for participating in the dosemetry measurements andhelpful discussions . The authors also acknowledge the skilful operation of bothaccelerators and thank Mr . Tomislav Lechpammer (M . Sci .) and the cyclotron team,Mr. Aleksandar Miran, chief operator of the Cockcroft-Walton accelerator . Theyare also grateful to Mrs . Marija Fiolic and Mrs. Ljiljana Krajcar, for their excellenttechnical assistance .

Les cellules L929 ont et& irradiees par des neutrons produits par le cyclotron et par desneutrons mono&nerg&tiques de 14,8 MeV . Pour la comparaison, les cellules ont &t& aussiirradiees par les rayons gamma de 60Co . Apres irradiation, les cellules ont et& traitees par unesolution &quimolaire de d&oxyribonucl&o sides et l'effet sur la survie des cellules a &te &value .

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Les resultats montrent que le traitement par les nucleosides est efficace apres une irradiation ATEL faible . Les courbes de survie apres les rayons gamma sont alterees par les deoxyribonu-cleosides dans le sens d'une augmentation significative du nombre d'extrapolation, sanschangement de la D0 . Les cellules irradiees par les neutrons provenant de l'une ou I'autresource ne reagissent pas A un traitement par les nucleosides et par suite, leur courbe de survien'est pas changee . Ces resultats montrent que 1'effet des nucleosides se manifeste apres uneirradiation A TEL faible, mais apparemment pas apres une irradiation A TEL eleve . Puisquedes nucleosides aussi bien que d'autres produits de la rupture cellulaire sont relaches dans lestumeurs irradiees et necrotiques, par suite de la destruction massive des cellules, un tel effetdes nucleosides pourrait aceroitre la survie et ainsi modifier le resultat de la radiotherapie .L'absence d'effet de nucleosides sans le cas d'irradiation A TEL eleve peut, par consequent,constituer un gain potentiel supplementaire de 1'emploi des neutrons en radiotherapie .

Zellen L929 wurden mit in Zyklotron erzeugten and mit monoenergetischen Neutronenvon 14,8 MeV bestrahlt . Fur Vergleichszwecke wurden die Zellen auch den Gammastrahleneiner 60Co-Quelle ausgesetzt . Die bestrahiten Zellen wurden mit aquimolarenDesoxyribonukleosidlosungen behandelt and die Wirkung auf das Uberleben der Zellenbestimmt. Die Ergebnisse zeigen, dass die Behandlung mit den Nukleosidlosungen nachniederen LET wirksam war : die Uberlebungskurven nach Gammabestrahlung wurden durchDesoxyribonukleoside in Form von signifikant vergrosserten Extraploationswerten veran-dert, jedoch ohne Anderung von D0 . Die Zellen, die mit Neutronen von einer der beidenNeutron enquellen bestrahlt wurden, reagierten nicht auf die Nukleosidbehandlung anddemgemass blieben die entsprechenden Uberlebungskurven unverandert . Diese Resultatezeigen, dass der Nukleosideffekt nach einer niederen LET-Bestrahlung in der Tat zustandekommt, offensichtlich jedoch nicht nach einer hohen LET-Bestrahlung . Da in bestrahitenand nekrotischen Tumoren, als Folge von starken Zellzerstorungen, Desoxyribonukleoside alsauch andere Produkte des Zellabbaues befreit werden, konnte solcher Nukleosideffektwahrscheinlich auch hier das Zelliiberleben fordern and sich so auf die Ergebnisse derRadiotherapie auswirken . Die Abwesenheit des Nukleosideffektes im Falle der hohen LET-Bestrahlung konnte also in der Radiotherapie einen zusatzlichen potenziellen Gewinn furNeutronen bedeuten .

ReferencesAYAD, S . R ., and Fox, M ., 1969, Int . Y. Radiat. Biol ., 15, 445 .BECAREVIC, A., JANKOVIC, V ., PETROVIC, S ., KANAZIR, D ., and JovicKI, G ., 1962, Bull . Inst .

Nucl . Sci . `Boris Kidric', Belgrade, 13, 35 .COLE, L . J ., and ELLIS, M . E ., 1955, Fed. Proc . Fedn Am. Socs exp . Biol ., 14, 29 .COLE, L. J ., FISHER, M . C ., ELLIS, M. E ., and BOND, V . P ., 1952, Proc . Soc . exp . Biol . Med. 80,

112 .CONGDON, C . C ., and LORENZ, E ., 1954, Am . Y . Physiol., 176, 297 .DVORNIK, I., and ZIVADINOVIC, M ., 1976, Third Internat. Summer School on Radiation

Protection, Herceg Novi, Yugoslavia, 24 August-3 September .HALL, E . J ., 1972, Br. Y . Radiol ., 4, 81 .ELKIND, M. M. and WHITMORE, G . F ., 1967, The Radiobiology of Cultured Mammalian Cells

(New York-London-Paris : Gordon Breach Science Publishers) .HORIKAWA, M ., SUGAHARA, T ., and DoIDA, Y ., 1964, Expl Cell Res., 34, 198 .KANAZIR, D. T., 1968, Studia biophys ., 7, 55 .KOVACEVIC, K ., STIPCIC, N ., PAIL, G .,SLAUS, I ., EMAN, B., PECUR, V ., and ANTIC, M ., 1978,

Nucl. Instrum . Meth ., 148, 291 .LEDOUX, L., 1972, Uptake of Informative Molecules by Living Cells, edited by L . Ledoux

(Amsterdam-London : North-Holland Publishing Company),MAISIN, J., DUMONT, P., and DUNJIc, A ., 1960, Nature, Lond ., 186, 487 .MASUDA, K ., 1971, Int. Y. Radiat . Biol ., 20, 85 .MILETIC, B., PETROVIC, D ., HAN, A., SASEL, Lj ., 1964, Radiat. Res ., 23, 94 .PANJEVAC, B ., RISTIC, G ., and KANAZIR, D ., 1958, Int . Conf. Peaceful Uses Atom . Energy, 23,

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PETROVIC, D ., 1968, Current Topics in Radiation Research, Vol . 4, edited by M . Ebert and A .Howard (Amsterdam : North-Holland Publishing Company), p. 251 .

PETROVIC, D., and FERLE-VIDOVIC, A ., 1968, Effects of Radiation on Cellular Proliferation andDifferentiation (Vienna: IAEA), p. 81 .

PETROVIC, D ., FERLE-VIDOVIC, A., HABAZIN, V., and VuKOVIC, B ., 1970, Int . Y . Radiat. Biol.,18, 243 .

PETROVIC, D ., FERLE-VIDOVIC, A ., and NAGY, B ., 1974, Studia biophys, 43, 13 .PETROVIC, S., KANAZIR, D., and BECAREVIC, A ., 1962, Proc. of the Symposium on the Biological

Effect of Ionizing Radiation at the Molecular Level, Brno (IAEA) .PETROVI6 ., D., MIJ,ETIC ., B ., and BRDAR, B., 1963, Int. J. Radiat . Biol ., 7, 131 .PETROVIC, D., MILETIC, B., FERLE-VIDOVIC, A., and HAN, A ., 1966, Radiat. Res ., 27, 41 .PETROVIC, D., and NIAS, A . H . W., 1966, Int. J . Radiat . Biol . 11, 609 .RADMAN, M., CORDONE, L., KRSMANOVIC-SIMJ6, D., and ERRERA, M ., 1970, J. molec . Biol .,

49, 203 .Rupp, W. D ., and HOWARD-FLANDERS ., P., 1968, J. Molec. Biol . 31, 291 .SO KA, J ., DRASIL, V., KARPFEL, Z., 1958, Int. Conf. Peaceful Uses of Atom Energy, 23, 34 .TALIAFERRO, W . H., and JAROSLOw, B . N., 1960, J. infect . Dis ., 107, 341 .TERASIMA, T., and TOLMACH, L. J ., 1963, Expl Cell . Res ., 30, 344 .

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