Document NE4ve7M19VVyNgL54ReoGdJDp

Chemical Leukemogenesis in Man c.ALFSSASDRFAORNIM, .D. & ESRWO \')GLIASl? M.D. EXHIBIT . Thr pzsibility that chcmicals as well as icinitinf radiations mipht play a ,lc in human lculemogrnesis was raised many decades ago and has been f$ ,n5i.icntls discused since then. h'unierous suhstances have bccn in- &* ii;nrin3trd, mostly on the basis of ringlc ohscr\.ed cases. Most of these , u ~ ~ l m c heasve in mmmon the peculiarity of inducing hone marrow hypo- +cia which may recover, p r d to aplasia, or terminate in leukemia. In rbrcr on this suhjccl. Cronkite (13) stated that 'an). drug or chemical i a p b l c of inducing hone marrou. damiigc must he absirnicd to he pmcnti- JI!3 lruhrmngcn'. H n w r ; ~ .Ihc l e u k c m r ~ ~ ~ e~fnfeicrt of most chcmicals In ni;in is difficult 10 prove. Observation of singlr cases may not he signifiCJnt: in somc cases exposure has heen to more than one substance; morc,,w. for most suspccted substances the real number of exposed subjects ic not known, so the incidence of leukemia in such subjects cannot be com- p r c d to that in the general population. The cbemical which has resisted a11 these criticisms is benzene, an oc- cupational poison which has given rise in the past IO epidemics of aplastic anemia and leukemia. 7bis chapter will therefore deal mostly with benzene, and will then consider some other chemicals presently suspected to be leut c m o p i c in man. Benzene, the first aromatic hydrocarbon, is a highly volatile solvent widely used in industq. in both pure and commercial forms. It evaporates at room temperature and enters the body mostly by inhalation. T h e vapor J in v c y high concentration is acutely toxic to the central nervous system, while chronic exposure to benzene induces hematologic changes. Workers sre exposed to benzene in the processes of extraction and purification, in thc chemical indunry for the production of its derivatives, and in the pharmaceutical industry for the manufacture of drups: in most of these ,/processes. however. cases of Dojsoniof a re veri. rare. due 10 the ust of 1irw.U equipment. 212 ALESSANDRA FORNl d ENRlCO C.\'ICLIANI TABLE 1 C u m of Irulrinio in rnbjrris with rhrnnir bm:tnr t r p m u r t , idrnrifird in t h t Paris rrpion lrom 1930 to 1963 ldcniifird cases (37 malts and 7 fcmrlo) Acuic lcultmir Chronic myeloid leulcmiu Chronic lymphorylic h~krnir 4 23 13 8 From Copucl ct .I. (31). Conversely, proccsscs involving the use of benzene as a mOjveD1 io, manufactum of m b b e r a n t a i n i n g p r d u c t s , in d o c manufacture, i ra gravure printing. clc., may expose workers to chronic inhalation ofbq in high conccnlratioas and may be responsible for the hemriologj~ of chronic poisoning. Therefore b e UK of benzene or of mixturer contr +high conctntrrtionr of benzene lor tbese purposes bas k c ; prohibik law in many countries and benzene bas ken r e p l e d by / molopues (toluene, xvlenc) or other rolvcnu whicb are aon-totic b *bone mamow. However, the risk of benzene poisoning is still prescl some couniries, espccially in small workshops where conditions of hyl are poor. Reduciion of the alkyl-lead conieni of gasoline tends io rai, benzcne pcrccniape. Once ii has entered the body, benzene is pan]) 1 to the tissues, especially those rich in lipids, and panly convened io liver io phenol and other oxidation products such as hydroquinone pyrocatechol, conjugated with sulphuric and glycuronic acids, and 1 excreied throuzh the kidncrs (see 5 and 66 for a list of references). The depressive effect of benzene on hcmopoicsis, resulting in xph anemia, has bcen k n o w since tbe lasf century. The mycloioxic &a benzene was also exploited in the treatment of chronic myeloid lede in the past. but such use is now only of hisiorical value. Only later was benzene recognized as hkemogenic. The first repor; a case of leulcmiz attribuied io hcnzcne was that by Dclorc d B o r p ~ ; : (17). In a crjtjcal review of the Iitcraturc on bcnzcne hernopath!, in IC Penaii 6. \'i_cliani ( 5 5 ) obsemcd that some cases diagnosed as zplr sncmia did noi ariually fii this diagnosis ouing i o myeloid proliferzt rspccially in the splrcn and h r r . and to a Irukcmoid rcaciion in ihc pci; cral hlond. \vhich was more su_r~csrivcof IruAemia ihan of aplastic ant:: Sincr thcn. various rcpnns of henzcne lcukcmia have appcarcd; bcsidr : numclous c a e s rrponcd by Viglimi 6: Saita (71): who summarized : / ; l t u a i i o n in 1tal)- up lo 1964, and the large groups collccicd by Cavip:: .. ..a ...- . * . . '\.. 0. CHEMICAL LEUKEMOGENESIS lh' M A N 213 i I I ,-,I. GoguCl et al. (31), and Girard et al. (30) in France, numerous single I -,ses or small groups of cascs have been reponed, especially in Europe (1, 6-j5, 22, 23, 34, 40, 45, 46, 62, 63, 64, 67, 74). An approximate esti- s31ion of the avai1ablc liierature, including cases collccicd by Browning I -;) and some unpublished Nonh-Italian cases of which we have knowlcdgc the h t i o n a ) Institute for Insurance against Accidcnts and Oc- ,.piimal Diseascs, put the number of k n o w cases of IeuAcmia attributed benzene at at least 150. n e most convincing cases of beruene leukemia arc &OK occurring in ~ ~ o r i we hsere outbreaks of chronic benzene poisoning have ken obsewed was true, for instance, for the rcries reponed by Cavigneaux (9) and bI Vigliani & Saita (71), where the incidence of leukemia in benzene hemopathy uas approximately 12-15 per cent. The cases observed in the provinces of Milan and Pavia occurred in shoe factories and in rotogravurc 3ants, where bcnrcne was used as a solvent for glues and inks rcspeciivcly. Iin these factories the concenirations of benzene vapor in the air of Ih tmrkinp environment were dcmonsiraied to be well above the thrcshol a /iimit (71). Buucne leukemias are mostly m a t e bemocylobfanic or myeloblastic. VOS!casts h a w v e q low` white cell counts or show only a moderate lcuko- cyosis with a small percentage of immature cells, excepr in the terminal %rage.The bone marrow shows infiltration by undiffercrrtiaied cells, moslly airrohemocytoblasts, or by myeloblasts which a r t someiirnts ztypical. There I* generally moderate or no s p k n or liver enlargement. Houevcr, in a f e u cases the bone marroa is poor in Icukcmic cells and there is leukemic in- lihration in other organs such as thc splccn and liver (71). Ofien IcuLcmia dcwlops in subjects with bcnzcnt-induced hyporegenerative ancmia or pan- Topcnia of long standing; in these mses. during the course of the discasc. :he bone marrow changes from a hypoplastic IO a Itukemic paticm (65). 214 ALESSANDRA FORNI b Eh'RICO C. VICLIANl ,~Gcnerally, aplastic anemia and IcuAcmia occur in subjccts who ale o!i.exposed 10 high wnccniraiions of bcnzcnc; howevcr, in a f c u caeS may be a long latency period (up to 15 years) betueen cessation posurc to puhlishcd bcnzcnc case that and wc occurrence of Icukcmia observd (C. A., female, (35, 71). 66 )cars In a old), re the pnalkt. who had suffered from bcnzcnt anemia in 1957, had sincc had a free h,. of 14 years with normal blood counts before thc onset of acute ]tu~Q, in 1972; shc died 5 months after lcukcmia was diagnosed. Acuic b induced lcukcmias are invariably fatal, even though thc a\,ailable cbrL, thcrapg and supponive treatment may somcwhat prolong survival. Among h e bcnzcnc Icukemias, at lcast 20 cases Of aCUtC crythr~lc~~~.. have bccn rcporlcd (13, 23, 27, 31, 54, 64) (see 5 and 66 for add&- rcfcrcnces). Among thc cases of bcnzcnc IcuLemia observed at the 1nSti::. of Occupational Health of the Universit). of Milan from 1943 lo 197:. , uere acuic hemocytoblastic or rnycloblastic 1euL.emias and 3 wcre qa. leukcmias. In view of the rarity of this discasc, lhis numbcr of cases Yrt to bc significanl. In the Frcncb reporis (9, 30, 31), cascs of chronic myeloid and chp. lpmphocytic IeuXcmia wcre attributed IO bcnzcnc, togethcr with tbt m,. numerous cases of acute leukemia (Tablc I). This is neither our cxp:fir:.. nor the cxpcricncc of the Institute of Occupational Health of the Univch of Pavia; in boih these series acute leukemia occurred at the same as casts of aplastic a -4.- n e -z m' i a kA t (Table 11). 7hcrcforc t h e possibilii! e benzenc might be responsible for chronic leukemia rcmains an opt0 qU> tion. For acute leukemias, Viglritni 8 Saita (71) calmlared that in Ipti=. exposed 10 b e n e in the provinces of Milan 8nd Pavia during Uu crt nineteen-sixties, tbc risk ofa m leukemia was 8t least 20 t b e s hi@mtk m tbc g e n c d population. Recently, in a retrospettivc statistical srud! t. severe hemopathies in Lyon hospitals, Girard & Rev01 (29) found a his:.-. of exposure IO benzenc and/or its homolop~csin the preceding 10 !c in a sjp~ificantljh. igher ploponion of paiicnts with acuic IcuXemia. chro: I!.nphncytic leukemia and aplastic anemia than among control sufrj:.' \i.iih disezsts other than of thc hemopoietic system;this was no1 the u\ hou.evcr, for chronic myeloid leukemia and othcr blood disorders. 11 is interesting that rhe hanning of benzene as a ink\. afier thc nutbreaks of aplsqiic anemia and a . 'Pa\.lp Icd to 2 sharp decrcasc in the nuniher of c2v* e n e poisoninp. In Ihc last fc\v years only occasional caw 1' curred in some small uorksnops, untie Den2enc u a s still beinr uscd W I I1 eI! subjects who are still CHEMICAL LEUKEMOGENESIS IN M A N 215 As previously reponed, however, occasional cases of aculc 1' - in a few cases thcrc t k z e n fessation of ex- lcukcmia with a long latency period may still be observed. Mi t b c epidemiologicrl data arc in favor d a kukemogtnic d o n af i'. 71). I Fears In 8 fCCCnt unold), the patient. I ii. since had a free period 1i-nset of acute leukcmi; in man, either directly or through &plasticchanges in bone mhfm.The results of animal experiments on this subject are scanty and con- filming ( 5 ) . It is also doubtful whether benzene is active per sc or through Iu metabolites (phenol, hydroquinone, pyrocatechol); expenmental data i:nosed. Acute benzcnc- d m to attribute the toxic effects on bernopoicsis especially to phenol and ii the available chcrne I p!rocarcchol (51). 1i.Aon_gsuwival. !:i acute erythroleukemia Other chemicals suspected of being leukemogcnic in man are drugs such ZI phenylbutazone and chloramphenicol, both capable of inducing bone i', and 66 for additional manow depression as a side effect. Jbscrvcd at the Institutc T h e first repon of six cases of leukemia in subjects who had received 1: I from 1943 to 1973, 8 ?hcnylburatone (from 10 grams to several hundred grams) for spondylitis k k s and 3 were crythre or arlhritis, was by Bean in 1960 (2). This repon was followed by several I !Is number ofcases secmb Ixic myeloid and chronic others which have been reviewed by Krogh Jcnsen & Roll (41) and Fraumcni (25). In the latter paper, 29 cases of leukemia attributed to phenylbutazone are collected from the literature and discussed; some cases did not I together with the mort iisneither our expenen= i a m to have an established diagnosis of leukemia; of the others, only 13 had a time interval of at least 18 months between the stan of drug treat- 1 Health of the URi\w3ir! ! men1 and the onset of leukemia. In a retrospective study of 55 patients with '-+'tcurred at the same tim i -le leukemia in Australia, 9 per cent had staned receiving phenylbutazone ' 1" high mncentrations O! irefore the possibility tb! f .Or v t hr i t i s from 2E-634 years before the onset of the disease and had received X-ray treatment; on the other hand, among 417 subjects with . a remains an open quer. hronic leukemias and lymphomas, only 1.2 per cent had rcceived phenyl- !-alculated that in subjcC!i . h t m n e prior to the onset of their disease; in one case of acute leukemia i!,jPavia during the ear'! dkr phenplbutazone, there was a history of bone marrow depression be- ileast 20 limes higher t h S the diagnosis of leukemia (72). In a shon-term follow up of 24 cases ' ective s1atistical study d in the United States because of bone marrow depression due to :evo(l29) found a histof? no cases of leukemia were found; 3 cases of acute myelo- In the preceding 10 yea^ not previously reponed to the Regist? of Advcrse Drug '.h aruic leukemia, chroni; ' " C h ~ s were collected during the time of study and all had evidrnsc , conlrol subjrb?' ! k n t marrow depression prior to the onset of leukemia (25). In the this was not the cast. blood disorders. . . Of data on the actual number of peoplc receiving phenjlbutazone, * difficult, on the evidence of sparse cases, to decide whethtr this drus -'1s a solvent for glues sJ ' Ileukemia in the provine leukemogenic or whether the occurrence of leukemia is coincidental. It hi 'Is0 been suggested that elderlg subjects with spondylitis or arthritis 1 the number of W e S Per sc be ai a higher risk of leukemia, indcpcndently of treatment, ?'only occasiona1 cases mnsjstent data on this point arc lacking. The same problems encoun- i,.s still being used des$" with phenylbutazone arise when one considers the possible leukcmo- role of chforamphcnicol, a n antibiotic which since the beginning of its use has been impljcaled in the causation of aplastic anemia ( r e t , ,;1 references 57 and 73). Accordrng to Yunis (73), this drug can induct different types of blood row, which may occur dyscrasias: (1) a in many subjects reversible depression of boor during treatment with Aargc E over a shon period oftime; (2) pancytopenia, which occurs very rare] rc mostly after prolonged use or repeated courses ofthe drug and is atl&)k.k either to a specific hypersensitivity or to an irreversible bone maRoy k, city. The first two cases of acute myeloblastic leukemia following amphcnicol treatment were reponed in 1955 and 1957 (43, SO). Fn\p, (25), in the follow up of 124 cases of bone marrow deprtssion npow the Registry of Adverse Drug Reactions of the America0 Media sociation, found three cases of leukmia, of which only one could prQ be attributed to chloramphenicol; in this case aplastic anemia ocanr months after the stan of treatment and evolved into acute myc]oblK lcukcmia 2 % years later. Three more casts in subjects not repstcrd, adverse drug reaction were rcponcd during the study; all patients ha,jt signs of bone marrow depression prior to the onset Of kukemia a d dcvelopcd from 2 IO 15 years after the s t a n 01 treatment. In discussing the possible role of chloramphenicol and phcnylbuk, in leukemogenesis, Fraumeni (25) stated that in order 10 SugEest a E; relationship between drug use and Icu'Lemia, the follovzing conditions de be met: ~rclativelyhigh dose of the adminisrered drug; no e4dena IeuLtmia at the time of drug use; and absence of diseases or other rfr: known or suspected to be leukemogcnkc; moreover, the q7ologic 1: should be chronic myeloid or acute leukemia and the intcr\.al between t of drug exposure and dtvclopment of leukemia should ex& appr, matcly 18 months, as with radiation IcuAcha. Five cases reponed b and 1966 actually had these characteristics (4, 13, 37). Cohcn b: CR, (I]), while discussing their case, underlined tbc chemical similari~! chloramphenicol to benzene and phenylbutazone, and the common p m y of inducing aplastic anemiz, and suggested that m a n o w aplasia is i; a condition from which leukemia can develop. All the cases attributed to chlorzmphenicol were reviewed in a x paper (26), including three more cases of acute lymphoblastic lcukemh children who had slaned receiving chloramphenicol 49, 49 and 6 mo:. respectivel!., prior to the onset of leukemia. Another inlcrcsting p i n 1 raised in the last few years is the occunr of lcukcmia in patients treated over a long period with c7.loJorir drup! cancer or for other diseases. Long-term mclphalan trcatmcnt has been implicated in the induclir acutc 1eul.ernia. mostly ofthe myelomonocytic type. in 15 cases ofmc'' CHEMICAL LEUKEMOGENESIS IN MAN 217 ausation of apkstic anemia (see b.: Yunis (73). this drug can induce tu. 1) a reversible deprrasion of bone mar. :as during treatment with large doc,. .)topcnia, uhich occurs very rarely x.: ;d courses of t h e drug and is attribu:L: i to an irreversible bone marrow toii :ii.eloblastic leukemia following chlor. in 1955 and 1957 (43, 50). Fraumcz *ibone marro\v depression reponed I. nions of the American Medical A. mia, ofwhich only one could pr~bah!: 1 this case aplastic anemia occurred : and evolved into acute myelohlast.. *re cases in subjects not registered b during the study; all patients had h s +or to the onset of leukemia and rh:. he stan of treatment. i chloramphenicol and phenylbutul'". ;3ted that in order to suggest a CJU.. akcmia, the following conditions shl?:.' fre administered drug; no evidcnce a * .ad absence of diseases or other agfn: zpnica; moreover, the cytologic t!F !cukemia and the inteneal bet\bi.Cn st'' t of leukemia should exceed 3 p P " I leukemia. Five cases reported in l!Pclerktics (4, 11, 37). Cohcn & CrG'. underlined the chemical sirnilant! ' Ienylbutazone, and the commt'n Pn*" Sugested that marrow aplasia is it*. 1 detelop. mphcnicol were reviewed in a -;>eS Of acute lymphoblastic l e u k ~ n r i J chbamphenicol 49, 49 and S mlln!'' Jkcmia. .in the last few years is the o~curr'r,'''-' a long period with cyioioxic dr@ " ?&ma (36, 42, 52); some of these cases had been treated also with X3t3. Two more cases quoted by Kyle et d. (42) had been treated with ; h e r alkylating agent, cyclophosphamide. These authors calculated that 3c frequency of acute leukemia obscmcd in 2 years (three cases among *-: proximately 250 patients with multiple .myeloma) was about 240 h e s .he expected rate. In his interesting editorial on the subject, Holland (36) +sed the questions of whether acute leukemia might be a natural evolution d multiple myeloma, formerly masked by early death; whether immune .apainnent in myeloma might cause failure to eliminate abnormal cell Jones; whether X-rays given together with chemotherapy might play a role a leukemogenesis in these patienls; whether recurrent episodes of bone XKOW depression might result in leukemia; and finally, whether the com51on factor of long-term chemotherapy may be responsible for the occur::nce of leukemia. These questions remain open at present. Single reports 4 acute leukemia in subjects receiving chemotherapy for malignant diseases b e appeared and are quoted by Davis et al. (14) and by Potolsky & Cnpr (61); the latter authors add three unpubIished personal observations 4 acute myeloid leukemia occurring in patients with lymphoma treated nly with cytotoxic drugs. T h e lack ofuntreated control cases w i h the same h s e and the same survival times makes it impossible to decide whether kuc leukemia is a consequence oftreatment or whether subjects with such :alignancies are at a higher risk of developing acute leukemia. The possibility that cytotoxic drugs might be leulremogenic is of special f t c r e ~ ~b,ecause of the increasing use of these drugs for nonmalignant !:*Xes, with an expected long sumival, and this risk should be kepi in mind 'hdeciding on their use in young people with non-fatal diseases. 11 hu been suggested that oxyneiholone, an androgen used in the treat-int Of aplastic anemia, might have been responsible for the development 'U t e m)-eloid leukemia in four cases (16),but the evidence seems scanty .'d has been questioned (44). As is well k n o w , a cenain number of *''UiC ancmias evolvc spontaneously into leukemia, and though it has been -9estcd that treatmcnt with conicostcroids or androgens facilitates the Of leukemia in aplastic anemia, this seems improbable since these '*ats do not increase the risk of leukemia in patients treated for other "-xs (44). h h rtgard to the mechanisms whereby chemicals can induce ieukemja, h t more than one hypothesis CM k suggested. The first is that tbe ha might aitcr the genome of the hemopoietic e l i s by interfering **Y with nucleic acid synthesis or the mitotic proccss, thus inducing b a t i o n of abnormal cell clones (sornodc rnvrorian). Data supporting have been collected for benzene. The effect d benzene or 1s E- c ! 218 ALESSANDRA FORNI & Eh'RICO C:VlGLlANI jbm-wJua' bacn fOr 8 k n g (19, 53). In the )as1 decade various papers have W e a r e d on chromosome eh in lymphocytes and bone marrow cells of subjccts with benzene hem m, %hhiesrtnoorpyatoh5fy9h,(e2m61,oi p5aa8nt)h.ydSuimnbajyleycmlsshpwohiowtchyi tpneassntoefexdspuobrsajuetecrestsloorfecbcoehvnrezoremendeosfborumotrawn~&, k, (24, 33, 69, 70). The chromosome changes observed (breaks an: rw% .~mcnts) are similar lo those induced by ionizing radiations, which are a rccognized leukemogenic agent ( s t t 21 for references). Such i n c r e w w- of chromosome changes mag persist for many years in long-lived p,% era1 b l d lymphoqtcs, thus indicating that damage has occurred; in a Ih cases abnormal cell clones in pcriphcral blood could be detectd in thCi. scnce of any sign of disease (21). The significance of these findings and their possiblc relationship lo late OccurrenCe of bcnzcnc leukemia are so lar not clear. H o w t ~@~. ~ , can speculaic that chromosomal damage to bone marrou cells, gibep. to stable changes, mighi be responsible for the fornation of abnormd cr clones, one of which mighht induce leukemia by selective advantage. hypothesis was put forward on the basis of the findings in a case of & hemopatby that evolved into acute mycloid leukemia and was followd L' by multiple cylogcnetic studies of peripheral blood and bone marrow. ptr i o rhe onset of leukemia, various stable chromosome changes wen is; in the bone marrow, bur as SOOD as lcukcmia developed a single aboor, cclone was present in the cells of bone marrow and pcriphcral blood Cell lines with abnormal karyotypes were described also in two 0th r. pons of benzene leukemia (23,67); the karyotype abnormalities were L ftrcnt in the three casts, which is in agreement with the manifold q'log:nc, pattern of acuie leukemia. Experimental animal studies have shown that benzene can severely d?S:- DXA synthesis in hemopoietic cells (49) and induce chromosome ak rations in bone marrow CeIIs (39, 56). 11 is of interest that another suspctr leukemogen, chloramphenicol, can induce structural chromosomal ab? maliiies (chromosomal vacuolization and bre.al;ap) both in vivo and in :1: in man and in experimental animals (7,8, 48). A single case of l c u k . associated with chloramphenicol rrcaiment, reponed o n l j a feu mor.: ago, showed a marker chromosome (10). Alkylating agenis arc also kD* IO induce chromosome breakage (for references, see 68). It is still doubtful whether chromomrne chanres art directly respan<' for malirnant transformation. Somc experimental studies sctm to SU; the hypothesis that both viral and chemical carcinogeneses are xned;: through changes in chromosome balance (35). CHEMICAL LEUKEMOGENESIS IN MAN 219 bs discussed in greater detail elsewhere in this issue. individuals with F,,coni's ancmia and Bloom's syndrome, who show incrcasd rates of 1 chrOmt)sOme breaks, art at 8 high risk for leukemia and other ntoplasias (3. 26); a high incidence ofleukemia is also nponed in Down's syndrome 21). Cells from these subjects show an increased transiormability try \*jmsin culture (18, 47). It.setms therefore that cells with chromosomal c jmba]ance or instability might be more susceptible to a possible leukcmoarenic virus; this mighl be lmc also for cells with acquired chromosomal d u e lo radiation or carcinogens. Another hypothesis is that of ,,rr;l.nrion oj u lurcnl leukemogenic virus by chemical carcinogens. Results animal cxperiments suppon this hypothesis (36). but no data are avail- able for human leukcmogencsis. Immune irnpairmcnr induced by the cytolosic chemicals might also be responsible for the development of Itukcmia. .4 ] ~. \ 4 a t i na~wnrs, for instance, which can cause both chromosome damage I Immunosuppression, might induce the formation of abnormal cell ClrintS, and these clones might proliferate as a result of the depressed im- nlune sun-eillancc. Benzene might act in a similar way, as suggested by one of US (22). T h e fact that in chemical, as well as in radiation leukemogenesis, only I +omc of the subjects exposed develop leukemia raises the question of in- dividual susceptibility to the l e u k e m o p ~ i cagents. The possibility that more I than one facior or mechanism might be implicated funhcr justifies the con- crpi of differing susccptibility, since a coincidence of various factors would - bc required for leukemia IO occur. In chemical leukemogenesis in the animal. recent experiments have suggested that 'host responses control the proliferation of tumor cells, ... and that o c c u n ~ n c eor non-occurrence of rhe disease is the end result of inieraciions between tumor cell and host rvhich probably involve physiological regulatoq processes, as well as im- munological responses' (32). This is probably true for man also, but proof awaits the development of new experimental and statistical methods. REFERES CES 1. AAcoy. M.. DinSol, K.$ Erdcm.S. d. DinSol. G.: Acute IeuArmia due i o chronic cipowrt i o hcnzcnc. Anirr. 1. hfrd. 52. 160, 1972. 2. R u n . R.H.D.: Phrn)lhuiazonc and leuhernia. A possible associarion. 6ri1. nrrd. 1. 2. 1 5 2 . 1960. 3. H l w m . G .E.. \Varncr, S.. Grnld, P.S. 6: Diamond. L. E.: Chromrwwnc ahnor- mhlilits in ronslilutianal aplastic anrmia. h ' r ~Engl. 1. Afrd. 274. 8, 1966. 4. Braucr. hi. J. d: DamtsheA, W.: Hypoptasiic anemia and m~rlohlaziit IeuLcrnia Cunt. 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