Smoking and cervical cancer: pooled analysis of the IARC multi-centric case–control study

Smoking and cervical cancer: pooled analysis of the IARC multi-centric case–control

study

Martyn Plummer1,*, Rolando Herrero2, Silvia Franceschi1, Chris J.L.M. Meijer3, Peter Snijders3, F. Xavier Bosch4,Silvia de Sanjose4 & Nubia Munoz1 for the IARC Multi-centre Cervical Cancer Study Group�1International Agency for Research on Cancer, 150 Cours Albert Thomas, F-69372 Lyon, France; 2ProyectoEpidemiologic Guanacaste, Costa Rica; 3Department of Pathology, VU Medical Center, Amsterdam, TheNetherlands; 4Epidemiology and Cancer Registry Service, Catalan Institute of Oncology (ICO), Barcelona, Spain

Received 24 February 2003; accepted in revised form 24 June 2003

Key words: Cervical cancer, human papillomavirus, smoking.

Abstract

Background: Smoking has long been suspected to be a risk factor for cervical cancer. However, not all previousstudies have properly controlled for the effect of human papillomavirus (HPV) infection, which has now beenestablished as a virtually necessary cause of cervical cancer. To evaluate the role of smoking as a cofactor ofprogression from HPV infection to cancer, we performed a pooled analysis of 10 previously published case–controlstudies. This analysis is part of a series of analyses of cofactors of HPV in the aetiology of cervical cancer.Methods: Data were pooled from eight case–control studies of invasive cervical carcinoma (ICC) and two ofcarcinoma in situ (CIS) from four continents. All studies used a similar protocol and questionnaires and included aPCR-based evaluation of HPV DNA in cytological smears or biopsy specimens. Only subjects positive for HPVDNA were included in the analysis. A total of 1463 squamous cell ICC cases were analyzed, along with 211 CIScases, 124 adeno- or adeno-squamous ICC cases and 254 control women. Pooled odds ratios (OR) and 95%confidence intervals (CI) were estimated using logistic regression models controlling for sexual and non-sexualconfounding factors.Results: There was an excess risk for ever smoking among HPV positive women (OR 2.17 95%CI 1.46–3.22). Whenresults were analyzed by histological type, an excess risk was observed among cases of squamous cell carcinoma forcurrent smokers (OR 2.30, 95%CI 1.31–4.04) and ex-smokers (OR 1.80, 95%CI 0.95–3.44). No clear pattern ofassociation with risk was detected for adenocarcinomas, although the number of cases with this histologic type waslimited.Conclusions: Smoking increases the risk of cervical cancer among HPV positive women. The results of our study areconsistent with the few previously conducted studies of smoking and cervical cancer that have adequately controlledfor HPV infection. Recent increasing trends of smoking among young women could have a serious impact oncervical cancer incidence in the coming years.

Introduction

Infection with certain types of human papillomavirus(HPV) has been established as a virtually necessary

cause of cervical cancer [1, 2]. This discovery hasimportant implications for both primary and secondaryprevention, with the prospect that the disease mayeventually be eliminated by prophylactic vaccination.Current knowledge of the aetiology of cervical cancer,however, remains incomplete. Cervical cancer is a rareconsequence of HPV infection and, when it does occur,it develops many years after initial infection. For thesereasons, current research efforts are oriented towards

* Address correspondence to: Martyn Plummer, Unit of Field and

Intervention Studies, International Agency for Research on Cancer,

150 Cours Albert Thomas, F-69372 Lyon, France. Ph.: +33-472-

738446/738402; Fax: +33-472-738345; E-mail: [email protected]�See Appendix 1 for details.

Cancer Causes and Control 14: 805–814, 2003. 805� 2003 Kluwer Academic Publishers. Printed in the Netherlands.

identifying cofactors that may influence the progressionfrom infection to cancer. Cofactors may be character-istics of the virus or host, or may be environmental.Between 1985 and 1997 the International Agency for

Research on Cancer conducted a series of case–controlstudies of cervical cancer; eight studies on invasivecancer [3–10] and two on carcinoma in situ (CIS) [11,12]. All studies used a similar protocol and question-naires and included an accurate evaluation of HPVDNA in cytological smears or biopsy specimens. Thisreport is part of a series of papers [13–16] that use thepooled data from these studies to evaluate possiblecofactors for cervical cancer. We currently believe thatthe appropriate way to evaluate potential cofactors,while controlling for the effect of HPV, is to restrict theanalysis to women with HPV infections.Smoking has long been associated with cervical cancer

risk [17]. However, it has been difficult to rule outresidual confounding, chiefly from sexual habits, as anexplanation for this association [18, 19]. In particular,few studies of smoking and cervical cancer havecontrolled for HPV infection. This study allows us toexamine the role of smoking while controlling for thestrong effect of HPV infection and, additionally, othercofactors that have been identified in the same popula-tions [13, 14].

Subjects and methods

This analysis concerns 1798 HPV positive cases and 254HPV positive controls from eight separate case–controlstudies of invasive cervical cancer (ICC) and two studiesof CIS. The ICC studies were conducted in eightcountries, which showed wide variation in incidencerates. Regions covered include Thailand [5] and thePhilippines [8] in Asia; Morocco [4] in Africa; Brazil[6], Peru [10], Paraguay [9] and Colombia [3, 7] in SouthAmerica and Spain [3, 7] in Europe. The two CISstudies were performed in Spain and Colombia atthe same time as the studies of ICC [11, 12]. These havealso been included in this analysis because CIS isaccepted as an immediate precursor of ICC, andbecause it shared the same risk factors in our studies[20].Methods and findings from each study have been

previously described. In summary, eligible patients werehistologically confirmed, incident ICC or CIS identifiedat the reference hospitals. Cases had no previoustreatment for the disease and gave informed consent toparticipate in the study. The histological diagnosis wasreviewed in each study either by a single expert, or by apanel. Cases were histologically classified as squamous

cell carcinoma, adenocarcinoma and adenosquamouscarcinoma. The last two categories have been combinedfor this analysis.For all studies except the two ICC studies in Spain

and Colombia, controls were hospital based, and werefrequency matched by age in five-year age groups.Diseases associated with known risk factors for cervicalneoplasia, including tobacco consumption, were exclud-ed. For the two CIS studies, the controls were popula-tion based and were individually matched by age anddate of cytology. The matching has been ignored in thecurrent analysis, with no material effect on the results.All consenting participants were administered a stan-

dardized questionnaire including information on socio-demographic factors, sexual behaviour, reproductivehistory, contraceptive practices, smoking habits, genitalhygiene, history of STDs and history of cervicalcytological screening. Face-to-face interviews were con-ducted in hospital by trained interviewers. In the part ofthe questionnaire on tobacco, subjects were first askedto classify themselves as life long non-smokers, currentsmokers or ex-smokers (defined as a former smoker whostopped smoking at least 1 year prior to the interview).Ever-smokers were then asked the age at which theystarted smoking, how many cigarettes per day theysmoked and the total time made up of periods (if any) inwhich they stopped smoking. Ex-smokers were askedthe age at which they finally stopped smoking. Furtherinformation on duration and number of cigarettes wascollected by type of tobacco. Information on durationand number of times per day was also collected forcertain chewing habits: chewing tobacco, taking snuffand chewing betel quid (with and without tobacco).After the interview, all subjects had a pelvic examinationperformed by a gynaecologist, in which exfoliated cellsfor cytological examination and HPV analysis weretaken. A tumour biopsy specimen was also taken inmost cases and kept frozen.

HPV DNA detection

Exfoliated cells were collected using wooden spatulaeand endocervical brushes. Two scrapes were obtainedfrom each woman. After preparation of one Pap smear,the remaining cells were eluted in phosphate-bufferedsaline (PBS), peleted in PBS and kept frozen at )70 �Cuntil shipment to a central laboratory for HPV testing.Detailed descriptions of the hybridization assays

employed are given in the individual publications ofthe various studies. Briefly, HPV DNA detection andtyping were performed in central laboratories usingPCR amplification methods targeting a small fragmentof the L1 gene. DNA quality was evaluated using

806 M. Plummer et al.

b-globin primers. In the studies from Colombia andSpain, MY09 and MY11 consensus primers were used[21]. In the remaining studies GP5+/6+ primers wereemployed. PCR products were assessed for HPV posi-tivity by low-stringency Southern blot hybridizationusing a cocktail of HPV-specific probes [22]. For viralgenotyping the PCR products were successively hybrid-ized with oligonucleotide probes for 33 HPV types aspreviously described [22, 23]. HPV DNA-positive spec-imens that did not hybridize with any of the 33 probeswere tentatively called HPV X.In a second step, E7 primers for 14 high-risk HPV

types [2] were used for reamplification of the followingspecimens: all case specimens positive for the b-globingene and classified as HPV X or HPV DNA negative, allspecimens from control women classified as HPV X, anda subsample of specimens from control women found tobe b-globin-positive and HPV DNA-negative. Standardprecautions were taken to minimize the risk of false-positive results in the PCR reactions as describedelsewhere [24]. All PCR assays were performed blindto the case–control status. Failure to detect HPV DNAwith a negative amplification of b-globin led to aninvalid result and these women have been excluded fromanalysis.Because some of the re-testing was performed at the

completion of the fieldwork in all study areas, numbersof HPV positive cases and controls of this analysissometimes differ from those reported in the originalpublications.

Statistical analysis

Logistic regression was used to estimate odds ratios(ORs) and 95% confidence intervals (95%CI). Likeli-hood ratio tests were used to assess the significance ofterms in the model. Heterogeneity of the OR betweencentres, and other stratifying variables, was tested byintroducing an interaction term between smoking andthe modifying factor.Most results are presented graphically, plotting the

summary ORs as a black square, whose size is inverselyproportional to the variance of the estimate. A hori-zontal line represents the 95%CI. Diamonds are used toplot the summary OR for squamous cell ICC, CIS andall studies together. The centre of the diamond repre-sents the pooled OR and the extremes show the limits ofthe 95%CI. A log scale is used for the OR in order topreserve symmetry of the CI.This report considers only tobacco consumption in

the form of cigarette smoking. Chewing of tobacco orother substances (coca, betel) and snuff taking was

reported in only a few centres. These exposures havebeen combined into a single variable for ‘chewing’ whichwas controlled for in the final analysis. Other confound-ers considered and their categories were age (in five yearbands), centre, education (none, primary, secondary+),number of sexual partners (0–1, 2, 3, 4+), age at firstintercourse (under 16, 17–21, 22+), years of oralcontraceptive use (never, 1–4, 5+ years), parity (0, 1–2, 3+) and screening history (never, ever, excludingscreening visits 12 months before the interview).

Results

Table 1 shows, for each study area, data on incidencerates of cervical cancer, prevalence of HPV infection incases and controls, and prevalence of tobacco consump-tion. Incidence rates are taken from Globocan 2000 [25].They show that Spain is a low-risk country, Morocco,Thailand and the Philippines are intermediate riskcountries, and the South American countries are highrisk. As expected, prevalence of HPV infection is veryhigh in cases, but low in controls, ranging from 6% inSpain to 21% in Morocco. Smoking prevalence incontrols is low in most countries, with the notableexceptions of Brazil (36% ever smokers), Colombia(28% ever smokers), and the CIS study in Spain (38%ever smokers). The average number of cigarettes per dayamong smokers is also low (3–11 cigarettes per day incontrols). Chewing is relatively common in only twocentres: Peru where 5% of controls were chewers,mainly of coca leaves, and Thailand where 20% ofcontrols were chewers, maily of betel.Figure 1 summarizes the OR of cervical cancer for

ever-smokers compared with never-smokers in eachstudy and the pooled results. ORs in this figure werecontrolled only for age and centre. Ever smoking ispositively associated with cervical cancer (OR¼ 2.17,95%CI 1.46–3.22). The results for each centre arerelatively homogeneous: the formal test for heterogene-ity has a p-value of 0.11. However, Morocco is a clearoutlier which shows a negative association (OR¼ 0.27,95%CI 0.05–1.45). This may be due to a design flaw.Cases in the Moroccan study were drawn from aspecialist cancer hospital whereas controls were recruit-ed from a general hospital with a smaller catchmentarea. In the original analysis of this data, an attempt wasmade to overcome this selection bias by controlling forsocial class and urban/rural residence [4]. The OR inMorocco controlling for education (as a marker of SES)and residence is 0.63 (95%CI 0.08–4.8). Morocco wasretained in the analysis, but with an additional adjust-ment for urban/rural residence.

Pooled analysis of smoking and cervical cancer 807

As a further test to ensure that the results are notunduly influenced by any single study, the pooled OR

was recalculated after leaving out each study in turn.The pooled OR remained fairly stable, ranging from

Table 1. HPV prevalence and smoking habits by centre

ASRa

(world)

HPV tested HPV positive Ever smokers (%)

Cases Controls Cases (%) Controls (%) Cases Controls

Invasive cancer

Paraguay 41.1 112 90 109 (97.3) 17 (18.9) 32.2 19.0

Peru 39.9 196 175 186 (94.9) 31 (17.7) 9.6 3.6

Colombia 32.9 110 126 86 (78.2) 22 (17.5) 42.4 28.1

Brazil 31.3 187 196 181 (96.8) 34 (17.3) 47.2 36.0

Philippines 22.7 364 381 349 (95.9) 35 (9.2) 20.4 7.2

Thailand 20.7 378 261 363 (96.0) 41 (15.7) 17.1 13.0

Morocco 18.8 188 176 182 (96.8) 38 (21.6) 2.8 4.4

Spain 7.2 159 136 131 (82.4) 8 (5.9) 20.4 14.3

Carcinoma in situ

Colombia 135 181 96 (71.1) 19 (10.5) 37.2 23.7

Spain 157 193 115 (73.2) 9 (4.7) 54.5 38.4

Total 1986 1915 1798 254

a Age standardized incidence rate from Globocan 2000 [25].b Among ever smokers.

Fig. 1. OR for ever smoking by study.


1.94 (95%CI 1.29–2.94) when the Philippines wereremoved to 2.37 (95%CI 1.54–3.67) when the ICCstudy in Colombia was removed.Figure 2 shows pooled results for various aspects of

smoking history for squamous cell carcinoma (includingCIS), controlling for age, centre, education, number ofsexual partners, age at first intercourse, oral contracep-tive use, parity and screening. Both current and ex-smokers showed elevated risks. However, among eversmokers, there is no evidence of increasing risk withyears of smoking, number of cigarettes per day or age atstarting smoking. In order to confirm this finding, trendtests were conducted for these three risk factors, treatingthem as continuous variables. The resulting p-valueswere 0.99 for duration, 0.90 for number of cigarettes perday and 0.52 for age at starting smoking. Adenocarci-

noma and adenosquamous carcinoma were also exam-ined but since the number of cases was small, the resultsare not presented in detail. There were 82 HPV positiveadenocarcinomas (2.0% of cases) and 42 adenosqua-mous carcinomas (1.0% of cases). When these werecombined, the OR for ever smokers was 1.64 (95%CI0.64–4.21). When adenocarcinomas were examinedon their own the OR was 2.19 (95%CI 0.74–6.47).The results for these histological types are thereforecompatible with no excess risk for smokers, but they arealso compatible with the results for squamous cellcarcinoma.Figure 3 shows the modifying effect of age and

number of sexual partners on the OR for ever smoking.Results are presented for all histological types com-bined. There is no strong evidence of heterogeneity

Fig. 2. ORs of squamous cell carcinoma of the cervix.

Fig. 3. ORs for ever smoking stratified by age and number of sexual partners.


across either age groups (p¼ 0.16 for heterogeneity test)or number of sexual partners (p¼ 0.17).The association with smoking was also tested after

further restriction to subjects infected with high riskHPV types (16, 18, 26, 31, 33, 35, 39, 45, 51, 52, 53, 56,58, 59, 66, 68, 73, 82). This subgroup comprised 93% ofthe HPV positive cases (1667 cases) and 63% of theHPV positive controls (161 controls). The associationwith ever smoking was not changed (OR¼ 2.14, 95%CI1.22–3.78, controlling for all confounders listed above).There was also no evidence of any dose–responserelationship (data not shown).In addition to analysing smoking as a risk factor for

cervical cancer among HPV positive women, smokingwas also examined as a risk factor for HPV acquisitionamong controls. The results are presented in Table 2and show no association between smoking behaviourand HPV.

Discussion

Our present pooled analysis of 1798 cases or cervicalcarcinoma and 254 control women who were infectedwith HPV shows that ever-smokers have an excess risk of

cervical cancer that persists after controlling for thestrong effect of HPV and for other potential cofactors ofprogression from infection to cancer. The low level ofsmoking in most of the study populations hampered ourability to study the presence of a dose-response effect.The hypothesis that smoking is a risk factor for

cervical cancer was first put forward in 1977 byWinkelstein [17], who also periodically reviewed thesupport for this hypothesis from subsequent epidemio-logical studies [26, 27]. Despite a consistent associationbetween smoking and cervical cancer after an adjust-ment for sexual behaviour, it was not generally agreedthat confounding could be ruled out as an explanationfor this finding, since sexual behaviour was evidently asurrogate for a sexually transmitted agent [18, 19, 28].The hypothesis that HPV is the causal agent was firstput forward by zur Hausen [29]. Now that HPV hasbeen identified as the principal cause of cervical cancer,it should be possible to resolve the controversy oversmoking. However, relatively few studies have con-trolled for HPV when examining smoking. Of these,some have used a relatively inaccurate HPV DNAdetection method [30] or anti-HPV antibodies [31], and/or have concentrated on a limited range of HPV types[30–33].

Table 2. ORs of HPV-positivity and 95% CI according to reproductive factors among control women

HPV prevalence (%) HPV positive/HPV negative ORa (95%CI)

Smoking status

Never 14 218/1369 1.00

Current 10 22/195 0.85 (0.52, 1.39)

Ex 13 14/97 0.85 (0.47, 1.56)

Test for heterogeneity p = 0.72

No. of years smoking

Never 14 218/1369 1.00

1–19 11 19/158 1.04 (0.61, 1.77)

‡20 11 15/127 0.65 (0.36, 1.16)

Test for trend p = 0.21

No. cigarettes/day

Never 14 218/1369 1.00

<6 12 17/129 0.84 (0.49, 1.45)

‡6 11 18/151 0.92 (0.53, 1.58)


Age started smoking

Never 14 218/1369 1.00

‡20 14 18/117 1.07 (0.63, 1.83)

15–19 10 13/111 0.93 (0.50, 1.72)

<15 8 5/59 0.45 (0.17, 1.18)


Years since quitting

Current smoker 10 22/195 1.00

1–10 15 10/55 1.52 (0.62, 3.74)

‡11 7 3/38 0.60 (0.16, 2.35)


a Controlling for study centre and age.


The evidence for pre-invasive cervical neoplasias islarger than for invasive ones. In a clinic-based case–control study of high grade dysplasia in the USA, Beckeret al. [34] found an increased risk for current smokersthat persisted after adjusting for HPV, age at first sexualintercourse, number of sexual partners and ethnicity(OR¼ 1.8, 95%CI 1.2–2.8). No excess risk was observedfor ex-smokers. Kjellberg et al. [35] conducted a popu-lation-based study of high-grade cervical intraepithelialneoplasia (CIN) in Sweden. An excess risk of CIN 2–3was observed for ever-smoking that persisted afteradjusting for HPV DNA positivity (OR 2.5, 95%CI1.3–4.9). Deacon et al. [36] conducted a nested case–control study of CIN 3 in England. In contrast to thepreviously reported studies, which used multiple regres-sion to control for the effect of HPV, Deacon et al.separately analyzed risk factors for progression amongHPV positive women and risk factors for presence ofHPV among control women. Ever smoking was a riskfactor for CIN 3 among HPV positive women (OR¼ 2.2,95%CI 1.44–3.35). The excess risk persisted after adjust-ing for age at first intercourse, number of sexual partners,spontaneous abortion and time since start of latestregular relationship. There was also strong evidence of adose–response effect. By contrast, smoking was notassociated with HPV DNA positivity among controls(OR¼ 0.90, 95%CI 0.60–1.35). Hildesheim et al. [37]conducted a population based study in Costa Rica usingprevalent cases of HSIL or cancer as an outcome. Bothcurrent smokers and ex-smokers were at increased risk,with ORs of 2.4 (95%CI 1.2–5.1) and 2.3 (95%CI 1.3–4.3) respectively, after controlling for age, HPV type(high-risk versus low-risk) and parity. Finally, Laceyet al. [38] conducted a multi-centre case–control study ofcervical adenocarcinoma in the USA, using communitycontrols and, additionally an age-matched group of SCCcases [38]. After controlling for HPV, education, numberof sexual partners and screening history, ever smokingwas not associated with adenocarcinoma (OR¼ 0.8,95%CI 0.5–1.2) or SCC (OR¼ 1.4, 95%CI 0.8–2.3).There was limited evidence that heavy smoking wasassociated with SCC (OR¼ 1.8, 95%CI 1.0–3.3 for‡1 pack/day) but not with adenocarcinoma (OR¼ 0.7,95%CI 0.4–1.3 for ‡1 pack/day). The results of thepresent analysis are therefore broadly consistent withother case–control studies that have controlled for HPVinfection using high quality HPV assays. In one of thefew available prospective studies, tobacco smoke wasassociated with progression from CIN I to CIN IIIamong HPV 16 positive women [39]. Significantly higherregression rates of cervical precancerous lesions have alsobeen reported for women who quit smoking compared tothose who continue with the habit [40].

Various possible mechanisms have been proposed forthe association between smoking and cervical cancer. Inanimal models, more than 50 years ago Rous andFriedwald [41] described the malignant conversion ofcottontail rabbit papillomavirus after exposure to me-thilcolanthrene or tar. More recently, malignant trans-formation has been observed when endocervical cellsimmortalized by HPV 16 are exposed to cigarette smokecondensate, whereas untreated cells remain non-tumori-genic in nude mice [42].Tobacco is able to induce its carcinogenic effect in

sites not directly exposed to cigarette smoke, as evi-denced by associations of smoking with pancreatic,kidney and bladder cancer [43]. In the cervix, it ispossible to detect nicotine derivatives like cotinine andtobacco specific nitrosamines (recently reviewed bySzarewski and Cuzick [44]). In addition, DNA adducts[45] and other evidence of genotoxic damage [46] aredetectable in cervical exfoliated cells. It has been shownthat smoking affects local immune mechanisms in thecervix, with reductions in the number of Langerhanscells and other markers of immune function [47].Another possibility is a systemic effect of smoking, bywhich smokers would have an altered metabolism offemale hormones compared with non-smokers. Theassociation presented here with smoking remains aftercontrolling for oral contraceptive use and parity, but wedo not have sufficient data to analyze the interactionsbetween these exposures.Our approach of restricting the analysis to HPV

positive women is based on our current understandingof HPV as a virtually necessary cause of cervical cancer.This implies that if smoking is a risk factor for cervicalcancer, it may act either by increasing the risk ofacquiring an HPV infection, or by increasing the risk ofprogression from HPV infection to cancer. We found noassociation between smoking and HPV infection amongcontrols. Conversely, we found an increased risk of cer-vical cancer for smokers among HPV positive women.Hence we conclude that smoking acts as a risk factor forprogression once HPV infection is acquired. Thisrestriction should be borne in mind when interpretingthe results: the ORs presented here cannot be extrapo-lated to the general population, but are restricted toHPV positive women. In order to check our conclusionsagainst a more traditional analysis, we re-examined therelationship between ever smoking and risk of cervicalcancer by multiple logistic regression using the full set ofsubjects on whom an HPV test result was available(1986 cases and 1915 controls). The OR for ever-smoking controlling for HPV, age and centre was 2.32(95%CI 1.78–3.04). Further control for education,screening, parity, number of sexual partners, oral


contraceptive use and age at first intercourse did notsubstantially alter this result (OR 2.31, 95%CI 1.72–3.11).One of the limitations of restricting the analysis to

HPV positive women in case control studies is that onlythe current HPV status is available. Ideally the analysisshould be restricted to women with chronic infectionssince this is the underlying risk factor. Whereas all caseswill necessarily have chronic HPV infections, this is notnecessarily true for all HPV positive controls. Thismeans that control for chronic HPV infection might beincomplete. We have examined this possibility by testingthe modifying effect of age and number of sexualpartners – two risk factors that may be associated withchronic infection. The null results of this test suggestthat chronic infection has been adequately controlled forin the statistical model.Apart from HPV infection, we have tried to control

for three separate causes of confounding: screening,sexual behaviour and reproductive factors. Screeninghas been observed to be protective against ICC in mostof these populations. Sexual behaviour is a surrogate forother sexually transmitted infections that may acceleratethe progression to cancer. Chlamydia and HerpesSimplex virus 2 are two such infectious agents that willbe investigated more closely in future papers. Parity andoral contraceptive have been previously identified inthese studies as hormonal cofactors of progression inthis pooled data [13, 14].Smoking is rare among women in developing coun-

tries and in Spain. Ideally, the relationship betweensmoking and cervical cancer should be addressed inpopulations in which smoking is more common amongwomen. However, opportunities to study large numbersof women with invasive cervical cancer are rare, and wehave taken advantage of the possibilities offered by thisseries of studies with a common protocol and accurateHPV measurements.Selection bias is always a potential problem in case–

control studies. This issue has been discussed in theoriginal reports for these studies, with the generallyreassuring conclusions that there is no problem. Theonly exception is Morocco, in which we have attemptedto overcome the evident selection bias by controlling forurban/rural residence. As smoking is associated with abroad range of diseases, hospital controls may includean excess of smokers compared to the source popula-tion. However, in our studies patients in hospital forsmoking-related diseases were not eligible as controlwomen.In conclusion, smoking increases the risk of cervical

cancer among HPV positive women. The present pooledanalysis adds to the limited number of studies that have

adequately controlled for HPV infection when examin-ing the association between smoking and cervicalcancer. Taken together, this body of evidence suggeststhat squamous cell carcinoma of the cervix should beadded to the list of tobacco associated cancers, while foradenocarcinoma, further data are warranted. Thisconclusion was supported by an expert working groupconvened by the International Agency for Research onCancer in June 2002 [48].Several countries are experiencing an increase in the

prevalence of smoking, particularly among youngerwomen [49]. This increase in smoking among womencould have a serious impact on the incidence of cervicalcancer in the future, emphasizing the need for intensi-fying preventive efforts [50].

Appendix 1

The group is composed of the following researchers:

S. Franceschi, M. Plummer, J. Smith, N. Munoz, IARC, Lyon,

France; F.X. Bosch, V. Moreno, S. de Sanjose, X. Castellsague X,

ICO, Barcelona, Spain; R. Herrero, Proyecto Epidemiologico Gua-

nacaste, Costa Rica; C.J.L.M. Meijer, J.M.M. Walboomers and P.J.F.

Snijders, Free University Hospital, Amsterdam, the Netherlands; K.V.

Shah, Johns Hopkins University, Baltimore, Maryland; S. Chichareon,

Prince of Songkla University, Hat-Yai, Thailand; C. Ngelangel,

University of The Philippines, Manila, the Philippines; N. Chaoki,

B. El Gueddari, Institut National d’Oncologie, Rabat, Morocco;

J. Eluf-Neto, Universidade de Sao Paulo, Sao Paulo, Brazil; P.A.

Rolon, Laboratorio de Anatomia Patologica y Citologia, Asuncion,

Paraguay; C. Santos, E. Cacares, Maes Heller Cancer Research

Institute, Lima, Peru; S. Bayo, Institut National de Recherche en Sante

Publique, Bamako, Mali; I. Izarzugaza, Euskadi Cancer Registry,

Vitoria Gasteiz, Spain; M. Gili, Catedra Reynals, Barcelona, Spain;

L.A. Tafur, University of Valle, Cali, Colombia; C. Navarro, Health

Council, Murcia, Spain; N. Ascunce, Breast Cancer Prevention Centre,

Pamplona, Spain; L.C. Gonzalez, Delegation of Social Welfare,

Salamanca, Spain; M. Santamaria, Navarra Hospital, Pamplona,

Spain; P. Alonso de Ruiz, General Hospital of Mexico, Mexico City,

Mexico; N. Aristizabal, Cali, Colombia; J. Deacon, Institute of Cancer

Research, Belmont, United Kingdom.

Acknowledgements

This research was funded by the European CommunityCI 1-0371-F(CD); The Fondo de Investigaciones Sani-tarias (FIS), Spain 86/753, 87/1513, 88/2049, 90/0901,95/0955; Preventiefonds, The Netherlands 28-1502.1;Programa Interministerial de Investigacion y Desarrol-lo, Spain SAF 96/0323, The Conselho Nacional deDesenvolvimiento Cientifico e Tecnologico, Brazil(CNPq) JEN-204453/88.7 and the Department of Re-productive Health & Research at the World HealthOrganization, grant no. 98.101.


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Smoking and cervical cancer: pooled analysis of the IARC multi-centric case–control study

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