This review of the prevalence and attribution of individual HPV types among U.S. female genital cancers builds upon the work of prior reviews(21
) through the incorporation of updated references, adjustments for histologic type and presence of multi-type HPV infections where feasible, and inclusion of vulvar and vaginal neoplasias in addition to cervical disease. We now discuss findings based on each of these study features in addition to the broader implications of the HPV typing results.
Among prior published reviews of the prevalence of HPV types in invasive and pre-invasive cervical disease, the worldwide meta-analysis of Smith et al. is the most recent, covering studies published through January 2006.(25
) However, a number of studies, particularly among those fully documenting multi-type HPV infections, have been published since. Five of 8 studies of CIN 2/3 lesions included in the present review were not part of this earlier study, representing 61% of all included CIN 2/3 cases. The same was true for 5 of 11 cervical cancer studies included here.
There were also studies included in these earlier reviews which we chose to exclude because the samples were based on exfoliated cervical cells, Pap smears or cervico-vaginal lavage.(21
) The decision to exclude these studies was not taken lightly, as it ultimately meant a reduction in sample size and statistical precision. However, in the early stages of our review, an examination of two U.S. studies utilizing exfoliated cervical cells to report HPV PCR typing for CIN 2/3 lesions revealed a very high rate of multi-type HPV infections. Adam et al.(57
) reported multi-type HPV infections among 21.0% of women inclusive of high-risk HPV type infections alone. MacLehose et al.(58
) observed 39 high or low-risk HPV infections among 21 women with HPV-positive lesions (an 86% overage relative to if all women had tested positive for a single HPV type). Similarly, lavage PCR data for U.S. women with CIN 2/3 reported by Ho et al. revealed multi-type HPV infections among 40% of study participants.(59
) Unlike the biopsy-based study excluded from this review,(39
) where multi-type infections with the same few types were consistently observed in multiple samples, HPV infections spanned a broad array of types in these non-biopsy studies, with no clear pattern for multi-type HPV infections reported. For comparison, across CIN 2/3 biopsy specimens included in the present review, where reporting of multiple HPV-types was available, just 6.5% tested positive for multiple types, representing a total overage of 7% among HPV-positive cases. On this basis, we concluded that although imperfect in their own right, limiting the review to biopsy specimens would produce results that were less likely to be biased by concomitant HPV infections distinct from the lesion of interest occurring elsewhere in the cervico-vaginal tract as ascertained by these other testing methods.
Even so, the proportion of multi-type HPV infections among biopsied specimens reported in this study was 7.2% among cervical lesions, 9.5% among vulvar lesions and 13.7% among vaginal lesions. To avoid multiple-counting when evaluating attribution among multi-type infected lesions, we introduced a multi-type adjustment method to estimate the proportion of cases accounted for by individual HPV types or groups of HPV types. HPV 16 and 18 are well known to be the two HPV types with the highest attribution to cervical cancers worldwide.(24
) Among female genital cancers and high-grade cervical pre-cancers, adjustment for multi-type HPV infections produced a relatively small impact on the estimated attribution of these types to disease. However, other HPV types less commonly observed in high-grade or invasive female genital neoplasias were disproportionately or exclusively observed in combination with HPV 16, HPV 18 or each other. For instance, among studies of squamous cell cervical cancers where the presence of multi-type infections could be assessed, HPV 16 was observed as a single type infection in 88% of HPV 16-positive specimens (284/324) whereas the same was true for HPV-58 in just 25% (1/4) of cases (p = 0.004). As a result, the elimination of multiple counting of multi-type infections had a more pronounced impact on the estimated attribution of non-HPV 16/18 type infections than for HPV 16 and 18. For example, crudely summing prevalence figures for the remaining 17 HPV types other than HPV 16 or 18 among squamous cell cervical cancers yields a figure of 17.4% which is 71% higher than the estimated attribution of these types after adjusting for multi-type HPV infections (10.2%). Thus, adjustment for multi-type HPV infections could potentially have implications for the estimation of the incremental cost-benefit associated with expanding HPV tests and vaccines to cover additional HPV types.(60
) Among prior international reviews, only Muñoz et al. adjusted crude prevalence figures for multi-type infections to estimate attributable fractions.(24
) However, unlike in the present study, all HPV types observed within multi-type infections were presumed to contribute equally to disease. In addition the study scope was limited to International Agency for Research on Cancer (IARC) studies. For the present analysis we felt it was helpful to differentially weight HPV types in multi-type infections according to their frequency in diagnosed disease. Thus, an HPV 6 and 16 co-infected cervical cancer was perceived to have more likely resulted from infection with HPV 16. It is interesting to note that there was a range in the proportion of lesions testing positive for multiple HPV types within a given grade and type of disease, which could be the result of purely statistical variation, but also potentially variation in HPV typing methods.
In this study, HPV 18 was estimated to account for a much larger proportion (4 times) of U.S. cervical adenocarcinomas than squamous cell cancers; a finding also highlighted in earlier reviews.(24
) The adjustment of HPV prevalence and attribution data across all cervical cancers to the proportion of adenocarcinomas vs. squamous cell carcinomas estimated for the U.S. population produced a very modest impact on results relative to no adjustment (data not shown) since the proportions were similar between U.S. population data (23.8%) and studies included in this review (23.6-27.6%). Depending on the available data, though, this may not always be the case in analyses for the U.S. or other countries. For instance, Smith et al. reviewed North American studies (U.S./Canada) in which a higher proportion (32%) of cervical cancer cases were adenocarcinomas and reported a higher prevalence of HPV 18 (22.2%) than that estimated in the present review (17.6%).(25
Unfortunately, data were insufficient for describing HPV typing results for vulvar or vaginal adenocarcinomas. Thus our review focused on squamous cell disease for these cancers and we were not able to report pooled data adjusted by histologic type for these cases. Based on data from the Surveillance Epidemiology and End Results (SEER) Program for 2000-2002, adenocarcinomas are estimated to comprise a very small proportion of vulvar cancers (2.4%).(12
) However, the fraction of adenocarcinomas is larger for vaginal cancers (16.4%) and investigation of HPV typing for these tumors represents an area for further research.(12
Most of the general findings of this study, such as that HPV 16 and 18 are the primary types observed in cervical and vaginal cancers(22
), HPV 16 is the most common type among vulvar cancers(61
) and that HPV 31 is the next most frequent type among U.S. cervical cancers(22
) have been reported by other authors previously. Also, similar to prior reviews,(21
) we found that HPV prevalence varied by grade of disease. For instance, HPV 16, 18, 31, 51, 52 and 66 were each prevalent in more than 5% of CIN 1 cases, compared to only HPV 16 and 18 observed in excess of 5% among invasive cervical cancers. However, for policy decisions, such as estimating the cost-effectiveness of expanding HPV vaccines and tests to include coverage of additional HPV types, or understanding the benefits conferred by HPV vaccine cross-type protection,(62
) accurate estimation of the absolute attribution of individual HPV types to these pre-cancers and cancers is essential. To this end, this paper has featured several methodological adaptations which have yielded some results that differ from earlier reviews.
First, after adjustment for multi-type HPV infections and the limitation of studies to only those based on cervical tissue specimens, the attribution of “high-risk” HPV types other than HPV 16 and 18 (types 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 66, 68, 70, 73 and 82) to cervical cancers was observed to be quite low (6.8%). This figure is much lower than what would be obtained were one to simply sum prevalence figures for these types from earlier studies for North America/Australia by Clifford et al. (12.4%),(22
) for North America by Smith et al. (15.0%)(25
), or from the data on HPV-type attribution for Europe and North America reported by Muñoz et al. (>16%).(24
) Although these types may have a relatively smaller attribution to cervical cancer incidence in the U.S., in some other regions of the world such as Asia, they may account for a much larger fraction of disease.(34
) For instance, in an international analysis of IARC studies, Muñoz et al. observed a pooled prevalence of single-type infected squamous cell cancers associated with these types of 19.7%, and a corresponding prevalence among adenocarcinomas of 11.7%.(24
) In addition, these types accounted for a much larger proportion of CIN 2/3 (23.5%) and CIN 1 (46.9%) lesions. An understanding of the relative merits of alternative coverage strategies for these types in vaccines and diagnostic tests requires formal cost-effectiveness analyses.
Conversely, the proportion of U.S. cervical cancer cases estimated to be attributable to HPV 16 and 18 infections (79.2%) is higher than the 70% attribution for these types that is typically referenced as a worldwide standard.(64
) Thus, while a 70% attributable fraction for these types could reflect a worldwide average, modest deviations may be observable for some countries and regions.
Second, although generally differentiated from these other HPV types as “low-risk”, HPV 6 was estimated to contribute to approximately 2.1% of U.S. invasive cervical cancers and 3.6% of vulvar cancers. Though low, these figures were above those estimated for HPV types such as 45, 52 and 58 which are commonly regarded as “high-risk types.” However, a causal role of HPV 6 in cervical cancers has previously been regarded as unlikely, based on its low malignant potential as demonstrated in laboratory studies,(66
) and the potential role of HPV 6 in these cancers merits further evaluation.
Third, HPV appears to be present in 65-75% of vulvar and vaginal squamous cell cancers. HPV 16 and 18 were estimated to account for over 50% of vulvar cancers and over 70% of vaginal cancers in the studies reviewed, with other types found in a minority of cases. While VIN 1 lesions bore some resemblance to anogenital warts in that HPV 6 and 11 were the predominant types detected,(33
) other HPV types such as HPV 68 and 16 seemed to play a more important role in these lesions than generally observed in anogenital warts. The representation of individual HPV types among VaIN 1 lesions was somewhat more similar to that of CIN 1 than VIN 1, in that a larger proportion of lesions tested positive for HPV 16 or 18 than HPV 6 or 11.
Our study has several limitations. First, the HPV testing in the studies reviewed may have had imperfect sensitivity and specificity for the HPV types reported and variation in the detection of specific types may exist across typing methods utilized.(68
) Newer assays detect a broader range of HPV types than older methods and some studies, regardless of publication year, elected to conduct and report typing data for particular sub-groups of potentially evaluable HPV types. The effect of including studies that assayed their samples for relatively fewer types would likely be to bias our results towards showing greater attribution to commonly assayed types, as the ability to detect and adjust downward for the presence of multi-type infections would be limited to a relatively smaller group of types. We attempted to minimize this potential bias by restricting our analysis of attribution to studies reporting typing results for a minimum of 8 HPV types and, in fact, only 2 of the 13 studies evaluated in the analyses of attribution conducted typing for fewer than 15 HPV types.
Second, because HPV infections are prevalent in the genital tracts of women without clinically evident disease, it is possible that a proportion of cases could have a given HPV type detected in a biopsy specimen by chance, without causal attribution to the lesion. Third, although our multi-type adjustment method may serve as a reasonable proxy for HPV type-specific attribution when multiple types are observed in a single specimen, and mitigate concerns regarding double-counting of lesions, it is not an exact method for identifying which HPV type or type(s) are responsible for a given lesion or lesions. The method implicitly assumes that if a given HPV type is rarely observed in single-type infected lesions, it is also rarely a causal type in multi-type infected lesions. Further studies using laboratory methods (e.g., in situ hybridization, assessment of integration or transcriptional activity) to isolate the causal HPV in multi-type infected lesions would be helpful. Fourth, the studies included in this and prior reviews do not constitute a truly representative random statistical sample of disease diagnosed within the general U.S. population. For instance, in some studies there could have potentially been under-representation of tumor specimens from women with late stage disease who were unable to participate or died before being contacted for participation. The impact of this potential limitation is not known. Finally, additional studies of HPV typing would be particularly helpful for vaginal precancers and cancers (both squamous cell and adenocarcinomas) given the limited numbers of U.S. specimens, HPV types, and assessments of multi-type infections reported to date for these lesions. Even for cervical cancers, where total sample sizes were much larger, the number of available studies cataloging the distribution of multi-type infections to date has been relatively limited and the external validity of our results depend upon the accuracy and generalizability of these studies.
In conclusion, this study has described the prevalence and attribution of HPV types to cervical, vaginal and vulvar precancers and cancers in the United States. For instance, among the findings, a slightly higher fraction of cervical cancers (79%) in the U.S. may be attributable to HPV 16 and 18 infection vis a vis pooled international data (71%).(24
) The analysis has built upon the work of prior reviews of cervical disease with an updated and expanded analysis, incorporation of new selection criteria for specimens, adjustment for histologic type among cervical cancers and accounting methods for multi-type HPV infections. To date, only a portion of U.S. studies of these precancers and cancers have specifically reported all HPV types for which testing was conducted, a complete characterization of single and multi-type infections and HPV typing by lesion histologic type. Consistent reporting of this information in future studies will enhance the value of the data for researchers and policymakers wishing to better understand the burden of HPV disease in the population and the potential health and economic impact of technologies for preventing, diagnosing and treating HPV infection and disease.