These three large Italian case series confirm some previously published results [7, 8]. The proportion of HPV-positive cases (96.0%) is slightly higher compared with that recently published by IARC (92.5%), meaning that the average quality of specimens and laboratory procedures was good.
Again, in line with de Sanjose’s study, HPV 16 and 18 were detected in about 77% of cervical cancers . This proportion is higher than that observed in preinvasive lesions (CIN2 and CIN3) in the same populations [9, 23], which is consistent with findings in almost all the previous studies [24, 25]. The proportions of HPV 18 and 45 are higher in adenocarcinomas than in squamous cancers, as previously reported in larger studies [7, 8, 26].
It is interesting that HPV 33 did not rank in the top five, in contrast with the worldwide distribution. Other studies have already reported higher prevalence of HPV 31 than 33 in southern Europe .
In our series, more than 1 out of 9 cases were adenocarcinomas, which is typical of high-moderate screening coverage , but the proportion of adenocarcinomas showed an increasing time trend during the study period, as expected, since Pap test coverage increased over the same period .
The main objective of this paper was to describe changes in the proportion of vaccine-covered HPV types 16 and 18 and in early onset HPV types 16, 18, and 45, according to the geographical area, age, and the calendar time.
Only few differences among geographic areas emerged and, even when statistically significant, they may have been due to chance, given the enormous number of statistical tests that can be performed comparing these distributions.
The most remarkable results in our analyses were the trends in HPV 16, 18, and 45 with age and calendar time. The multivariate analysis clearly shows that the two trends are independent and not due to reciprocal confounding, to the increase in adenocarcinomas proportion in recent years, or to different PCR techniques adopted for typing in the three studies.
The relation with age was expected since these are known to be early onset types [7, 9, 20, 29], particularly HPV16, which showed a higher probability of progression [10, 11]. In fact, if we assume a multi-step model in cancer progression, a higher probability of progression will produce a higher proportion of cases in younger ages, as sexual activity (and with it, possible infection) has only recently begun. Differently from the large study by de Sanjose and collaborators , our trend was due to an excess of HPV 16, 18 and 45 HPV-related squamous cancer in women below age 35, but we did not have enough statistical power to accurately describe the shape of the curve.
On the contrary, the association with calendar time was not observed in other large series [30, 31] and we must therefore assume it is a context-specific finding. It must be noted that the screening coverage in Italy has increased over the last decade, from 50% in 1994 to 73% in 2004. The increase in screening coverage has the immediate effect of decreasing the overall incidence of cervical cancer, as illustrated by the dramatic decrease in incidence in Italy, from 9.2/100.000 in 1996 to 7.7/100.000 in 2005 . However, Pap-test preventive efficacy is not the same for all the cancers. Indeed, it is well known that the efficacy of screening is stronger for squamous cancers than for adenocarcinomas [15–17], and it has recently been demonstrated that the efficacy is very low for early onset cancers . In line with these observations, screening might be less effective for those cancers caused by HPV 16, 18 and 45, virus types known to be associated with early onset. Consequently, in a highly screened population, the few cases occurring are in part those occurring in the under-screened or not covered minority. Further, a relevant proportion is also made up of the few cases occurring during the interval between two screening episodes. The cases escaping the Pap-test control, according to this hypothesis, are more likely to be linked to HPV 16, 18, or 45.
The possibility of a more rapid progression associated with HPV 16 and its variants is supported by the Guanacaste Cohort Study, a 7-year nested case–control study conducted on 10,049 women . The study showed that HPV 16 viruses, particularly non-European HPV 16 variants (mainly Asian American, AA), are significantly associated with higher risk of developing invasive cervical cancer [OR, 6.3 (95% CI, 1.6–24.6)]. Similar data have also been reported for HPV 16 AA variants in the Italian population, with a relative risk of 24.5 of developing invasive cervical cancer .
Another hypothesis is that HPV 16/18/45-related cancer precursors occur at a younger age, are far superior in number in comparison with other high-risk HPV-related cancer precursors, are not easily detected by cytology screening, and the transit time from precursor to cancer is not necessarily quicker. Recent population-based studies have shown that by using very sensitive screening tests such as HPV DNA testing, the number of precancerous lesions missed by cytology proves to be considerable . This lack of cytology screening sensitivity is particularly evident in the younger age groups, where HPV testing may detect high-grade CINs more frequently than cytology . It is already known that high-grade CINs occurring at a young age are mostly related to HPV 16/18 [9, 29]; the trend in distribution of HPV genotypes in invasive cancers in different calendar years observed in the present study may therefore be the result of the faster and more frequent formation of high-grade CINs in young girls and the inability of cytology screening to detect most of these lesions, typically related to HPV 16/18/45. The large number of high-grade CINs left in situ could then generate invasive lesions between two screening intervals more frequently than would the other high-risk genotypes, whose ability to persist and generate CIN3 lesions is less efficient than HPV16 and, albeit less markedly, than 18.
Both these hypotheses consider the three HPV types related to early onset cancers as a unique group, but the evidence available about speed of progression from infection to CIN3 and cancer and about screening sensitivity are different for HPV 16 and HPV 18, and are almost absent for HPV 45.
In conclusion, the proportion of HPV 16 and 18 types may be higher in highly screened populations because these types more frequently escape Pap test control. Consequently, the dramatic decrease in cancer incidence after screening introduction may be slightly less relevant for HPV 16- and 18-related cancers. Unfortunately, the effectiveness of Pap test as prevention cannot easily be increased by shortening screening intervals; a large body of evidence shows that reducing the interval from 3 years to 1 has a small effect on cancer incidence and a strong effect on the number of screening-induced treatments [35, 36]. On the contrary, by shifting to HPV as primary screening test, we can imagine a protocol that increases protection and decreases over-treatment, adopts longer intervals for HPV-negative women, and modulates the follow up for HPV-positive women according to the genotype.
As the present study has several limits, we can only confirm previously observed data or formulate hypotheses to be confirmed in other studies.
The first limitation is that we did not perform a central pathology review, even if all the cases were specifically reviewed for the original study in each center [9, 19, 20].
As typing was performed on archival paraffin-embedded samples, some trends could be due to reduced analytical accuracy of typing methods on older samples, but this is not very likely to change the proportion of types.
Our archives do not report some of the information that might have been crucial to testing our hypothesis. In particular, we do not know if the cases had had a previous negative Pap test. Furthermore, we do not know the stage at diagnosis. It could be extremely interesting to identify the micro-invasive cancers (FIGO IA1), since these cancers are virtually all asymptomatic and are thus mostly screen-detected .
Finally, our series do not come from population-based registries and thus may not represent the universe of cancers in Italy. Furthermore, in some of the areas with higher screening coverage, i.e., northern Italy and Rome, the participating centers are not reference centre for screening programs, while in the areas with lower screening coverage, i.e. southern Italy, they are. It is likely that the proportion of screening-detected cancers in our archives does not represent the proportion in the cancer population in each geographic area. This may explain why we did not observe an effect of the geographic area on the proportion of HPV 16, 18, or 45, despite the broad differences in screening coverage among the areas.