Developing effective screening tools for early cancer detection has long been a pressing interest for the oncology community due to the poor prognosis and survival associated with advancing cancer stage1. Identifying individuals at the subclinical or asymptomatic stage provides the opportunity for early intervention that has shown to improve survival2. In a general population with relatively low prevalence of cancer, a clinically useful screening test should be broadly accessible as well as specific and sensitive. A screening test should be specific to minimize overdiagnosis-related psychological and financial burden and risks associated with unnecessary follow-up treatments and sensitive to prevent missed or interval cases. For these reasons, to date, only a handful of age-based single-cancer screening modalities such as colonoscopy for colorectal cancer3, mammogram for breast cancer4, low-dose computerized tomography (LDCT) scan for lung cancer5, and pap test for cervical cancer6 are supported by United States Preventative Taskforce. The current single-cancer screening tools, albeit effective, are not without limitations. Issues including lack of adherence to screening recommendations7–9, low positive predictive value (PPV) or high false-positives10, and missed or interval cancer cases11 are of concern. Additionally, for many of the cancers with poor prognosis or high late-stage diagnosis rate, there are no presently accepted screening tools used for cancer detection in asymptomatic individuals.
In this context, blood-based multicancer liquid biopsy tests using analytes such as cell-free DNA (cfDNA) for population-based early cancer screening is gaining traction12–21. Liquid biopsy tests are considered minimally invasive and easily accessible, with perceivably better patient compliance12,13. Several recent studies have started to explore the feasibility of such approaches for early multicancer detection in a limited clinical setting14–21. Of note is a recent 12-month prospective interventional cohort study involving 10,006 females aged 65-75 years old without a personal history of cancer20. DETECT-A (Detecting cancers Earlier Through Elective Mutation-based blood Collection and Testing), their multicancer liquid biopsy test, had an overall sensitivity of 27.1% at 98.9% specificity with a corresponding PPV of 19.4% and negative predictive value (NPV) of 99.3%20. Although the future of the multicancer liquid biopsy tests is promising, such approaches are unlikely to be implemented for the general population with fairly low risk of cancer due to costs and limited performance of current methods. To improve the risk-benefit balance, one can employ a risk-modeling approach and target individuals for screening who exceed certain risk thresholds22. In the multicancer scenario, however, population risk stratification becomes more challenging as one needs to consider risk factors across a number of common cancers.
In this article, we use data from the prospective UK Biobank (UKBB) cohort study and the US population cancer incidence rate to develop a multicancer model that allows estimation of future risk for developing at least one of the eight most common cancers in females. First, from the from the 2013-2017 United States Cancer Statistics (USCS) database, we identified the top ten incident cancer types for females23. We retained eight (breast, colorectum, endometrium, kidney, lung, melanoma, non-Hodgkin's lymphoma, ovary) cancer types with sufficiently large and publicly available genome-wide association studies (GWAS) for developing polygenic risk scores (PRSs). Our analysis involved 160,586 UKBB unrelated female participants of White British ancestry aged 40-71 with 6,886 incident cancer cases over the course of follow-up. Data were split into 2/3 training and 1/3 test set. We used the training sample to fit a Cox proportional hazard model with the outcome as the first incidence of any cancer.
The model specified a baseline hazard as a function of age and assumed multiplicative effects of the risk factors24. The set of risk factors include two major lifestyle related exposures, namely smoking (status and pack years of smoking) and body mass index (BMI), that are known to influence risk across multiple cancers; family history of the most common cancers (breast, colorectum, lung, and prostate) in first-degree relatives and the PRSs for each of the individual cancers. We further calibrated the estimate of the age-specific baseline hazard function so that overall incidence rates match those reported for the US population in the NCI-SEER cancer registry25. We then computed pan-cancer risk scores (PCRSs) as the weighted sum of the predictors included in the model. Performance of the PCRSs were evaluated using standardized hazard ratio for instantaneous risk and area under the curve (AUC) using up to 5-years follow-up data.
We assumed that the probability of an individual carrying an asymptomatic but screen detectable cancer is proportional to the risk of incident cancer over a small time interval. We observe that use of a window of 11-month for calculation of underlying absolute risk led to a projected PPV for the DETECT-A test in the age group 65-75 to match with what has been reported for this group from a recent prospective study20. Thus, we calculate 11-month absolute risk of incident cancer across PCRS percentiles and age groups and use them to approximate the underlying probabilities of screen detectable asymptomatic cancers. Then using Bayes theorem, and the reported sensitivity (27.1% overall and 23.5% for breast, colorectal, and lung cancer) and specificity (98.9%) of the DETECT-A test, we compute the expected PPVs and NPVs across the PCRS percentiles and different ages. We also projected PPVs and NPVs for the test for three individual cancer types (postmenopausal breast, colorectum, and lung) based on corresponding cancer-specific risk models.
PCRSs were strongly associated with risk of developing at least one cancer during the follow-up of the UKBB study in females (HR: 1.40 per 1 SD, 95% CI: 1.35 - 1.46) (Supplementary Table S4). AUC associated with the PCRS at 5-years follow-up was 0.61 (Supplementary Table S4). Fig. 1 illustrates the estimated 11-month risk of developing at least one of the eight cancers for females by age and PCRS strata. Overall, we observe a strong degree of stratification by the combined effects of age and PCRS. The range of risk between a 50-year-old person who is at the lowest centile of PCRS and an 80-year-old who is at the highest centile of PCRS was 0.14%-3.25% for females. The level of stratification of absolute risk due to PCRS was stronger with increasing age and vice versa due to multiplicative nature of their joint effects26. The projected PPVs for the liquid biopsy test also varied substantially according to the level of underlying risk of the population strata. For example, 75-year-old females in the 90th-95th PCRS percentile (AR: 1.91%) will have a 2.7-fold risk compared to the same-aged female in the 5th-10th PCRS percentile (AR: 0.72%) (Fig. 1a). This corresponds to a PPV value of 32.4% and 15.2%, respectively, translating to a 17.2% (95% CI: 16.7%-17.5%) PPV difference (Fig. 1b). NPV across all risk percentiles was reasonably high (min NPV: 97.6%) across all strata (Fig. 1c).
If a fixed threshold for PPV is used to recommend multicancer screening, then the eligibility will strongly vary by both age and PCRS. For example, at a threshold for 25% PPV, females could be eligible for the test as early as age 58 if they were at the highest centile of PCRS. On the other hand, women who were at or below the 40th and 20th percentiles of respective PCRS distributions, will not achieve the required PPV even at the oldest age groups. If a higher PPV threshold is used, such as 40%, we observe that only a small number of risk groups will be eligible for screening. No women who are younger than 71 year or have PCRS value lower than 99th percentile of PCRS will meet the 40% PPV threshold.
To further elucidate how multicancer liquid biopsy tests could be useful for persons with a high risk of a specific type of cancer, we built cancer-specific prediction models for breast, colorectal, and lung cancers including additional cancer specific risk factors (see Supplementary Figure S3-S5). As expected, individual cancer models have higher discriminatory ability compared to the multicancer model (Supplementary Table S4). However, as the absolute risk of any individual cancer is lower compare to that for any cancer, the level of the PPV values for individual cancers are correspondingly lower (Fig. 2). Nevertheless, we do observe strong stratification of absolute risk and PPV by age and underlying risk score for each individual cancer. The range of PPV between the lowest and highest risk groups were 0.9%-27.6%, 0.3-11.9% and 0.2%-52.3% for cancers of breast, colon, and lung in females. Very strong stratification for PPV is seen for lung cancer due to the underlying stratification of lung cancer by smoking.
In summary, we conducted a first of a kind study to highlight the potential for a risk-based approach in early multicancer screening. Specifically, we demonstrated that for females, the combination of age and PCRS can provide substantial stratification of absolute risk of developing at least one of several common cancers within short time intervals. As a result, one would expect the PPV for multicancer screening tests, such as that of DETECT-A, to vary substantially across different risk categories. In particular, we observe that while age is an important determinant of PPV due to higher incidence of cancer cases with increasing age, the other risk factors are also expected to play an important synergistic role. For example, we estimated that the average PPV for all cancers for a test like DETECT-A to be about 13.2% and 19.7% for a 60- and 70-year-old female, respectively. The range of PPV between the 1st-99th percentile of PCRS for these two age groups were 5.7%-28.2% and 10.7%-44.1%, indicating that the PPV yield of the test can be lower (or higher) for the older (or younger) age group for a substantial fraction of the population due to risk associated with PRSs, BMI, smoking, and family history of cancer. As the risk-benefit balance for population screening depends heavily on underlying PPV (and NPV)27,28, our analysis indicates that multicancer risk models are likely to play an important role in the implementation of population multicancer liquid biopsy tests.
Our analysis has several limitations. We assumed that the reported sensitivity and specificity of a test like DETECT-A would be applicable across all age, sex and risk groups. Given the increase in observed sequence alterations in cfDNA resulting from clonal hematopoiesis in older individuals29–32, improvements in cfDNA analyses will be needed to overcome these challenges, potentially through use of mutation- agnostic methods15,16,32. Additional empirical data are needed to explore potential heterogeneity of the diagnostic accuracy of liquid biopsy tests by age and other risk factors. Second, in our multicancer risk prediction models, we included a limited set of risk-factors, namely two major life-style related factors that influence the risk of multiple cancers, family history of the most common cancers, and PRSs for each individual cancer types. We also assumed a proportional hazard model for all cancer risk, assuming multiplicative effects of age and all the other risk factors. Additional efforts are needed to build and validate more refined multicancer risk models in prospective cohort studies by including additional risk factors and interaction effects. Further, models33,34 that incorporate extensive family history information and carrier status for rare high-penetrant mutations would be important to consider for individuals in high-risk families that show strong clustering of related cancers. Third, our model building effort was restricted to women of White British ancestry in the UK Biobank study, due to not only limited sample size of other groups in the UK Biobank itself, but also lack of large-scale GWAS for other groups required to build PRS that are as predictive as those for European origin populations. Large studies of diverse populations are urgently needed, both to study the accuracy of multicancer liquid biopsy tests and to build and validate robust multicancer risk prediction models across different racial and ethnic groups. Finally, we have focused on exploring the potential utility of a multicancer risk model to determine initial screening eligibility for liquid biopsy test. A secondary application of the individual cancer specific risk models could be to improve identification of target sites to select follow-up procedures among individuals who test positive in the initial multicancer screening. Additional studies are merited to explore these opportunities.
In conclusion, our study shows the promise for the potential of risk-based approach to increase the risk-benefit balance of multicancer screening using liquid biopsy tests. In the future, well powered empirical studies are needed in diverse populations to prospectively evaluate the PPV and NPV of these tests, overall and stratified by risk groups.