Tumor Genetics Blog

By: Eetu Mäkinen

Suomennos: Komorbiditeetti ja syöpä

Comorbidities, i.e. co-existing health conditions, affect both a person’s cancer risk and, with some diseases, cancer treatment and thus cancer survival. Since most chronic health conditions become more common with age, the role of comorbidities will become even more prominent in relation to cancer as the population ages.

One significant research theme for the Finnish Cancer Registry in the Finnish Center of Excellence in Tumor Genetics has been the assessment of the associations between comorbidities and the risks of various cancers from a large population sample.

Statistical significance does not always indicate causality

In retrospective cohort studies, we are not necessarily able to draw direct conclusions about the independent causal effects of comorbidities on cancer risk but can only detect statistically significant associations. Therefore, caution is required when interpreting the observed results. The interpretation may further be complicated by risk factors common to the cancer and comorbidity under investigation, especially if the confounders cannot be adjusted for in the model.

In statistical analyses there is always a risk of bias. Depending on the comorbidity, patients often have a higher diagnostic intensity compared to the healthy population, especially in the early stages of a newly diagnosed illness, when, for instance, a suitable medication balance is being sought for the patient. In this case, it is possible that an already existing asymptomatic cancer is detected earlier than otherwise would have been. A significantly increased risk of cancer soon after a comorbidity diagnosis has been observed in many studies, but it is more likely to be due to detection bias. For this reason, a short period after the diagnosis of comorbidity is often excluded when estimating relative risks. On the other hand, adding an exclusion period may add survival bias to the estimates, as only those in the exposed cohort who are still alive and who were not diagnosed with cancer during the excluded period are still included in the follow-up afterwards. In other words, the follow-up population has passed through a filter.

Individual risks are highlighted at population level

As is the case with other risk factors, higher or in some cases lower cancer risks associated with comorbidities are more relevant at the population level than at the individual level. For instance, take the study published in early autumn 2025, in which we found that people with type 2 diabetes have an average 26 percent higher risk of developing colorectal cancer compared to other population. If we assume that the risk of colorectal cancer in a healthy person over a fixed period of time was 1 percent, the corresponding risk in a person with type 2 diabetes of the same age and sex would then be approximately 1.26 percent. From an individual perspective, the difference is impossible to detect. But if we consider the situation at the population level, on average, we observe 260 colorectal cancers more for every 100,000 people with diabetes than in the rest of the population (in a situation where all-cause mortality would be the same as in general population). This has an impact on the burden on healthcare.

A part of interpreting the risk ratios is reporting them. The purpose of studying cancer risks is to increase awareness among both the general population and healthcare professionals. Simultaneously, we must not guilt the individuals possessing the comorbidity or lifestyle linked to a higher cancer risk.

Finland has one of the oldest populations in Europe. 11 percent of Finns were over 75 years old in 2023, and nearly 40 percent of those diagnosed with cancer were over 75 years old. Better understanding of the links between comorbidities and cancer will better enable action to be taken to curb the growing burden on healthcare through cancer prevention and early detection.

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