Age-specific relative and absolute burden of subsequent primary non-gynecologic cancers after endometrial cancer: a population-based SEER study

When a woman survives cancer, the journey does not end with the treatment. As medical care improves and more women live longer after an endometrial cancer diagnosis, a new question arises: are they at higher risk for developing a different, unrelated cancer later in life? This is the study of subsequent primary cancers. To understand this risk, scientists often look at two different ways of measuring danger. One way asks how much more likely a specific group is to get cancer compared to the general population, a measure of relative risk. The other asks how many extra cases actually occur in a specific group over time, a measure of absolute burden. While the first method highlights how unusual a risk is for a small group, the second reveals where the most actual cases are happening in the real world. Distinguishing between these two perspectives is crucial for public health, because a group might show a very high relative risk simply because they are young and cancer is rare for them, even if the total number of extra cases remains small.

Researchers set out to explore this exact distinction using a massive database of cancer records from across the United States. They focused on women who had been diagnosed with endometrial cancer between the years 2000 and 2022. To ensure they were looking at new cancers rather than a return of the original disease, the team began tracking these women one year after their initial diagnosis. They followed over 192,000 women, watching for the development of any new malignant tumors outside the female reproductive system, such as cancers of the breast, colon, or lung. The study covered a substantial amount of observation time, allowing the team to compare the number of cancers that actually appeared against the number that would be expected to occur in a similar group of women who had never had endometrial cancer.

The analysis revealed a complex picture that changes depending on how you look at the data. When the researchers compared the observed cancers to the expected number, they found a slight overall increase in risk. For every 10,000 women followed for a year, there were about 4 extra cases of a new cancer compared to what the general population would experience. However, this average number hid significant differences based on age. When the team looked at the relative risk—the proportional increase—they found it was highest among the youngest women. Those diagnosed with endometrial cancer between the ages of 20 and 39 were significantly more likely to develop a second cancer than their peers in the general population. In contrast, for women diagnosed between the ages of 40 and 49, the relative risk was lower, but the actual number of extra cases was much higher.

This is where the two ways of measuring risk diverge. The group of women aged 40 to 49 contributed nearly half of all the extra cancer cases found in the entire study, even though their relative risk was not the highest. Because there are simply more women in this age group and cancer is more common in the general population as people get older, a moderate increase in risk translates into a large number of actual cases. The youngest group, while showing the highest proportional jump in risk, contributed a much smaller share of the total extra cases because the baseline number of cancers in young women is naturally low. The researchers also looked at what happened more than ten years after the initial diagnosis. They found that for the entire group of women, the risk of developing a new cancer eventually dropped slightly below the level expected for the general population. While women diagnosed before age 50 still showed a small, persistent increase in risk even after ten years, the data did not support a strong, lasting difference in risk patterns between younger and older survivors over the long term.

The study concludes that while there is a measurable increase in the risk of developing a new cancer after endometrial cancer, the overall burden on the population remains modest. The findings highlight that the group with the highest relative risk is not necessarily the group that carries the heaviest absolute burden of disease. This distinction matters for how we understand population health, but the researchers caution that these broad statistics cannot be used to predict risk for an individual woman or to set specific rules for medical surveillance. The data does not prove that the treatment for the first cancer causes the second, nor does it identify a specific biological mechanism. Instead, it serves as a clear map of where the extra cases are occurring, showing that the largest number of additional cancers appears in women diagnosed in their forties, while the highest proportional risk appears in the youngest survivors.