Why doctors have changed their minds about the thymus, the organ considered useless after puberty: link to longevity and cancer risk

For a long time, the thymus was treated as an organ that loses its role after adolescence – a gland that atrophies with age and that, in many surgeries, was removed without major consequences. New data, however, radically changes this perspective.

In adults with a healthy thymus, the risk of premature death from any cause is halved, and lung cancer is less common. Two studies published in 2026 in the journal Nature, plus an earlier review in the New England Journal of Medicine, challenge the idea that the thymus, the gland hidden behind the sternum, becomes useless after puberty.

Risk of death cut in half

Adults with a healthy thymus after age 50 have about a 50% lower risk of all-cause mortality, a 63% lower risk of cardiovascular death, and a 36% lower chance of developing lung cancer, compared to those with severe thymus decline. The discovery was made in two studies published in 2026 in the journal Nature, coordinated by Hugo Aerts, professor at Harvard Medical School and director of the program of artificial intelligence in medicine at Mass General Brigham. The researchers used a deep learning model applied to routine CT scans to measure the condition of the thymus in more than 27,000 adults followed for more than 10 years.

The first of the two studies revealed that in the cohorts of the National Lung Screening Trial (25,031 participants) and the Framingham Heart Study (2,581 adults), the state of the thymus varied greatly from person to person, and the differences accurately predicted who would get more seriously ill and who would have a shorter life expectancy. The second study followed 3,476 cancer patients undergoing immunotherapy treatment with checkpoint inhibitors (drugs used in cancer immunotherapy that allow the immune system to act effectively against tumor cells). For those with non-small cell lung cancer, a healthier thymus was associated with a higher survival rate and a reduced risk of disease progression.

A gland absolutely necessary for health

The thymus is shaped like two united lobes and is located behind the sternum, above the heart. The ancient Greeks believed it to be the seat of the soul. In 1960, a Nobel laureate, Sir Peter Medawar, called it “a graveyard of cells, an evolutionary accident of no great significance”.

For a long time, medical textbooks described it as a gland active in childhood that atrophies at puberty, gradually turning into fatty tissue. Precisely because it didn't seem to do much in adulthood, cardiothoracic surgeons often remove it when operating on the heart or mediastinum.

A study published in 2023 in the New England Journal of Medicine changed the way doctors look at the thymus in adults. A team of researchers from Massachusetts General Hospital compared two groups of patients undergoing cardiothoracic surgery. 1,420 of them no longer had the thymus, while 6,021 had it. Five years after the intervention, the risk of dying from any cause was more than twice as high in the group without a thymus, and the incidence of cancer doubled. After excluding patients who already had immune diseases, infections or cancer at the time of surgery, the researchers also observed a significant increase in autoimmune diseases in patients without a thymus.

“By studying people who had their thymus removed, we found that this gland is absolutely necessary for health. If it's missing, the risk of death and the risk of cancer doubles,” said David Scadden, lead author of the paper and director of the Center for Regenerative Medicine at Massachusetts General Hospital, in an interview with the Journal of the American Cancer Society.

Thymus status, a possible explanation for anticancer treatment failure

Hugo Aerts' team built a deep learning model that analyzes routine chest CT scans and calculates a thymus health score based on the size, shape and proportion of fat tissue that has replaced active tissue. The algorithm was trained on images from 5,674 people and later validated on more than 27,000 participants from the National Lung Screening Trial and the Framingham Heart Study. The score divides the population into three groups: 25% with thymus in good shape, 50% with thymus in average shape, 25% with severely degraded organ.

In the first group, mortality 12 years after the start of the study was 50% lower, cardiovascular deaths were reduced by 63%, and lung cancer occurred 36% less often. The team compared the radiological score with blood markers of chronic inflammation and found that a degraded thymus was associated with higher levels of systemic inflammation. Chronic inflammation, smoking and extra pounds have been linked to worse thymus health, though the researchers caution that they haven't tested whether lifestyle changes can reverse the process.

“The thymus has been overlooked for decades and may be one of the explanations for why people age differently and why cancer treatments fail in some patients,” Aerts said in a Mass General Brigham statement. The researcher points out that the results need to be confirmed by other teams and that the imaging method is not yet ready for routine clinical use.

How immunotherapy works in cancer and how it relates to the thymus

Modern oncology immunotherapy, especially immune checkpoint inhibitors such as pembrolizumab or nivolumab, work by reactivating T lymphocytes, cells of the immune system trained by the thymus to recognize threats. If the body has a smaller supply of new T lymphocytes, the treatment has fewer cells to mobilize against the tumor. The response may thus be weaker.

The model developed by Aerts' team, applied to 3,476 cancer patients, supported this hypothesis. In non-small cell lung cancer patients, a better thymus score was linked to a lower risk of disease progression and death, regardless of tumor type.

The thymus, often overlooked in imaging reports, could become a useful indicator before immunotherapy. Its appearance could help doctors estimate the chances of response to treatment, regardless of the type of tumor. If the thymus reserve is very low, the oncologist might consider a combination regimen or another strategy. For now, these remain research hypotheses, not clinical recommendations.

The thymus, the gland that ages early but could be regenerated

“T lymphocytes leave the bone marrow and reach the thymus where they multiply, expand. Then those that could attack the own tissues are eliminated,” explained Andri Lemarquis, a doctor and researcher at the City of Hope oncology center in California, to The Washington Post. This filter is the basis of immune tolerance, the mechanism by which the body learns not to attack its own tissues.

Paola Bonfanti, a cell biologist at the Francis Crick Institute in London and a professor at University College London, has been studying the thymus for more than ten years and has noticed that the organ degrades early but contains stem cells similar to those in the skin, a tissue that constantly renews itself. “It contains stem cells identical to those in the skin, and the skin regenerates itself every three weeks,” Bonfanti noted.

In 2023, his team first identified these stem cells in human thymic tissue. The next step would be to find out if they can be activated so that the adult thymus, gradually replaced by fat tissue, regains at least some of its function.

Paola Bonfanti also coordinated a study published in 2020 in Nature Communications, in which she succeeded in reconstructing a functional human thymus starting from stromal cells and a natural organ structure. In the long term, researchers are exploring the idea of ​​a lab-grown thymus from a donor's cells, which could help transplant patients better tolerate the recipient's organ. Classical immunosuppressants protect the graft but have severe adverse effects, from opportunistic infections to secondary tumors.

Why surgeons sometimes remove the thymus

Thymectomy is practiced in several situations: to access the heart in cardiothoracic surgery, to resect a thymoma (primary thymus tumor), but also as a therapeutic intervention in myasthenia gravis, a neuromuscular autoimmune disease. In approximately 80% of patients indicated for thymectomy for myasthenia gravis, disease remission is achieved, which is why the intervention remains a valid therapeutic option.

The results of the new research do not cast doubt on thymectomy itself. When the organ must be removed because of a thymoma, myasthenia gravis or for surgical access to the heart area, the decision remains a medical one. What is changing is the focus on the post-op period. If the thymus continues to participate in immunity in adulthood, patients without this organ may need closer long-term monitoring.

Researchers are now trying to understand why the thymus degrades more quickly in some people and more slowly in others. Among the questions being pursued are the factors that accelerate its aging, the possibility of stimulating gland regeneration through treatments, and the use of the radiological score in immunotherapy trials. Paola Bonfanti is also studying the connection of the thymus with autoimmune diseases and the elderly's response to vaccines.