Thymoma, a rare cancer that affects the thymus

Definition of a thymoma

What is the thymus?

To understand thymoma, you must already have in mind what the thymus is: it is an intrathoracic organ located in front and above the heart, in the upper part of the thorax, just behind the sternum . It is part of the immune system and reaches full maturity during puberty, before atrophying in adulthood and being replaced by fatty tissue. The thymus serves to filter white blood cells (or lymphocytes), eliminating those which are not sufficiently reactive towards the non-self and those which, on the contrary, are too reactive towards the self, explains Professor Benjamin Besse. The formation of a thymoma results from the transformation into cancer cells of the epithelial cells that make up the thymus. “Thymoma is therefore a cancer of the thymus, more precisely a carcinoma derived from the epithelium of the thymic gland; if the cancerization process affects the lymphocytes which also make up the thymus, we speak of lymphoma. In this case, it is a cancer of a completely different nature.”specifies the specialist.

Is thymoma a cancer? Can we speak of a benign thymoma or is it necessarily malignant?

Despite the origin of its name, which comes from the association of the root thym- for thymus and the suffix -ome usually attributed to benign tumors, “a thymoma is necessarily a malignant tumor, in other words a cancer”indicates Professor Besse.

Since 2004, thymomas have been classified into 6 main categories: A, AB, B1, B2, B3, and thymic carcinomas, depending on the nature and shape of the cancer cells, and the infiltration of lymphocytes. “Type A thymomas, composed of spindle and oval cells, have a very good prognosis; conversely, type C thymomas, synonymous with thymic carcinomas, are much more aggressive and have a prognosis more comparable to that of a lung cancer“explains Professor Besse.

Is thymoma a common cancer?

Thymoma is one of the rare diseases. In Europe, between 350 and 400 new cases are diagnosed each year. A low incidence but nevertheless increasing since 2020, in parallel with the increase in the number of chest imaging examinations. “Since Covid, we have achieved a lot more chest scanswhich led us to discover twice as many thymomas”underlines the oncologist.

What are the causes of thymoma?

We do not know the causes of a thymoma. “There is no genetic predisposition and no risk factors have been identified.”confirms Professor Besse.

This thymus tumor can develop in both women and men. Extremely rare in children, it appears more in adults a little before the age of 50.

What are the symptoms of thymoma?

A minimally symptomatic thymus cancer

Thymoma is one of those cancers discovered most of the time incidentally, following an imaging test carried out for another reason. It causes few symptoms when localized; its progression is slow and generally localized; It is often only when it affects neighboring organs, particularly the pleura, that this thymus cancer can cause shortness of breath and chest pain.

What are the links between thymoma and myasthenia gravis?

Thymoma nevertheless has the particularity of being often associated with an autoimmune disease (around 20% of cases), and it is myasthenia gravis in 2/3 of cases. Myasthenia gravis is a chronic autoimmune neuromuscular disease that manifests in varying degrees as excessive weakness and fatigability of skeletal striated muscles, those that are activated voluntarily. “While there is undeniably a correlation between thymoma and myasthenia gravis, the cause and effect mechanism remains enigmatic. But the fact remains that when faced with a patient suffering from myasthenia gravis, we must systematically look for a thymoma., explains the oncologist. Other patients may have another autoimmune syndrome such as systemic lupus erythematosus, rheumatoid arthritis, or hematologic syndromes such as red blood cell aplasia.

How to diagnose a thymoma?

CT scan, reference examination for the diagnosis of a thymoma

Doctors will rely on imaging exams to diagnose a thymoma, primarily the chest CT scan which is the gold standard exam. More precise than chest X-ray, the CT scan is generally coupled with the injection of a contrast product in order to determine the stage of the tumor as well as the therapeutic strategy. A PET scan then makes it possible to check the presence or absence of metastases, and thus define the degree of aggressiveness of the thymoma. “This examination also makes it possible to guide towards a differential diagnosis and to rule out other pathologies such as lymphoma or germ cell tumors of the mediastinum in men”. A CT or surgical biopsy must be performed to confirm or refute the diagnosis of thymoma most of the time.

A biological test to look for an autoimmune disease

Although there is no biological marker for this cancer, a blood test is generally prescribed to look for myasthenia gravis. “We systematically look for anti-RACH or anti-MUSK antibodies and we order a blood test to check for the possible existence of other autoimmune diseases., specifies Professor Besse. Likewise, in cases of myasthenia gravis, a chest CT scan should be performed to look for a thymoma.

How to treat a thymoma?

As with any rare tumor, surgery for a thymoma should preferably be carried out in an expert center, warns the oncologist. “The slightest therapeutic error risks causing the dissemination of cancer cells and invasion of the pleura, which cannot be corrected afterwards. The stakes are high.”. Regional or inter-regional expert centers are listed on the Rythmic.org website. Coordinated by Professor Besse and run with Professor Girard, this site was set up in 2010 and then certified by the National Institute for the Fight against Cancer (INCa), which finances it; it allowed the definition of a common reference framework which is updated every two years, and constitutes, thanks to all the cases registered in the database (>4000), an invaluable resource for advancing research on this cancer, underlines the oncologist.

How to treat a thymoma?

Thymoma surgery

Thoracic surgery, with resection of the tumor, constitutes the main treatment for thymoma. “Schematically, for a tumor larger than 5 cm, we favor sternotomy, an approach whose operative consequences are quite simple since we do not touch the ribs; for tumors less than 5 cm, we can opt for a minimally invasive approach, for example bilateral thoracoscopy. The latter can also be practiced with the help of a robot, a trend that is growing more and more. For tall tumors, which are much rarer, we perform a cervicotomy, in other words an incision in the neck.”

Complementary treatments

Surgery can be combined with other treatments, such as pre-operative chemotherapy (we speak of neoadjuvant chemotherapy), and post-operative radiotherapy (adjuvant radiotherapy), but the therapeutic strategy must always be discussed in a consultation meeting. multidisciplinary (RCP) from the RYTHMIC network according to the size of the tumor, its histological subtype, its grade but also the overall state of health of the patient. Immunotherapy, on the other hand, is contraindicated for treating a thymoma because this approach risks flaring up a potential latent autoimmune disease.

To avoid the risk of decompensation of their myasthenia gravis, the patients concerned receive immunoglobulins for prevention, indicates the expert. “But treating thymoma will not cure their autoimmune disease.”.

How are patients monitored?

Patient monitoring is relatively simple but must be done over time to prevent the risk of recurrence. “Recurrences are relatively infrequent (they depend on the histological type and stage), and in 20% of cases, they occur more than ten years after treatment. Fortunately, patients can be operated on several times.”, explains Professor Besse. For 15 years, they will have to pass a control scan every year.

Other good news, “lThymoma is a cancer that we know how to treat, we cure many patients.”.