Thymic Tumours Pathology
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Direct answer
Behind the sternum, in the anterior mediastinal compartment that also hides germ cell tumours and lymphomas, the thymus gives rise to thymoma — an epithelial neoplasm infested to a variable degree with non-neoplastic lymphocytes — and, at the malignant end, thymic carcinoma. Thymoma is the commonest tumour of the anterior mediastinum in middle-aged adults and is famous for its paraneoplastic company: roughly a third to nearly a half of thymoma patients develop myasthenia gravis, while pure red cell aplasia and hypogammaglobulinaemia (Good syndrome) define the rarer associations. The WHO classification from type A through B3 grades epithelial atypia and lymphocyte content, type B2 being the classic myasthenia-associated subtype, and encapsulation versus invasion — the Masaoka staging — drives prognosis more faithfully than the histological grade itself.
What you must remember
- Demographics and address: adults in the fourth to sixth decade, anterior mediastinum; thymoma is the commonest anterior mediastinal tumour of this age group, and the mass that pairs with myasthenia in every examination pairing.
- WHO types in one line each: A — spindle epithelial cells, few lymphocytes, indolent; AB — mixed; B1 — lymphocyte-rich, resembles normal cortex; B2 — mixed, plump epithelial cells, the classic myasthenia association; B3 — epithelial-predominant, atypical, aggressive; type C — thymic carcinoma, overtly malignant cytology.
- Thymic carcinoma markers: CD5 and CD117 (c-KIT) positivity distinguishes thymic carcinoma from lung primaries — a modern immunohistochemistry point worth quoting at postgraduate level.
- The myasthenia numbers, both directions: about 30 to 45 per cent of thymoma patients develop myasthenia gravis, and about 10 to 15 per cent of myasthenia patients harbour a thymoma — thymectomy is part of myasthenia management in selected patients even without tumour.
- The rarer syndromes: pure red cell aplasia (a small but classic minority, anaemia with absent erythroid precursors) and Good syndrome — hypogammaglobulinaemia with recurrent infections, diarrhoea and opportunistic organisms.
- Staging logic: Masaoka stages I (encapsulated) to IV (pleural or distant spread); invasion of the capsule predicts recurrence, so completeness of excision is the surgeon's and the pathologist's shared project.
- Treatment: complete surgical excision is the anchor, with adjuvant radiotherapy for invasive thymoma, chemotherapy for advanced disease, and thymic carcinoma treated on its own more aggressive protocol.
Working up the anterior mediastinal mass
A 47-year-old woman has ptosis and diplopia worsening through the day; chest computed tomography shows a 5-centimetre lobulated anterior mediastinal mass. The "five Ts" organise the compartment: thymoma (the demographic fit here), teratoma and other germ cell tumours (young adults, markers), thyroid goitre with substernal extension (continuity on imaging), terrible lymphoma (including T-cell lymphoblastic lymphoma of adolescents), and a fifth T variously given as thoracic cysts. Step two assigns likelihood: her age, sex and myasthenia make thymoma the working diagnosis. Step three sends serum markers when germ cell disease is plausible (alpha-fetoprotein, beta-human chorionic gonadotropin) and assesses resectability with contrast imaging.
Step four is the operation itself, because in resectable disease the diagnosis and treatment are the same event: thymectomy with the tumour, via sternotomy or thoracoscopy, and the pathologist reports type (B2, say), capsule integrity and Masaoka stage. Step five closes the loops that students forget — the myasthenia may improve after thymectomy over months but not immediately, the haemoglobin and immunoglobulins should be checked before the diagnosis is allowed to rest, and follow-up imaging continues for years because late recurrence defines thymoma's personality.
Where the exam sets its traps
The first trap is calling thymoma benign by default: even encapsulated type A tumours recur, and malignancy in thymoma is judged by invasion (Masaoka), not by cytology alone — hence the phrase "benign thymoma" has been retired in favour of stage-driven language. The second is the direction of the myasthenia statistics: quoting "all myasthenia patients have thymoma" is false — most myasthenia is non-thymomatous with lymphoid follicular hyperplasia — but a tenth to a sixth do, which is precisely why every myasthenia patient gets a chest scan. The third is the anaemia association: a thymoma patient with a normocytic anaemia and reticulocytopenia has pure red cell aplasia until marrow examination shows the absent erythroid series, and the tumour removal can cure it — one of oncology's neatest cause-effect stories.
Frequently asked questions
Which WHO thymoma subtype is classically associated with myasthenia gravis?
Type B2, with its plump epithelial cells among dense immature lymphocytes — though myasthenia occurs across B subtypes.
What percentage of myasthenia gravis patients have a thymoma?
About 10 to 15 per cent, which is why chest imaging is part of the myasthenia workup, while a larger fraction show lymphoid follicular thymic hyperplasia.
What is Good syndrome?
Thymoma with hypogammaglobulinaemia, producing recurrent bacterial infections, diarrhoea and opportunistic infections such as mucocutaneous candidiasis.
How is malignancy determined in thymoma?
By invasion — capsular, mediastinal, pleural or distant — reflected in Masaoka staging, rather than by cytological atypia alone.
Which immunohistochemical markers support thymic carcinoma?
CD5 and CD117 (c-KIT) positivity in the appropriate morphological and radiological setting, distinguishing thymic carcinoma from metastatic lung carcinoma.