# Thymic Tumours Pathology > Thymic tumours for MBBS Pathology: thymoma WHO types, anterior mediastinal mass workup, myasthenia gravis, red cell aplasia and Good syndrome. - Canonical URL: https://prepelephant.com/topics/mbbs/pathology/thymic-tumours-pathology - Exam / course: MBBS · Subject: Pathology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Thymic Tumours Pathology", PrepElephant, https://prepelephant.com/topics/mbbs/pathology/thymic-tumours-pathology ## 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.