Thymoma
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Direct answer
A young adult with ptosis, diplopia and fatigability who has an anterior mediastinal mass on CT has a thymoma until excision proves otherwise — thymoma is the commonest thymic epithelial tumour, and myasthenia gravis accompanies it in roughly a third to half of cases. Its behaviour is graded by the Masaoka(-Koga) staging system — stage I macroscopically encapsulated, II microscopic capsular (IIa) or macroscopic trans-capsular (IIb) invasion, III invasion of adjacent organs, IVa pleural or pericardial deposits, IVb lymphatic or haematogenous metastases — and by WHO histology from indolent type A and AB through B1, B2, B3 to thymic carcinoma (type C). Complete (R0) resection, classically via median sternotomy removing the whole thymus with its perithymic fat, is the cornerstone of cure and yields about 95 per cent 10-year survival for stage I, falling steeply for stage III–IV disease, where induction chemotherapy, radiotherapy and debulking combine.
What you must remember
- Masaoka staging (the surgicopathological system examiners want): I — encapsulated, no capsular invasion; IIa — microscopic capsular invasion; IIb — macroscopic invasion into surrounding mediastinal pleura or pericardium; III — invasion of adjacent organs (lung, great vessels, phrenic nerve); IVa — pleural or pericardial dissemination; IVb — distant metastases.
- WHO histological types: A (spindle, medullary), AB (mixed), B1 (organoid, cortical), B2, B3 (epithelial-rich, atypical), C (thymic carcinoma); A and AB behave indolently with near-100 per cent long survival, B3 worse, type C clearly malignant (5-year survival in the region of 50–60 per cent).
- Myasthenia gravis: present in a substantial share of thymoma patients (and about 10–15 per cent of myasthenia patients harbour a thymoma); the tumour epithelium drives autoimmunity; anti-acetylcholine receptor antibodies, and anti-striated muscle antibodies mark thymoma-associated disease — stabilise myasthenia medically before surgery (pyridostigmine, sometimes plasmapheresis or IVIg for severe bulbar symptoms).
- Other paraneoplastic associations: pure red cell aplasia and hypogammaglobulinaemia (Good's syndrome, with recurrent infections) — both may remit after thymectomy.
- Surgery: median sternotomy with total thymectomy including all perithymic fat (ectopic thymic rests reach the lower neck and aortopulmonary window); VATS or robot-assisted thymectomy is accepted in experienced hands for early-stage disease; phrenic and recurrent laryngeal nerves define the lateral limits of dissection.
- Stage-adapted therapy: stage I — surgery alone; stage II — surgery with consideration of adjuvant radiotherapy for IIb or WHO B types; stage III — induction chemotherapy (platinum-based) or surgery with en bloc resection of invaded lung, pericardium or vessel followed by radiotherapy; stage IV — chemotherapy, selected debulking, and radiotherapy; thymoma is one of the few solid tumours where debulking is credited with benefit in stage IVa pleural disease.
A typical case walked through
A 42-year-old woman develops fluctuating ptosis and diplopia worse towards evening; neostigmine test and acetylcholine receptor antibodies confirm myasthenia gravis, and contrast CT of the chest shows a 5 cm lobulated anterior mediastinal mass with a hint of contact on the pericardium. The reasoning: any myasthenia patient needs chest imaging, and any thymic mass in a myasthenic is a thymectomy indication — even independent of neoplastic risk. She is optimised medically (pyridostigmine dosing, anaesthesia planned to avoid depolarising relaxants). At median sternotomy the tumour excises with the whole gland and perithymic fat; the capsule is intact on the operative field but histology reports microscopic transcapsular extension into mediastinal fat — Masaoka IIb, WHO type B2. She receives adjuvant radiotherapy (typical for IIb with B-type histology) and her myasthenia improves, though anticholinesterases are tapered slowly, not stopped abruptly. Now shift the same patient to CT showing encasement of the ascending aorta and vena cava with a pleural nodule — stage III/IVa: the answer becomes induction chemotherapy (cisplatin-based), restaging, then resection of residual tumour en bloc with involved pericardium and lung, plus radiotherapy — a multidisciplinary sequence, not a solo sternotomy.
Where students slip
Two errors recur. First, staging confusion: Masaoka is a surgicopathological system — you cannot assign it from CT alone, and stage II hinges on microscopic capsular invasion the surgeon and pathologist must seek. Second, students resect "the tumour" instead of the thymus: leaving perithymic fat with ectopic thymic rests causes myasthenic persistence and mediastinal recurrence, which is why total thymectomy is the operation. The viva trap: the anaesthetic line — suxamethonium causes prolonged paralysis and worsened weakness in myasthenia, so non-depolarising agents (reduced dose) are used.
Frequently asked questions
What is Masaoka stage II thymoma?
Stage IIa is microscopic capsular invasion; stage IIb is macroscopic trans-capsular extension into mediastinal pleura or pericardium — an R0 resection remains curative, with adjuvant radiotherapy considered for IIb.
Which WHO type denotes thymic carcinoma?
Type C — histologically overt carcinoma with markedly worse prognosis than A, AB or B types; it is staged and treated as an invasive malignancy from the outset.
Why is total thymectomy rather than tumourectomy performed?
Thymoma arises within the thymus, and ectopic thymic tissue scattered in perithymic fat causes recurrence and persistent myasthenia — the entire gland plus fat is removed.
How does thymoma spread?
By direct local invasion and pleural or pericardial "drop" metastases (stage IVa); distant haematogenous spread is late and unusual — hence prolonged CT surveillance for pleural recurrence.
Which haematological paraneoplastic syndromes accompany thymoma?
Pure red cell aplasia and hypogammaglobulinaemia (Good's syndrome with recurrent sinopulmonary infection); both may improve after thymectomy.