Thymic tumour

The aetiology of thymic tumours is unknown. Certain paraneoplastic syndromes such as myasthenia gravis are strongly associated with thymoma, however, there is no evidence to date indicating a causal relationship.

Large or invasive thymomas may compress or invade mediastinal or chest wall structures, causing chest pain or cough. Invasive thymomas can involve the phrenic nerve, causing marked dyspnoea, particularly when supine. Rarely, superior vena cava (SVC) syndrome occurs due to compression or invasion of the SVC by the tumour or as a result of a tumour thrombus.

Many immune-related disorders have been associated with thymomas: most commonly, myasthenia gravis.[10]Suadjian J, Enriquez P, Silverstein MN, et al. The spectrum of diseases associated with thymoma: coincidence or syndrome? Arch Intern Med. 1974;154:374-379. http://www.ncbi.nlm.nih.gov/pubmed/4602050?tool=bestpractice.com Patients with myasthenia gravis have antibodies directed against the acetylcholine receptor of the neuromuscular junction, resulting in muscle weakness. Other rare associations include red-cell aplasia, pemphigus, peripheral nerve hyperexcitability syndromes, Good syndrome, lichen planus, scleroderma, autoimmune haemolytic anaemia, hypogammaglobulinaemia, polymyositis, systemic lupus erythematosus, rheumatoid arthritis, thyroiditis, and Sjögren syndrome.[15]Wightman SC, Shrager JB. Non-myasthenia gravis immune syndromes and the thymus: is there a role for thymectomy? Thorac Surg Clin. 2019 May;29(2):215-25. http://www.ncbi.nlm.nih.gov/pubmed/30928003?tool=bestpractice.com These syndromes variably improve following resection of an associated thymoma/thymus gland.[16]Zhao J, Bhatnagar V, Ding L, et al. A systematic review of paraneoplastic syndromes associated with thymoma: treatment modalities, recurrence, and outcomes in resected cases. J Thorac Cardiovasc Surg. 2020 Jul;160(1):306-314.e14. https://www.doi.org/10.1016/j.jtcvs.2019.11.052 http://www.ncbi.nlm.nih.gov/pubmed/31982129?tool=bestpractice.com

WHO histological classification[3]Detterbeck FC. Clinical value of the WHO classification system of thymoma. Ann Thorac Surg. 2006;81:2328-34. https://www.annalsthoracicsurgery.org/article/S0003-4975(05)02367-2/fulltext http://www.ncbi.nlm.nih.gov/pubmed/16731193?tool=bestpractice.com [4]Shimosato Y, Mukai K. Tumors of the mediastinum. In: Rosai J, Sobin LH, eds. Atlas of tumor pathology. Washington, DC: Armed Forces Institute of Pathology; 1997:93-115.[5]Travis W, Brambilla E, Burke A, et al. Tumours of the thymus. In: WHO classification of tumours of the lung, pleura, thymus and heart. 4th ed. Lyon, France: IARC Press; 2015:184-298.[6]Marx A, Chan JKC, Chalabreysse L, et al. The 2021 WHO classification of tumors of the thymus and mediastinum: what Is new in thymic epithelial, germ cell, and mesenchymal tumors? J Thorac Oncol. 2022 Feb;17(2):200-13. https://www.jto.org/article/S1556-0864(21)03258-5/fulltext http://www.ncbi.nlm.nih.gov/pubmed/34695605?tool=bestpractice.com

Thymoma (majority of thymic tumours)

• Type A: medullary, spindle cell

• Type AB: mixed

• Type B1: predominantly cortical, organoid

• Type B2: cortical

• Type B3: well-differentiated thymic carcinoma, squamoid.

Type C represents thymic carcinomas and comprises approximately 15% of thymic neoplasms:

• Tumours with low malignant potential include well-differentiated squamous cell carcinoma, basaloid carcinoma, and muco-epidermoid carcinoma.

• Tumours with more aggressive malignant potential include high-grade lympho-epithelial-like carcinoma, undifferentiated/anaplastic carcinoma, sarcomatoid, and clear-cell carcinomas.

Neuro-endocrine tumours of the thymus (NETT) are extremely rare:

• Small cell carcinoma

• Carcinoid

• Atypical carcinoid.