Residency · Residency · Cardiothoracic Surgery

Mediastinal Masses and Thymoma

Overview

Mediastinal masses are classified by compartment: anterior (prevascular), middle (visceral), and posterior (paravertebral). The differential diagnosis is guided by compartment, patient age, and clinical presentation. Thymoma is the most common primary anterior mediastinal neoplasm in adults, and surgical resection is the primary treatment for most resectable mediastinal tumors.

Mediastinal Compartments and Differential Diagnosis

Anterior (Prevascular) Compartment

The classic "4 T's" mnemonic guides the differential diagnosis: Thymoma (the most common in adults over 40 years), Teratoma and other germ cell tumors (mature teratoma is benign; seminoma and non-seminomatous GCTs occur in younger males), Terrible lymphoma (both Hodgkin nodular sclerosis type and non-Hodgkin varieties), and Thyroid (retrosternal goiter with substernal extension from the neck).

Middle (Visceral) Compartment

The middle compartment harbors lymphadenopathy (reactive, lymphoma, metastatic, or sarcoidosis), bronchogenic cysts, pericardial cysts, esophageal duplication cysts, and tracheal tumors.

Posterior (Paravertebral) Compartment

Neurogenic tumors are the most common posterior mediastinal masses and include nerve sheath tumors (schwannoma, neurofibroma), sympathetic ganglia tumors (ganglioneuroma, ganglioneuroblastoma, neuroblastoma), and paraganglioma. Other posterior compartment lesions include meningocele, extramedullary hematopoiesis, and thoracic vertebral body lesions.

<image>Cross-sectional diagram of the mediastinal compartments (anterior, middle, posterior) with representative masses listed for each compartment</image>

Workup of Mediastinal Masses

Imaging

CT chest with IV contrast is the first-line modality and characterizes location, size, density, and invasion. Fat and calcification suggest teratoma, a homogeneous soft tissue mass with a lobulated contour suggests thymoma, and a cystic lesion suggests a bronchogenic or pericardial cyst. MRI is superior for posterior mediastinal masses, particularly neurogenic tumors and assessment of spinal canal extension. PET-CT is useful for lymphoma staging and differentiating benign from malignant masses.

Laboratory Studies

Serum tumor markers include AFP and beta-hCG (for germ cell tumors; elevated AFP rules out pure seminoma) and LDH (for lymphoma). Acetylcholine receptor antibodies should be checked for myasthenia gravis association with thymoma. Thyroid function tests are obtained for suspected retrosternal goiter. Flow cytometry is ordered if lymphoma is suspected.

Tissue Diagnosis

A tissue diagnosis is not always required before surgery, especially when imaging is classic for thymoma or teratoma. Core needle biopsy is preferred for suspected lymphoma because architectural assessment is needed for subtyping. CT-guided or EBUS-guided biopsy is used for inaccessible masses, and anterior mediastinotomy (Chamberlain procedure) or VATS biopsy provides larger specimens. Importantly, needle biopsy of suspected thymoma should be avoided if complete resection is planned because of the risk of seeding. Lymphoma requires tissue for subtype classification — fine-needle aspiration alone is insufficient.

Thymoma

Epidemiology and Associations

Thymoma is the most common primary anterior mediastinal neoplasm in adults, with peak incidence at 40-60 years and no sex predilection. Paraneoplastic associations are important: myasthenia gravis is present in 30-50% of thymoma patients (and 10-15% of MG patients have a thymoma), pure red cell aplasia in 5%, hypogammaglobulinemia (Good syndrome) in 5-10%, and other autoimmune conditions including SLE, polymyositis, and thyroiditis.

WHO Histologic Classification

The WHO classification stratifies thymomas by histology. Type A (medullary, spindle cell) carries an excellent prognosis. Type AB (mixed) has a good prognosis. Type B1 (predominantly cortical, lymphocyte-rich) resembles normal thymus. Type B2 (cortical) has an increased proportion of epithelial cells. Type B3 (well-differentiated thymic carcinoma) shows epithelial predominance. Type C (thymic carcinoma) is aggressive and is no longer considered thymoma by some classifications. The prognostic hierarchy progresses from A (best) through AB, B1, B2, B3, to C (worst).

Masaoka-Koga Staging

Stage I tumors are completely encapsulated with no microscopic capsular invasion. Stage IIA shows microscopic transcapsular invasion. Stage IIB demonstrates macroscopic invasion into surrounding fat or gross adherence to pleura or pericardium. Stage III involves macroscopic invasion into neighboring organs (lung, great vessels, pericardium). Stage IVA features pleural or pericardial dissemination (drop metastases). Stage IVB denotes lymphogenous or hematogenous metastases.

WHO Histologic Classification of Thymoma

Masaoka-Koga Staging and 5-Year Survival

5-Year Survival by Stage

Stage I achieves 95-100% 5-year survival, stage II 85-95%, stage III 70-85%, stage IVA 50-70%, and stage IVB 30-50%.

<image>Masaoka-Koga staging system for thymoma showing progressive degrees of capsular invasion, organ involvement, and metastatic spread</image>

Surgical Management of Thymoma

Principles

Complete surgical resection (R0) is the cornerstone of treatment. En bloc thymectomy — removal of the entire thymus with the tumor and surrounding mediastinal fat — is performed. The tumor capsule must not be violated, as this risks pleural and pericardial seeding. Extended resection for stage III disease includes invaded structures such as pericardium, lung, phrenic nerve, and innominate vein.

Surgical Approaches

Median sternotomy is the gold standard for open thymectomy, providing excellent exposure of the entire anterior mediastinum. VATS thymectomy is appropriate for early-stage (I-II), small (less than 5 cm), well-encapsulated tumors; it can be performed via a unilateral or bilateral approach and must achieve complete thymectomy (not just excision of the mass). Robotic thymectomy is gaining adoption, with 3D visualization aiding dissection and similar oncologic outcomes to open in early-stage disease, and may reduce length of stay and postoperative pain. Transcervical thymectomy is primarily used for non-thymomatous MG and has a limited role for thymoma.

Phrenic Nerve Management

Phrenic nerve involvement by tumor is common in stage III disease. Sacrifice of one phrenic nerve may be necessary for R0 resection, but bilateral phrenic nerve sacrifice is generally avoided because it causes ventilatory failure. If the nerve must be sacrificed, adequate contralateral diaphragm function should be confirmed preoperatively.

Adjuvant Therapy

Radiation is recommended for stage III (invasive) and incompletely resected tumors (R1/R2) but is not routinely recommended for completely resected stage I-II. Chemotherapy is reserved for unresectable, metastatic, or recurrent disease, using regimens such as cisplatin plus doxorubicin plus cyclophosphamide (PAC) or cisplatin plus etoposide. Thymoma has a propensity for late recurrence (10-20 years), and lifelong surveillance is required.

Thymectomy for Myasthenia Gravis

Indications

Thymoma-associated MG always requires thymectomy for treatment of the tumor. For non-thymomatous MG, the MGTX trial (2016) demonstrated the benefit of thymectomy plus prednisone versus prednisone alone, with better outcomes for generalized MG (not purely ocular) and the greatest benefit in anti-AChR antibody-positive patients under 65 years. Benefit was seen up to 5 years after thymectomy. Preoperative optimization of MG includes pyridostigmine, corticosteroids, IVIG, or plasmapheresis. Myasthenic crisis prevention requires avoiding perioperative triggers such as aminoglycosides and neuromuscular blockers.

Other Anterior Mediastinal Tumors

Germ Cell Tumors

Mature teratoma is benign, and complete resection is curative; it contains ectodermal, mesodermal, and endodermal elements. Seminoma is radiosensitive and chemosensitive, treated with cisplatin-based chemotherapy plus radiation, with surgery for residual mass. Non-seminomatous GCT is aggressive, treated with neoadjuvant cisplatin-based chemotherapy then surgical resection of residual mass; AFP and/or beta-hCG are elevated.

Lymphoma

Tissue diagnosis before treatment is essential, with architecture required for subtyping. Hodgkin lymphoma often presents as an anterior mediastinal mass in young adults and is highly curable with chemotherapy with or without radiation. Primary mediastinal large B-cell lymphoma is treated with chemotherapy plus immunotherapy. The role of surgery is diagnostic biopsy only, not resection.

Thymic Carcinoma

Thymic carcinoma is an aggressive malignancy distinct from thymoma, with squamous cell carcinoma as the most common histologic type. It has higher recurrence and mortality compared to thymoma. Treatment includes surgery when resectable, adjuvant radiation, and possibly chemotherapy. Unlike thymoma, thymic carcinoma has no paraneoplastic associations.

<image>CT scan demonstrating a well-encapsulated anterior mediastinal mass (thymoma) with clear fat planes from surrounding structures versus an invasive thymic carcinoma with loss of fat planes and vascular encasement</image>

Clinical Pearls

The "4 T's" mnemonic is useful but imprecise — lymphoma and germ cell tumors are common anterior mediastinal masses that do not begin with "T." A suspected thymoma should never be biopsied if complete resection is planned because capsule violation risks pleural seeding. Lymphoma requires core or incisional biopsy for accurate subtyping — FNA alone is insufficient. Acetylcholine receptor antibodies should always be checked in any patient with an anterior mediastinal mass to screen for myasthenia gravis. Thymoma recurrence can occur decades after initial resection, necessitating lifelong CT surveillance (annually for 10 years, then every 2 years). Preoperative optimization of myasthenia gravis is essential to avoid myasthenic crisis postoperatively. Complete thymectomy (not just tumor excision) is required for both thymoma treatment and MG benefit.

References

• Detterbeck FC et al. "The Masaoka-Koga Stage Classification for Thymic Malignancies." J Thorac Oncol. 2011.
• Wolfe GI et al. "Randomized trial of thymectomy in myasthenia gravis (MGTX)." N Engl J Med. 2016.
• Marx A et al. "The 2015 WHO Classification of Tumors of the Thymus." J Thorac Oncol. 2015.
• Filosso PL et al. "Outcome of surgically treated thymic carcinoma." Eur J Cardiothorac Surg. 2014.
• NCCN Clinical Practice Guidelines: Thymomas and Thymic Carcinomas. Version 2024.

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