Timoma

01Pathophysiology

Thymoma arises from thymic epithelial cells, and understanding this origin is the key to understanding its entire clinical behavior. The thymus gland is located in the anterior mediastinum and plays a central role in T-lymphocyte maturation and selection. In a normal thymus, developing T-cells undergo positive and negative selection: they learn to recognize foreign antigens (positive selection) and are eliminated if they react to self-antigens (negative selection). This process ensures immunologic self-tolerance.

In thymoma, the neoplastic epithelial cells disrupt normal T-cell education. The tumor microenvironment allows the escape of autoreactive T-cells that would normally be deleted during negative selection. These autoreactive T-cells enter the peripheral circulation and attack the body's own tissues, explaining why thymoma is strongly associated with paraneoplastic autoimmune syndromes. The most common of these is myasthenia gravis (MG), occurring in roughly 30-50% of thymoma patients. In MG, autoantibodies target the nicotinic acetylcholine receptors (AChR) at the neuromuscular junction, blocking signal transmission and causing fatigable skeletal muscle weakness.

Patients with thymoma may experience symptoms associated with autoimmune paraneoplastic diseases such as myasthenia gravis, which typically do not occur in patients with thymic carcinoma. This distinction is critical: thymoma preserves enough thymic architecture to generate autoreactive T-cells, while thymic carcinoma is so poorly differentiated that it loses this capability.

Beyond MG, thymoma can produce other autoimmune conditions through similar mechanisms of disordered T-cell selection:

Pure Red Cell Aplasia (PRCA) results from autoimmune destruction of erythroid precursors in the bone marrow, presenting as severe normocytic anemia with reticulocytopenia and absent erythroid precursors on bone marrow biopsy, while the myeloid and megakaryocyte lineages remain intact.

Good Syndrome (Thymoma-Associated Immunodeficiency) is a combined B-cell and T-cell immunodeficiency characterized by hypogammaglobulinemia in the setting of thymoma. These patients are profoundly immunocompromised, prone to recurrent sinopulmonary infections, opportunistic infections (CMV, mucocutaneous candidiasis), and autoimmune cytopenias. Unlike MG, Good syndrome does not improve after thymectomy.

The tumor itself grows slowly and tends to invade locally rather than metastasize distantly. When symptoms arise from mass effect, they reflect compression of anterior mediastinal structures: the SVC (causing SVC syndrome), the phrenic nerve (causing diaphragm paralysis), and the recurrent laryngeal nerve (causing hoarseness). Distant hematogenous metastases are uncommon; instead, thymoma tends to spread by pleural dissemination ("drop metastases"), which is a pattern unique to this tumor.

02Classification and Clinical Manifestation

WHO Histological Classification

The five main subtypes (A, AB, B1, B2, and B3) can be broadly divided into thymomas containing spindled neoplastic epithelial cells (A, AB) and thymomas composed of epithelioid neoplastic epithelial cells (B1-B3).

The frequency of invasion to neighboring organs increased according to tumor subtype in the order A (0%), AB (6%), B1 (19%), B2 (25%), B3 (42%), and C (89%). The examiner is testing whether you can connect histologic subtype to clinical aggressiveness: Types A and AB are indolent; B2, B3, and C carry the worst prognosis.

Masaoka-Koga Clinical Staging System

Thymoma staging and classification are commonly done at the time of surgery, using the Masaoka Staging System, based on the degree of tumour infiltration at the time of operation.

Paraneoplastic Syndromes Associated with Thymoma

03Diagnostic Workup

The diagnostic workup for thymoma follows a logical sequence driven by clinical context.

Step 1: Detection. Most thymomas are found incidentally on chest imaging performed for other reasons. On chest X-ray, you may see a mediastinal widening or a lobulated opacity projecting over the anterior mediastinum. However, chest X-ray is insensitive, and many thymomas are missed.

Step 2: Characterization with CT. The best initial imaging test is a contrast-enhanced CT of the chest. The classic appearance is a well-circumscribed, homogeneous, soft tissue mass in the anterior mediastinum, often demonstrating smooth or lobulated margins. CT also evaluates for local invasion (into the lung, pericardium, or great vessels), pleural implants, and lymphadenopathy. This is the single most important test for determining resectability.

Step 3: Rule out the Differential. The anterior mediastinum houses the "4 T's": Thymoma, Teratoma (and other germ cell tumors), Terrible lymphoma, and Thyroid (substernal goiter). Laboratory tests help narrow the differential: serum AFP and beta-hCG (to rule out germ cell tumors), LDH and flow cytometry (to evaluate for lymphoma), and thyroid function tests (to evaluate for substernal goiter).

Step 4: Evaluate for Paraneoplastic Syndromes. Every patient with a suspected thymoma must be clinically evaluated for myasthenia gravis and other paraneoplastic conditions. Order anti-AChR antibodies, CBC with reticulocyte count (for PRCA), and serum immunoglobulins (for Good syndrome).

Step 5: Tissue Diagnosis. Tissue diagnosis with core needle biopsy is recommended for locally advanced or metastatic thymic masses. However, if the mass appears clearly resectable and the clinical suspicion for thymoma is high, the preferred approach is to proceed directly to surgical resection without biopsy, because the surgery itself provides both the tissue diagnosis and the treatment. Pre-operative biopsy is reserved for cases where neoadjuvant therapy is being considered (unresectable tumors) or when lymphoma is a significant concern.

04Management and Treatment

Surgical Resection is the cornerstone of thymoma treatment. The goal is complete (R0) resection of the entire thymus and surrounding mediastinal fat through a median sternotomy or, in select early-stage cases, minimally invasive (robotic/VATS) thymectomy. Complete resection is the single most important prognostic factor. Despite successful resection of the thymic tumour, roughly 15-30% of patients experience recurrent thymoma, with a disease-free phase of approximately 60-80 months.

Adjuvant Radiation Therapy (PORT) is recommended for Masaoka stage II or higher, particularly when margins are positive (R1 resection) or close. For completely resected stage II thymoma with negative margins, the role of adjuvant RT remains debated, but many centers administer 45-50 Gy. For incompletely resected or stage III disease, doses of 54-60 Gy are used.

Chemotherapy is reserved for advanced, unresectable, or metastatic disease. The first-line treatment suggested by guidelines for thymoma is called CAP, and it consists of the combination of cisplatin, doxorubicin, and cyclophosphamide. The standard dosing of the CAP regimen is:

• Cisplatin 50 mg/m${}^2^2$ IV on day 1

• Doxorubicin 50 mg/m${}^2^2$ IV on day 1

• Cyclophosphamide 500 mg/m${}^2^2$ IV on day 1

• Repeated every 21 days for 2-4 cycles

Patients received two to four cycles (repeated every 3 weeks) of cisplatin, doxorubicin, and cyclophosphamide followed by a total dosage of 54 Gy to the primary tumor and regional lymph nodes for patients with a stable, partial, or complete response to chemotherapy.

Alternative regimens include ADOC (doxorubicin, cisplatin, vincristine, cyclophosphamide) and PE (cisplatin + etoposide).

Management of Concurrent Myasthenia Gravis is a critical peri-operative concern. MG must be optimized before surgery to minimize the risk of myasthenic crisis. This involves:

• Pyridostigmine (cholinesterase inhibitor) for symptomatic control

• Immunosuppression (corticosteroids, azathioprine, mycophenolate)

• Pre-operative plasma exchange (PLEX) or IVIG to reduce circulating antibodies and stabilize the patient for surgery

• Post-operative monitoring in an ICU setting, with careful attention to respiratory function

Important note on MG post-thymectomy: Thymectomy improves MG symptoms in the majority of patients, but the improvement is gradual and may take months to years. MG rarely resolves immediately after surgery.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students get thymoma questions wrong is by failing to connect the anterior mediastinal mass to myasthenia gravis. The exam vignette will often not tell you the diagnosis outright. Instead, it will describe a middle-aged patient with progressive ptosis, diplopia, and fatigable weakness who "had a CT scan showing an anterior mediastinal mass." The expected answer is thymoma with concurrent MG. Many students focus on the neurological symptoms and miss the mediastinal mass, or they see the mass and jump to lymphoma.

A second common trap involves the distinction between MG and Lambert-Eaton syndrome (LEMS). Both cause weakness. Both are paraneoplastic. But MG (thymoma) involves post-synaptic antibodies against AChR, worsens with repeated activity, and starts with the ocular muscles. LEMS (small cell lung cancer) involves pre-synaptic antibodies against voltage-gated calcium channels, improves with repeated activity, and begins with proximal limb weakness. If the vignette says "weakness that gets better with use," it is LEMS, not MG, and the mass will be in the lung, not the anterior mediastinum.

A third trap is the management sequencing question. For a resectable thymoma, the answer is surgery first, not biopsy. A biopsy is only indicated when the tumor is unresectable and you need a tissue diagnosis before starting neoadjuvant chemotherapy, or when lymphoma is a competing diagnosis. Choosing "biopsy" for an obviously resectable anterior mediastinal mass with MG is the wrong answer.

Be prepared for the association between thymoma and pure red cell aplasia. If the vignette describes severe isolated anemia with absent reticulocytes in a patient with an anterior mediastinal mass, the diagnosis is thymoma-associated PRCA. The bone marrow will show absent erythroid precursors with preserved myeloid and megakaryocytic lineages. This is different from aplastic anemia, which involves pancytopenia and a hypocellular marrow.

Finally, remember Good syndrome. If the vignette describes a patient with a thymoma who has recurrent sinopulmonary or opportunistic infections and low immunoglobulin levels, the diagnosis is Good syndrome. Unlike MG and PRCA, Good syndrome does not improve after thymectomy.

The core competency being tested across all thymoma questions is whether you can: (1) localize a mass to the anterior mediastinum, (2) connect it to its classic paraneoplastic syndromes, (3) correctly sequence the diagnostic and therapeutic workup, and (4) distinguish it from its closest mimics using discriminating clinical details.