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C37 covers primary malignant neoplasms arising in the thymus, an anterior mediastinal organ. Thymic epithelial tumors are uncommon and range from relatively indolent thymomas to aggressive thymic carcinomas.
Common Histologies
- Thymoma, classified by WHO types A, AB, B1, B2, and B3
- Thymic carcinoma, most often squamous cell carcinoma
- Less common primary thymic malignancies, when documented as arising in the thymus
Malignant carcinoid tumor of the thymus is excluded from C37 and is classified under C7A.091.
Common Symptoms
Many tumors are discovered incidentally on chest imaging. Clinical manifestations may include:
- Chest pain, cough, dyspnea, or dysphagia
- Superior vena cava obstruction or other compressive symptoms
- Myasthenia gravis, especially with thymoma
- Pure red cell aplasia
- Hypogammaglobulinemia/Good syndrome
- Other autoimmune or paraneoplastic disorders
Diagnosis
Evaluation generally includes contrast-enhanced CT of the chest to define the anterior mediastinal mass, local invasion, nodal disease, pleural deposits, and distant spread. MRI may help assess vascular, cardiac, or chest-wall involvement; FDG-PET/CT is used selectively, particularly for suspected thymic carcinoma or metastatic disease.
Histologic confirmation may be obtained by core biopsy when the diagnosis is uncertain or neoadjuvant/systemic therapy is planned. In a clearly resectable lesion strongly suspected to be thymoma, biopsy may be deferred in favor of surgical resection after multidisciplinary review. Pathology should distinguish thymoma from thymic carcinoma, lymphoma, germ-cell tumor, metastatic disease, and other anterior mediastinal lesions.
Staging
Thymic epithelial malignancies may be described using:
- AJCC/UICC TNM staging, based on local invasion, nodal involvement, and distant or pleural/pericardial metastases
- Masaoka-Koga staging, historically used for thymoma and based largely on capsular invasion and spread
Resectability, invasion of adjacent mediastinal structures, pleural or pericardial dissemination, nodal disease, and completeness of resection are major prognostic factors.
Molecular Markers
Routine molecular testing is not required for every thymic tumor. Findings may include:
- GTF2I mutations, enriched in some thymomas and associated with more indolent histologies
- KIT expression in many thymic carcinomas, although activating KIT mutations are uncommon
- PD-L1 expression in some thymic epithelial tumors
Biomarker-directed or immune-checkpoint therapy requires specialist interpretation. Immune checkpoint inhibitors can cause severe immune-mediated toxicity in this population, particularly in patients with thymoma or pre-existing autoimmune disease.
Common Treatments
- Complete surgical resection, when feasible, is the principal treatment for localized disease
- Postoperative radiotherapy for selected patients based on histology, stage, invasion, margin status, and recurrence risk
- Neoadjuvant chemotherapy, with or without radiotherapy, for selected locally advanced tumors
- Platinum-based systemic therapy for unresectable, recurrent, or metastatic disease
- Radiotherapy for unresectable local disease, residual disease, recurrence, or palliation
- Selected targeted or immunologic therapies for advanced thymic carcinoma under specialist guidance
Management is typically multidisciplinary because treatment differs between thymoma and thymic carcinoma and depends strongly on resectability and associated autoimmune conditions.
Scope Note
C37 identifies a malignant primary tumor of the thymus by site. It does not encode the specific histology, WHO subtype, grade, TNM or Masaoka-Koga stage, resection status, molecular profile, or associated paraneoplastic syndrome. Secondary metastatic involvement of the thymus or mediastinum is not coded as a primary C37 malignancy. Malignant carcinoid tumor of the thymus is specifically excluded and classified under C7A.091.