Mediastinal seminomas or mediastinal germinomas are primary malignant germ cell tumors of the mediastinum.
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Epidemiology
Mediastinal seminomas are rare mediastinal tumors and account for up to one-third of primary malignant mediastinal germ cell tumors 1. They are almost only found in males ≥10 years with the median age being in the fourth decade 1.
Diagnosis
The final diagnosis is based on pathology, gender and the absence of a testicular lesion on imaging 1.
Diagnostic criteria
Diagnostic criteria according to the WHO classification of thoracic tumors (2021 - blue book) 1:
- small sheets or clusters of polygonal epithelioid cells with a clear, pale or eosinophilic cytoplasm
- lymphocytic and/or granulomatous environment
- no testicular lesion on imaging
- male patient
Further desirable criteria include 1:
- occurrence after puberty
- positivity of OCT4 and KIT/CD117
- 12p abnormalities on FISH studies
Clinical presentation
Clinical signs and symptoms are frequently chest-related and include dyspnea, chest pain and cough. Systemic symptoms and signs include weight loss, nausea, fever and gynecomastia 1-3. They might also present with complications such as superior vena cava syndrome or might be found incidentally on imaging studies 1,2. β-hCG and/or LDH might be elevated 1-4, whereas AFP is usually normal 1-4.
Complications
Mediastinal seminomas might cause the following complications 1,4:
- superior vena cava obstruction
- pulmonary stenosis
- nodal and distant metastases
Pathology
Mediastinal seminomas are germ cell tumors that originate from the thymus 1.
Pathogenesis
Primary mediastinal seminomas are most commonly associated with genetic abnormalities that is amplification or isochromosome 12p 1.
Location
Mediastinal seminomas are usually located in the anterior or rarely in the middle mediastinum 1.
Macroscopic appearance
Macroscopically mediastinal seminomas are lobulated to nodular tumors with a smooth and shiny cut surface and a white to light tannish color. Areas of necrosis, hemorrhage or cystic spaces are uncommonly seen 1,4.
Microscopic appearance
Microscopically mediastinal seminomas are characterized by the following histological features 1:
- confluent sheets, clusters or cords of round to polygonal tumor cells
- large prominent nucleoli
- clear to mildly eosinophilic cytoplasm
- distinct tumor cell membranes
- lymphocytic infiltration and/or non-necrotizing granulomatous background
- variably cellular pleomorphism
- variably areas of necrosis
- less commonly thymic remnants or florid lymphoid hyperplasia
- rarely prominent cystic changes, syncytiotrophoblasts or intercellular edema
Immunophenotype
Mediastinal seminomas usually express OCT4, KIT/CD117 and SALL4 on immunohistochemistry stains 1. They are also frequently positive for D2-40, PLAP and CD99 1. Weak positivity for keratins might be observed 1.
Radiographic features
Mediastinal seminomas usually appear as bulky, homogeneous lobular masses. Sometimes they might have cystic, hemorrhagic or necrotic components.
CT
On thoracic CT mediastinal seminomas do usually not show any calcifications 4. They appear as bulky, lobulated fairly homogeneous soft tissue masses with irregular contours and mild contrast enhancement 1,3-5.
MRI
Descriptions of MRI appearances of mediastinal seminomas are scarce in the current literature. They have been described as bulky masses with contrast enhancement. Cystic and necrotic lesions have been also described.
Nuclear medicine
PET-CT shows a high uptake of FDG and has been used to monitor treatment results 6,7.
Radiology report
The radiological report should include a description of the following:
- location and extent of the tumor
- relation to the heart and great vessels
- associated complications (e.g. superior vena cava obstruction)
- nodal disease
- distant metastases
Treatment and prognosis
Chemotherapy is the main treatment modality with radiotherapy and/or surgery being additional options 2.
Prognosis is mostly dependant on the existence of visceral non-pulmonary metastases. The majority of patients with mediastinal seminomas have a good prognosis with the remainder having an intermediate prognosis 1. Progression-free survival after 5-years has been reported to be 75% or better 1,2.
History and etymology
Mediastinal germ cell tumors were first described by an American pathologist, Lewis B Woolner, and colleagues in 1955 2,8.
Differential diagnosis
Condition or tumors that might mimic the appearance of mediastinal seminomas include 1:
- lymphoma
- thymoma
- thymic carcinoma
- other mediastinal germ cell tumors
- nodal mediastinal metastases