Mediastinal seminoma

Seminomas of the mediastinum, mediastinalMediastinal seminomas or mediastinal germinomas are primary malignant germ cell tumours of the mediastinum.

Epidemiology

Mediastinal seminomas are rare mediastinal tumours and account for up to one-third of primary malignant mediastinal germ cell tumours 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 bone tumours (2020 - 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 dyspnoea, chest pain and cough. Systemic symptoms include weight loss, nausea fever or gynaecomastia ^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 tumours 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 tumours with a smooth and shiny cutsurface  andcut surface and a whithewhite to light tannish colour. Areas of necrosis, haemorrhage or cystic spaces are uncommonly seen ^1,4.

Microscopic appearance

Microscopically mediastinal seminomas are charactersisedcharacterised by the following histological features ¹:

• confluent sheets, clusters or cords of round to polygonal tumour cells
• large prominent nucleoli
• clear to mildly eosinophilic cytoplasm
• distinct tumour 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 oedema

Immunohistochemistry

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, haemorrhagic 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 tumour
• relation to the heart and great vessels
• associated complications (e.g. superior vena cava obstruction)
• nodal disease
• distant metastases

Treatment and prognosis

Prognosis is mostly dependant on the existanceexistence of visceral non-pulmonary metastases. The majority of patients with mediastinal seminomas have a good prognosis with the remainder having an intermediate prognosis ¹. Chemotherapy is the main treatment modality with radiotherapy and/or surgery being additional options ². Progression free-free survival after 5-years has been reported to be 75% or better ^1,2.

History and etymology

Mediastinal germ cell tumours have been firtsfirst described by LB Woolner and colleagues in 1955 ^2,8.

Differential diagnosis

Condition or tumours that might mimic the appearance of mediastinal seminomas include ¹:

lymphomas

thymomas

• lymphoma
• thymoma
• thymic carcinoma
• teratoma
• other germ cell tumours
  • embryonal carcinoma
  • yolk sac tumour
• nodal mediastinal metastases