eMedicine Specialties > Thoracic Surgery > Tumors

Mediastinal Seminoma

Author: Shabir Bhimji, MD, PhD, Locum Cardiothoracic and Vascular Surgeon, Saudi Arabia and Middle East Hospitals
Contributor Information and Disclosures

Updated: Oct 31, 2008

Introduction

Germ cell tumors of the mediastinum are uncommon. Less than 5-7% of germ cells occur outside the gonads, but of the extragonadal sites, the mediastinum is the most common location for germ cell tumors. Other areas where germ cell tumors can occur are the retroperitoneum, the intra-abdominal cavity, and the chest. The exact mechanism whereby germ cell tumors originate in the mediastinum remains unknown.

Germ cell tumors in the mediastinum were first reported nearly 50 years ago. Pathological studies from autopsy data revealed that the tumors were usually large and frequently locally invasive.

The first theory on extragonadal malignant germ cell tumors postulated that the tumors developed from primitive germ cells in the endoderm of the yolk sac or from the urogenital ridge. These cells normally move into the scrotum during development, but when this migration fails, the cells may remain localized to either the mediastinum or the retroperitoneum. Other researchers hypothesized that these totipotential cells become detached during embryogenesis and result in primitive masses, which may develop into germ cell tumors. So far, however, no theory is proven.

No research documents that these cells metastasize from gonadal tissue. Mediastinal germ cells tumors are now postulated to be autonomous oncologic entities.

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Frequency

Malignant germ cell tumors of the mediastinum are rare, accounting for approximately 10% of all mediastinal tumors. These tumors occur almost exclusively in males, and the age at presentation is generally 20-35 years.¹ More than a third of all malignant germ cell tumors are pure seminomas.

All types of malignant germ cell masses are more prevalent in males, and most of these males are symptomatic. Benign germ cell tumors, on the other hand, are distributed equally between the sexes, and most develop in the third decade of life. In children, mediastinal germ cell tumors can occur at any age and are equally divided between boys and girls.

Etiology

Experimental evidence suggests that the cells responsible for mediastinal tumors are derived from germinal cells that transmigrate to the mediastinum from the urogenital ridge during embryonic life; however, not all authorities agree with this view of presupposed metastatic disease. Nevertheless, it is always important to make sure, through physical examination of the groin and scrotum, that no gonadal tumors are simultaneously present. (Click here to complete a Medscape CME activity on synchronous bilateral testicular seminoma.) Ultrasound is sensitive for examination of the scrotal area, and CT scanning is essential to evaluate the retroperitoneum.

Although the cause of these tumors is not known, men with Klinefelter syndrome have an increased prevalence of them. Individuals with this syndrome, consisting of an extra X chromosome, are known to develop mediastinal germ cell tumors at least 10 years earlier than persons without the syndrome. Further workup in these patients has shown abnormally low levels of testosterone and high levels of luteinizing hormone and estradiol. These discoveries suggest a primary germ cell defect, which may cause faulty spermatogenesis and a predisposition to extragonadal malignancies.

Pathophysiology

The histology of mediastinal tumors is very similar to that of tumors in the gonadal tissues; however, most authorities believe that metastases from the gonadal area are not responsible for the growth of these tumors in the mediastinum. In adults, germ cell tumors are the third most common type of tumor in the anterior mediastinum; in children, they are the second most common anterior mediastinal mass. Fortunately, most of these lesions are benign teratomas. Of the germ cell tumors, benign teratomas (ie, dermoid cysts) are the most commonly diagnosed mediastinal mass and are present in 50-70% of infants and children with germ cell tumors. Seminomas are the predominant malignant lesions, accounting for nearly 50% of mediastinal lesions.

Pathology

Mediastinal seminomas are generally bulky tumors and tend to infiltrate into adjacent structures early in the growth process. The cells are large and contain variable amounts of glycogen. These tumors are composed of large cells with multiple nuclei, which closely resemble syncytiotrophoblasts.

Unlike other germ cell tumors, seminomas tend to remain localized in the chest, and only occasionally invade adjacent structures. However, these tumors are sometimes discovered late, and extrathoracic spread is sometimes observed. The usual route of metastases is hematogenous, and the major organs to which metastases occur are the lungs, liver, and bones.

Presentation

The clinical manifestations of seminomas depend on their histology. In most benign cases, seminomas are discovered on a routine chest radiograph. Patients with malignant seminomas usually present with symptoms of compression or invasion of surrounding structures and, uncommonly, systemic effects of the tumors. Constitutional symptoms (eg, pain, weight loss, fever, fatigue, dyspnea) have all been described. Occasionally, mediastinal adenopathy and superior vena cava syndrome may occur. Unlike nonseminomas, most seminomas have a localized effect and remain intrathoracic. They are rarely associated with other syndromes.

Indications

Surgery is not the prime treatment modality, and more than 50% of patients are deemed to have unresectable tumors. For this reason, surgery is usually reserved for only small mediastinal masses in asymptomatic patients. Even in this situation, surgery has been associated with a high rate of recurrence and must therefore be accompanied by some form of adjuvant therapy, even if the resection appears to be complete.

When complete excision is not possible, a biopsy is performed to confirm the diagnosis and an alternative treatment is started. Because these tumors are quite responsive to radiation, performing high-risk surgery (with its potential for injuring mediastinal structures) is unnecessary.

Relevant Anatomy

Seminomas usually develop in the anterosuperior mediastinum and can develop into fairly large tumors. The tumors usually grow at the juncture of the innominate vein and the superior vena cava. As a tumor grows, it can compress the above structures and invade the adjacent fatty tissues. When a seminoma is large, differentiating the mass from the thymic fat and surrounding pericardium is difficult.

Contraindications

Surgery should not be undertaken without a tissue diagnosis because (1) seminomas respond poorly to surgery and have a high rate of recurrence and (2) the mediastinal mass may be a lymphoma (more common in young males) instead of a seminoma and lymphomas usually respond to chemotherapy. Furthermore, if the patient has severe tracheomalacia from prolonged compression of the trachea by the tumor, anesthesia is not recommended. Finally, if the patient has metastatic disease or numerous medical conditions, surgery is best avoided.

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