Anterior Mediastinum Anatomy

• Benign or malignant mediastinal masses can develop from: structures normally in the mediastinum, structures that pass through it during development, or metastases of malignancies from elsewhere in the body.
• The mediastinum is the chest region located between the pleural cavities, excluding the lungs, and contains the heart and other thoracic viscera
• Lateral border of the mediastinum is parietal pleura
• Anterior border of the mediastinum is sternum
• Posterior border of the mediastinum is vertebral column and paravertebral gutters
• Superior border of the mediastinum is thoracic inlet
• Inferior border of the mediastinum is diaphragm

Superior Mediastinum

• The superior border is the thoracic inlet
• The inferior border is the transverse thoracic plane
• The anterior border is the sternal angle
• The posterior border is IV disc T4 & T5

Interior Mediastinum

• The superior border is the transverse thoracic plane
• The inferior border is diaphragm

Anterior Mediastinum

• Everything lying forward of and superior to the heart shadow
• Boundaries include: Sternum, first rib, imaginary curved line following the anterior heart border and brachiocephalic vessels from the diaphragm to the thoracic inlet
• Contents include: Thymus gland, substernal extension of the thyroid and parathyroid gland and lymphatic tissues

Middle Mediastinum

• Dorsal to the anterior mediastinum, extends from the lower edge of the sternum along the diaphragm and then cephalad along the posterior heart border and posterior wall of the trachea
• Contents include: the Heart, pericardium, aortic arch and its major branches, innominate veins and superior vena cava, pulmonary arteries and hila, trachea, group of lymph nodes, phrenic and upper vagus nerve

Posterior Mediastinum

• Occupies the space between the back of the heart and trachea, the front of the posterior ribs, and paravertebral gutters
• Extends from the diaphragm cephalad to the first rib
• Contents include: Esophagus, descending aorta, azygos and hemiazygos vein, paravertebral lymph nodes, thoracic duct, lower portion of the vagus nerve and the symphathetic chain

Mediastinal Masses by Location

Antero-superior mediastinum

• Can be caused by thymic tumors
• Can be caused by lymphomas
• Can be caused by germ cell tumors
• Can be caused by endocrine tumors
• Can be caused by mesenchymal tumors

The middle mediastinum

• Can be caused by lymphomas
• Can be caused by cysts
• Can be caused by mesenchymal tumors.
• Can be caused by tracheal tumors.
• Can be caused by cardiac and pericardial tumors
• Can be caused by vascular tumors
• Can be caused by Lymphadenopathy

The posterior mediastinum

• Can be caused by lymphomas
• Can be caused by Neurogenic tumors
• Can be caused by Mesenchymal tumors
• Can be caused by Esophageal tumors and cysts
• Can be caused by Hiatal hernias
• Can be caused by Thoracic duct cyst
• Can be caused by Meningocele

Regionalization

• The anterior mediastinum can be affected by Teratoma, Thymus, Ectopic Thyroid, and Adenopathy
• The middle mediastinum can be affected by Adenopathy, Bronchogenic Cysts, and Esophageal Duplication Cysts
• The posterior mediastinum can be affected by Neurogenic Tumors and Esophageal Duplication Cysts

Anterior Mediastinum Lymphoma

• Usually presents in older children
  • Hodgkin's presents at 14 years
  • Non-Hodgkin's presents at 9 years
• Can be associated with other symptoms and adenopathy
• Can frequently lead to Airway compromise

Symptoms of Mediastinal Mass Effects

• Direct involvement or compression of normal mediastinal structures causes a wide range of symptoms.
• Symptoms can include: cough, stridor, hemoptysis, shortness of breath, pain, dysphagia, hoarseness, facial and/or upper extremity swelling due to vascular compression (e.g., superior vena cava syndrome), hypotension due to tamponade or cardiac compression, and Horner syndrome due to sympathetic chain involvement.
• Systemic symptoms such as fever, night sweats, and weight loss can be present in the case of lymphoma or may be due to a variety of paraneoplastic syndromes, such as myasthenia gravis with thymoma.

Diagnosis

• Tumor markers are useful in the diagnosis of thymoma or germ cell tumor
• Anti-acetylcholine receptor antibodies identify thymic tumors indicating myasthenia gravis (more than 75% have thymic abnormalities)
• Alpha-fetoprotein (AFP) identifies malignant germ cell tumors, 60 to 80 % of nonseminomatous dysembryomas
• Beta-human chorionic gonadotropin (beta-hCG) is associated with seminoma (10%) and nonseminomatous (30 to 50%) germ cell tumors.
• Lactate dehydrogenase (LDH) is elevated in patients with nonseminomatous dysembryoma, but not as specific as AFP or beta-hCG, and is also elevated in patients with lymphoma.
• Chest scans are the imaging procedure of choice. Determine thymic enlargement because most enlarged thymus glands on CT scan represent a thymoma.
• Preferred scanning is CT with intravenous contrast dye to show the relationship between the thymoma and surrounding vascular structures, define the degree of vascularity, and guide the surgeon in removal of a large tumor, possibly involving other mediastinal structures
• Perform a biopsy if a patient presents with atypical features or an invasive tumor is found and induction therapy is a consideration.
• Biopsy can be via a limited anterior mediastinotomy.

Thymomas

• Thymic lesions account for about one-half of all anterior mediastinal masses and include a spectrum of benign and malignant histologies.
• They occur in patients of all ages, with peak incidence between 40 and 60 years, the gender distribution being approximately equal
• Jordan University hospital study: 50% of patients presented with masses with thymic gland in origin and another 50% diagnosed with other masses that were not thymic in origin, with mass origin being the most common
• The Jordan Study: Thymic masses appear in younger patients with an average age of 36 years, compared with the non-thymic masses which were found in older patients with an average age of 42 years (p value of 0.04); 75% of patients had benign masses.
• Thymomas are associated with a variety of paraneoplastic syndromes
• Myasthenia Gravis is associated with thymomas in 30% of patients
• Conduct the anti-acetylcholine receptor antibodies test for patients with a thymic mass who have not been evaluated for myasthenia gravis and perform a CT scan on the patient.
• All thymomas originate from epithelial thymic cells
• 4% of them consist of a pure population of epithelial cells
• Most have mixed populations of lymphoid cells to a varying extent
• Approximately 50% are asymptomatic, discovered incidentally on CXR or at autopsy
• 30% local symptoms are related with pressure or local invasion: SVC syndrome, cough, chest pain, dysphonia, or dysphagia
• 20%-70% associated with an autoimmune disease: Myasthenia gravis, Pure red cell aplasia, Polymyositis, or hypogammaglobulinemia

Masaoka Classification System for Thymomas

• Stage I
  • Encapsulated tumor with no gross or microscopic invasion
  • Treat via Complete surgical excision
• Stage II
  • Macroscopic invasion into the mediastinal fat or pleura or microscopic invasion into the capsule
  • Treat via Complete surgical excision and postoperative radiotherapy to decrease local recurrence
• Stage III
  • Macroscopic invasion of the pericardium, great vessels, or lung
  • Treat via Complete surgical excision and postoperative radiotherapy to decrease local recurrence
• Stage IVA
  • Pleural or pericardial metastatic spread
  • Treat via Surgical debulking, radiotherapy, and chemotherapy
• Stage IVB
  • Lymphogenous or hematogenous metastases
  • Treat via Surgical debulking, radiotherapy, and chemotherapy

Thymoma Prognosis and Treatment

• Benign tumors are noninvasive and encapsulated
• Malignant tumors are defined by local invasion into the thymic capsule or surrounding tissue.
• The Masaoka staging system of thymomas is the most commonly accepted system. Preponderance of evidence indicates that all thymomas, except completely encapsulated stage 1 tumors, benefit from adjuvant radiation therapy. The prognosis of a person with a thymoma is based on the tumor's gross characteristics at operation, not the histological appearance.
• If the tumor is small and appears readily accessible, perform total thymectomy with contiguous removal of mediastinal fat
• If the tumor is invasive, perform total thymectomy in addition to en bloc removal of involved pericardium, pleura, lung, phrenic nerve, innominate vein, or superior vena cava
• Resect one phrenic nerve; however, if both phrenics are involved, do not resect either nerve, and debulk the area
• Clip areas of close margins or residual disease to assist the radiation oncologist in treatment planning
• Commonly used chemotherapy drugs in the treatment of thymoma:
  • doxorubicin (Adriamycin, Rubex)
  • cisplatin (Platinol)
  • cyclophosphamide (Cytoxan, Neosar)
  • etoposide (VePesid, Etopophos, Toposar)
  • ifosfamide (Ifex, Holoxan)
• Chemotherapy typically combines cyclophosphamide, doxorubicin, and cisplatin, or etoposide and cisplatin. Adjuvant radiation therapy in completely or incompletely resected stage III or IV thymomas is standard of care. Use of postoperative radiation therapy in stage II thymomas is questionable. Thymomas are indolent tumors and may take at least 10 years to recur, therefore, short-term follow-up will not depict relapses accurately.

Germ Cell Tumors

• The mediastinum is the most common location for extragonadal germ cell tumors in adults.
• They can be benign (teratomas, dermoid cysts) or malignant (seminomas, nonseminomatous germ cell tumors). Seminomas are more common than nonseminomatous germ cell tumors
• Test for alpha-fetoprotein (AFP), lactate dehydrogenase (LDH), and beta-human chorionic gonadotropin (beta-hCG) in all patients with a mediastinal mass that could be a germ cell tumor prior to any therapy
• Characterized histologically, as mature, immature, teratoma with malignant transformation, seminoma, and nonseminoma
• Originates from two to three germ cell layers

Enlarged/Ectopic Thyroid

• Intrathoracic thyroid tissue typically causes shortness of breath or dysphagia
• The intrathoracic mass is usually continuous with the thyroid gland in the neck; only 2 percent of cases are separate from the cervical thyroid.

Middle Mediastinum

• Lymphadenopathy is the most common lesion presenting as a mass in the middle compartment of the mediastinum; common causes are lymphoma, sarcoid, and metastatic lung cancer
• Remnants of embryonic foregut (trachea & esophagus) more common in patients under two years of age
• Pericardial cysts
• Esophageal Duplication Cyst
  • Adjacent to or embedded in wall of esophagus
  • Can have respiratory or Gl epithelium
  • May either obstruct or erode through esophageal wall
• Benign cystic tumor
• Bronchogenic cysts comprise 20% of cystic masses
  • Bronchogenic cysts most common in men
  • Lesions identified d/t: substernal pain, cough, recurrent symptoms of a infection or dyspnea
  • Typically located in the right paratracheal region and in the subcarinal location
• Cardiovascular aneurysms or other anomalies, such as thoracic aortic aneurysm, pulmonary artery aneurysm, and vascular rings, can present as mediastinal abnormalities
• Advanced esophageal tumors may be identified on imaging as a mediastinal mass, location and dysphagia symptoms, with associated weight loss, unlike lymphadenopathy or other middle mediastinal masses
• Imaging Studies: Contrast esophagram

Posterior Mediastinum

• Neurogenic tumors represent more than 60% of masses within this anatomical compartment, with classification depending on the neural origin
• Schwannomas and neurofibromas compose 90% of adult neurogenic tumors and are benign lesions that arise from the intercostal nerve sheath
• Neuroblastomas and ganglioneuroblastomas are malignant tumors that occur most commonly in children and originate from the sympathetic ganglia
• Ganglioneuromas are benign lesions that arise from the sympathetic ganglia and are most common in young adults
• Neuroblastoma: Very good prognosis, especially Stage I & II
  • Paraplegia implies compression of the spinal cord, should be followed up with MRI & an urgent laminectomy

Surgery

• Two Techniques: Open median sternotomy and Video Assisted Thoracoscopic Surgery (VATS)
• the most common, allows for complete removal of the thymus.
• Video Assisted Thoracoscopic Surgery (VATS): Minimally invasive thymectomy for Myasthenia Gravis