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Mediastinal Masses
Anatomy
A precise understanding of the anatomy of the mediastinum is critical for the
localization, safe biopsy, and possible extirpation of any primary mediastinal mass.
1) anterior mediastinum: extends from the manubrium and first ribs superiorly, to
the diaphragm inferiorly. The anterior border is the posterior sternal table, which
extends posteriorly to the pericardium and innominate vein.
Contents: thymus, aortic arch vessels, and associated LN. parathyroid and thyroid
glands may be found or extend to the anterior mediastinum.
2) middle mediastinum: extends only as superior as the pericardial reflection,
inferiorly to the diaphragm, and posteriorly to the anterior border of the spine.
Contents: heart, pericardium, intrapericardial aorta, trachea and associated LN, both
pulmonary hila and SVC.
3) Posterior mediastinum: extends from the superior aspect of the first thoracic
vertebral body superiorly, to the diaphragm inferiorly, anteriorly is the ventral aspect
of the verebral bodies, which extends posteriorly to the articulation with each rib, this
incorporates the costovertebral sulci.
Contents: segmental nerve roots, sympathetic chain, esophagus, vagus nerves,
thoracic duct, azygos vein and descending aorta.
Epidemiology and Incidence
Secondary involvement from direct infiltration of an intrathoracic primary or
metastatic disease from elsewhere occurs more frequently than primary mediastinal
tumors.
Regarding primary mediastinal masses, Lesions of the posterior mediastinum
dominate pediatric series, and masses in the anterior mediastinum are more common
in adults.
If all mediastinal lesions are considered in a pediatric series, neurogenic tumors
that arise in the posterior mediastinum are most common. Most are neuroblastomas, a
malignant neoplasm most common among children 3 years of age and younger. The
second most common malignant neoplasm in pediatric age group is lymphoma,
which is usually found in anterior mediastinum, these tumors are more common in
adolescent years. Germ cell tumors are the second most common anterior mediastinal
mass in children, most of them being benign teratomas.
In adult series, anterior compartment lesions predominate with thymic neoplasm
leading the list, adult series are comprised of fewer posterior lesions. Thymoma is the
most common neoplasm of the thymus and may be associated with MG, lymphomas
are the second and the primary germ cell tumors are third.
The mediastinum may be involved by metastatic tumor mimicking a primary
mediastinal lesion. In elderly people, this is the most common cause of mediastinal
lymphadenopathy.
type
percentage
Thymic
Lymphoma
Germ cell tumor
endocrine
46
24
15
15
Anterior med. masses in adults
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Anterior med. Masses in children
Anterior Mediastinal Masses
Thymoma
This is the most common mediastinal tumor accounting for 25 per cent of the
total. These are tumors of the thymic epithelial cells of Hassall’s corpuscles and are
sited in the anterior and superior compartments. They generally occur after childhood
and present as lobulated, occasionally calcified, masses in the anterior mediastinum.
They may appear encapsulated and are often associated with the autoimmune diseases
like myasthenia gravis, red cell aplasia and hypoglobulinemia. The tumors vary in
their behavior from completely benign to aggressively invasive. The only reliable
indicator of malignancy is capsular invasion. Diagnosis and treatment are best
achieved by complete thymectomy, but radiotherapy may be the only treatment option
if the lesion is advanced.
Rarer still is the thymic carcinoma. This is not associated with myasthenia and
resembles SCC or an undifferentiated large cell tumor. Surgical excision is the
treatment of choice when there is no evidence of widespread dissemination. The
prognosis is generally poor in spite of combination treatment of excision, radiotherapy
and cytotoxic therapy.
Lymphoma
Mediastinal lymphoma usually presents as a component of more diffuse systemic
process, however; primary mediastinal lymphoma occurs approximately in 10% of
time. Lymphoma is a common cause of a mediastinal mass lesion, particularly the
anterior mediastinum, leading to obstruction of the superior vena cava. Lymphomas
arise from the thymic lymph tissue or the lymph nodes of the mediastinum. They can
be classified into Hodgkin’s and non-Hodgkin’s types. Hodgkin’s lymphomas
presenting in the thymus tend to be localized and usually have favourable histology
and a favourable response to treatment. Non-Hodgkin’s lymphomas are usually high
grade and are more common in young to middle-aged females. A tissue diagnosis is
essential so that the appropriate treatment can be planned. The overall prognosis in
non-Hodgkin’s lymphomas is poor.
Lymphoma is better to be treated non surgically, but still a tissue diagnosis is
essential in order to plan an appropriate treatment.
Germ Cell Tumors
Primary germ cell tumors that arise in extragonadal sites are uncommon in people
of all ages. nevertheless; it is important to exclude a gonadal primary lesion with a
physical exam and scrotal US whenever a mediastinal germ cell tumor is diagnosed.
Germ cell tumors are usually found in the anterior mediastinum. They tend to occur in
young adults and 75 % are benign and cystic (benign teratomas, dermoid cysts).
type
percentage
Lymphoma
Germ cell tumor
Thymic
mesenchymal
41
23
21
15
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They contain elements from all three cell types (mesoderm, endoderm and ectoderm)
and are best treated by surgical excision. Malignancy is suspected if elevated levels
of serum alpha-fetoprotein, human chorionic gonadotrophin and carcinoembryonic
antigen are detected. They are classified into:
Seminoma: any evidence of tumor below the diaphragm may suggest systemic or
advanced disease.
Elevation of βHCG in serum.
Treatment is usually non surgical.
Nonseminomas: elevation of both AFP and βHCG in serum.
Treatment is usually non surgical.
Mesenchymal Tumors
A small number of mesenchymal tumors occurs within the mediastinum and
approximately half are malignant. Lipomas are common in the anterior mediastinum,
whereas the malignant form, liposarcoma, tends to occur posteriorly. Fibroma,
Fibrosarcoma and mesothelioma may also occur.
Endocrine
Ectopic thyroid tissue (and parathyroid) may be found in the anterior
mediastinum. Neoplasia and hyperplasia may occur but these are uncommon. More
often the tumor is merely a mediastinal extension of a thyroid lesion.
Middle Mediastinal Masses
They are mostly cystic lesions, whether acquired or congenital, some are very
rare, may be asymptomatic, or produce symptoms due to pressure effect, infection, or
perforation, usually diagnosed by radiological methods, and treatment options range
from observation, aspiration, to excision depending on symptoms, nature and
diagnosis of such cysts, and on possible complications (infection or malignant
transformation).
posterior Mediastinal Masses
Neurogenic Tumors
These may be derived from the sympathetic nervous system or the peripheral
nerves or peripheral ganglia and are more prevalent in the posterior mediastinum.
They may be asymptomatic, or cause symptoms related to pressure effects on
neighboring structures or may be hormonally active giving rise to related symptoms.
Treatment is by complete surgical excision, usually through posterolateral
thoracotomy except when there is intraspinal extension (dumbbell tumors).
A) Tumors of the autonomic nervous system:
are more common in young patients less than 10 years of age, and the cells of
origin are the sympathetic ganglion cells, a spectrum exist from benign to malignant
cells. .
Ganglioneuroma: It is benign encapsulated tumor, and generally has a good
prognosis.
Ganglioneuroblastoma: It is mixed encapsulated tumor, This tumor has an
intermediate prognosis.
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Neuroblastoma: It is malignant and rarely have capsule, It metastasizes widely and
has a poor prognosis.
B) nerve sheath tumors:
typically are asymptomatic or cause pressure symptoms,
Adults are more prone to develop Schwannomas (neurilemmoma) and
neurofibromas. The histological appearances are similar to those found in
conventional sites and they present as a mass lesion with pressure effects or as
asymptomatic radiographic findings, these are benign tumors.
Tumors of the nerve sheath and fibers (neurofibromas and Schwannomas) have a
wide range of presentations and behaviors. Multiple neurofibromas may be part of a
familial syndrome (Von Recklinghausen’s disease, neurofibromatosis). Tumors in the
paravertebral gutter may have a component within the intervertebral canal (a so-called
dumb-bell’ tumor).
Neurofibrosarcoma and malignant schwannoma are malignant, they are friable
with more tendency to produce symptoms at presentation.
C) paraganglionic tumors:
Chemodectoma: tumor of the aortic body, resection is the rule if the tumor is not
locally invasive.
Phaeochromocytomas: These arise from the aorticosympathetic paragangliomas and
produce the characteristic endocrine syndrome.
Treatment is best achieved by complete excision.
Symptoms and Signs of mediastinal masses in general
May be asymptomatic, and symptoms may be chest pain, dyspnea (due to the
bulk of the tumor compressing, or invading the trachea, pleural effusion, or
pericardial effusion), and cough. other less common symptoms are dysphagia,
hoarseness, Horner’s syndrome, palpitation, malaise, weakness, and weight loss.
Signs are lymphadenopathy, distended neck veins, plethora and hyper-reflexia.
Neurogenic lesions may encroach on the spinal canal giving signs of cord
compression. Systemic syndromes are the hallmark of endocrine process which
include hyper or hypothyroidism, paroxysmal malignant hypertension, and MG.
Rare mesenchymal lesions such as mesothelioma and fibrofarcoma are known to
produce an insulin like substance , leading to hypoglycemia.
Most asymptomatic mediastinal masses discovered by routine chest radiography will
be benign. In contrast, masses presenting with symptoms, in particular pain, are much
more likely to be malignant.
Superior vena caval obstruction
Tumors located behind the sternum have little space in which to grow and
consequently the low-pressure superior vena cava is the first to be compressed.
Venous engorgement of the upper extremities and face occurs and persists even when
the patient sits upright. The most common cause of this syndrome is carcinoma of the
bronchus and radiotherapy usually provides good symptomatic relief.
Tracheal and esophageal compression
This may be extrinsic or from mural invasion; symptoms include dysphagia,
dyspnea and occasionally stridor. Radiotherapy or intraluminal stenting may provide
some relief.
Neural invasion
A left-sided hilar lesion may infiltrate the recurrent laryngeal nerve and paralyze
the vocal cords leading to hoarseness and a bovine cough. Paralysis of the phrenic
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nerve causes a raised hemidiaphragm on the affected side and indicates irresectabiliry.
Horner’s syndrome is a result of invasion of the sympathetic chain superiorly.
Pericardial invasion
Direct invasion may cause changes similar to pericarditis with arrhythmias and
ECG changes. Chronic tamponade may occur from the slow accumulation of fluid in
the pericardium.
Neural tumors
Invasion of the spinal cord may lead to progressive paraplegia.
Investigation
Investigation of a mediastinal lesion follows the same pattern as investigation for
pulmonary lesions with more emphasis on radiology, mediastinotomy and
mediastinoscopy.
Blood work
Β HCG and AFP are used as adjunctive measures in the diagnosis of anterior
masses, and for follow up treatment.
Conventional radiography
CXR, barium esophagography, myelography.
CT scan
As a staging modality.
MRI
It is better than CT in detecting soft tissue and vascular extension of the tumor,
and it has become the diagnostic modality of choice for evaluation of neurogenic
tumors, suspected vascular anomalies, and processes that involve aortic arch and it’s
branches.
Biopsy
FNA: less useful here because of the mass being central, or because the diagnosis
of many tumors need large biopsy tissue to be precise.
Mediastinoscopy and thoracoscopy (which is an excellent approach for
inaccessible posterior masses).
Sternotomy or thoracotomy.
Treatment
If a resection is planned, the best approach to the anterior and superior
mediastinum is through a median sternotomy. The posterior mediastinum can be
reached through a postero-lateral thoracotomy at the appropriate level. Any operation
must be carefully planned because the pitfalls are numerous and may be difficult or
impossible to counter.
Dumbbell tumors are approached intraspinally, then by thoracotomy(in same
cession).
Chemotherapy and radiotherapy are used for unresectable tumors, or to control
local pressure or invasive symptoms.