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Small mediastinal masses may not be visible or result in only subtle radiographic findings. Large lesions may result in a focal opacity or abnormalities involving the mediastinal lines, stripes, and interfaces. The lateral chest radiograph is beneficial for demonstrating abnormalities in the retrosternal space or overlying the upper thoracic spine which may not be visible on the posteroanterior radiograph. There are multiple signs which have been described on chest radiographs that can be useful in suggesting the presence of a mediastinal abnormality.

One of these signs is known as the silhouette sign, and describes the loss of normal borders of intrathoracic structures. For example, a mass in the left anterior mediastinum may obscure the left heart border. Another helpful sign is the hilum overlay sign, which may help differentiate a mediastinal lesion from other abnormalities such as cardiomegaly or enlarged pulmonary vessels.

The imaging modality of choice for identifying, localizing, and characterizing most mediastinal masses is CT.

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A study of prevascular mediastinal masses of various etiologies by Tomiyama and colleagues showed that CT was equal or superior to MR imaging in correctly diagnosing most of the lesions, except for thymic cysts Specific information that should be identified on CT and reported in the clinical report include the following: I location, size, morphology, shape, and margins; II density, heterogeneity and enhancement; III fat, cystic components, soft tissue, and calcification; and IV connection with or invasion of adjacent structures.

Although all of these features are important for thorough characterization, some of them are more diagnostically relevant than others. As an example, the presence of fat in a prevascular mediastinal mass on CT narrows the differential diagnosis considerably, and includes lesions such as mature teratoma, thymolipoma, and, less commonly, lipoma and liposarcoma. In contrast, calcifications are nonspecific and may be associated with benign or malignant mediastinal lesions.

MR imaging is not routinely performed to evaluate all mediastinal masses; however, it is the best modality for distinguishing cystic from solid masses e. A variety of MR imaging protocols have been developed to evaluate the mediastinum, most of which include T1-weighted, T2-weighted, T2-weighted fat-saturated, in-phase and out-of-phase gradient echo GRE , and pre- and post-gadolinium enhanced sequences 23 , Although breath hold or respiratory gating techniques may be utilized, the former is usually preferred given its greater reliability in halting respiratory motion and preventing artifacts.

How to Interpret a Chest X-Ray (Lesson 5 - Cardiac Silhouette and Mediastinum)

Cardiac gating can be achieved with either electrocardiogram ECG gating or peripheral gating, the former of which is preferred due to more reliable halting of cardiac motion and elimination of pulsatility artifacts 23 , The presence of microscopic or intravoxel fat is best evaluated with chemical shift imaging which includes in-phase and out-of-phase GRE sequences. Thymic hyperplasia may manifest as a focal soft tissue mass in the prevascular mediastinum and mimic soft tissue neoplasms such as thymic epithelial tumors; however, chemical shift imaging can distinguish between these entities, as thymic hyperplasia and normal thymus lose signal on out-of-phase sequences due to the suppression of microscopic fat interspersed between non-neoplastic thymic tissue whereas soft tissue neoplasms do not 25 - Thymic hyperplasia and normal thymus typically show CSRs of 0.

In contrast, thymic epithelial neoplasms, lymphoma, and other soft tissue neoplasms exhibit CSRs of 0. Values reached are the percentage of signal drop on out of phase images. DWI and DCE imaging have shown promise in early studies in differentiating between types of thymoma and other types of prevascular mediastinal masses. In one study, an apparent diffusion coefficient ADC value of 1.

Benveniste and colleagues evaluated 38 mediastinal masses with DWI and found that a mean ADC value was statistically significant in differentiating benign from malignant masses, benign thymic lesions from thymic epithelial neoplasms, and thymic epithelial neoplasms from non-thymic mediastinal neoplasms However, differentiation between low-grade thymomas, high-grade thymomas and thymic carcinomas and early from late stage thymomas with ADC values was not possible.

Sakai and colleagues evaluated 59 patients with sequential imaging at 30 seconds for 5 minutes following the administration of IV gadolinium contrast material and reported that low-risk thymomas Masaoka stages I and II showed rapid time-to-peak enhancement with a mean time of 1. In contrast, the mean time to peak enhancement for stage III thymomas was 2. Other etiologies such as thymic carcinoma, lymphoma, malignant germ cell tumor and thymic carcinoid demonstrated gradual enhancement over time with a mean time-to-peak enhancement of 3.

Such patients should undergo MR imaging with specific fluid-sensitive sequences, not unenhanced chest CT. Jerushalmi and colleagues showed that FDG uptake in thymic hyperplasia is highly variable and in can significantly overlap with that of mediastinal malignancies, with SUVmax as high as 7.

Multiple studies have been performed to investigate its ability to differentiate between malignant and benign lesions and between specific types of malignant tumors. In one study, Kubota et al. The mean FDG uptake of malignant neoplasms was significantly higher than that of benign tumors.

The authors suggested that a cutoff value of 3. A threshold SUVmax of 4. The limited specificity Sung et al. Localization of mediastinal abnormalities to specific compartments on imaging examinations is crucial; however, this may be difficult when large lesions involve multiple compartments or extend from one compartment to another.


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The center method states that the center of a lesion, defined as its center point on the axial CT image on which the lesion has the greatest diameter, localizes the mass to a specific compartment. When large lesions are present that displace organs from other mediastinal compartments, the structure displacement tool is particularly helpful. For instance, a very large visceral mediastinal mass may result in the anterior displacement of prevascular compartment structures such as the thymus or mediastinal fat.

The true incidence of prevascular mediastinal masses is difficult to establish for multiple reasons. The most significant confounding factor is the fact that different mediastinal division systems have been used in published studies Another is the variability with which both non-neoplastic abnormalities such as thymic hyperplasia and thymic and pericardial cysts and other neoplastic lesions such as lymphoma have been included and the nomenclature used The most common primary tumors of the prevascular mediastinal compartment include thymic epithelial neoplasms thymoma, thymic carcinoma, and thymic neuroendocrine tumors and lymphoma.

Thymoma is the most common mass and primary neoplasm of the prevascular mediastinum, with the highest incidence in middle-aged patients. Mature teratoma, nonteratomatous germ cell malignancies such as seminoma and nonseminomatous germ cell neoplasms, and metastatic disease represent other benign and malignant neoplasms that may occur. Nonneoplastic lesions include substernal extension of thyroid goiter, thymic hyperplasia, cystic lesions such as thymic and pericardial cysts, and vascular-lymphatic abnormalities.

Substernal extension of a thyroid goiter and an ectopic thyroid goiter occurring in the mediastinum manifest as heterogeneous prevascular mediastinal masses that demonstrate continuity with the cervical thyroid gland, are intrinsically hyperdense with HU values of 70—85 due to the presence of iodine , and show intense and sustained enhancement following the administration of IV contrast Figure 1. Internal regions of low density due to cystic changes and foci of high density representing calcifications may be present.

Direct connection with the cervical thyroid gland is typically appreciable in the case of substernal extension. The most common lesion is a mature teratoma, a benign germ cell neoplasm that typically demonstrates a combination of fat, fluid, calcification, and soft tissue Figure 2. Bone and tooth-like elements have been reported within these lesions 40 , Although one or more fat-fluid levels are highly specific for mature teratoma, this is much less common than gross macroscopic fat 42 Figure 3.

Other fat-containing abnormalities in the prevascular mediastinum include neoplasms such as thymolipoma, lipoma, and liposarcoma and non-neoplastic abnormalities such as thymic hyperplasia. Thymolipomas usually occur in a cardiophrenic angle, grow very large average reported size of 20 cm at presentation , and demonstrate a direct connection with the thymus. Patients may be asymptomatic or report clinical symptoms due to local mass effect.

INTRODUCTION

On CT, lipomas manifest as encapsulated lesions composed principally of fat and a small amount of soft tissue and blood vessels. Liposarcomas are malignant neoplasms that typically display aggressive features that can help distinguish them from lipomas, thymolipomas, and other fat-containing lesions such as a greater proportion of soft tissue components, local invasion, intrathoracic lymphadenopathy, and metastatic disease 45 , A variety of cystic lesions, defined as those that have water or fluid density with HU values between 0 and 20 on CT, may be encountered in the prevascular mediastinum.

One of the most common of these is a thymic cyst, which appears as a well-circumscribed homogeneous lesion near the thymic bed that is round, oval, or saccular in configuration. Thymic cysts may be congenital or acquired, the latter of which are much more common and are associated with inflammation; iatrogenic processes such as surgery, radiation therapy, or chemotherapy; and malignant neoplasms.

Prevascular masses that are purely cystic based on HU values with no soft tissue components or internal septations most likely represent unilocular thymic cysts 47 Figure 5. Other thymic cysts may demonstrate regions of much higher attenuation due to the presence of hemorrhagic or proteinaceous components. In this setting, MR imaging should be performed, as it is superior to CT in its ability to distinguish between cystic and solid lesions and identify cystic and solid internal components within complex lesions Figure 5.

In the clinical setting of symptoms related to myasthenia gravis or other paraneoplastic syndromes, especially in men and women older than 40 years, the diagnosis of cystic thymoma should be strongly considered. Although mature teratomas may demonstrate internal fat at multidetector CT, a large percentage of these lesions manifest as predominantly or entirely unilocular or multilocular thin-walled cystic masses in the prevascular mediastinum. In contrast to simple cysts, cystic teratomas are typically associated with additional features such as internal septa and soft-tissue components and may enhance after administration of intravenous contrast material.

The Mediastinum: Radiologic Correlations with Anatomy and Pathology

A well-circumscribed unilocular mass of fluid attenuation with thin or imperceptible walls localized to a cardiophrenic angle can be confidently diagnosed as a pericardial cyst. These benign nonneoplastic lesions arise from aberrations in the formation of coelomic or somatic cavities and more commonly arise in the right costophrenic angle than in the left, although they may be seen as high as the pericardial recesses at the level of the proximal aorta and pulmonary arteries Although pericardial cysts are always connected to the pericardium, only a small number of cases demonstrate this communication at the time of surgery.

Although CT findings may be sufficient to suggest a specific diagnosis or narrow the differential diagnosis for some prevascular mediastinal abnormalities, in other cases a combination of imaging and clinical information is necessary to guide further management. This is most common when prevascular masses are predominantly soft tissue, as several benign and malignant conditions can result in such an appearance.

Normal thymic tissue is usually only observed in young patients and decreases with advancing age. Complete fatty replacement is typically seen by 40 years of age. In the setting of new uniform thymic enlargement or soft tissue with the normal bilobed triangular configuration, thymic hyperplasia should be strongly considered. In these patients, CT demonstrates diffuse symmetric thymic enlargement.


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  • Thymic lymphoid follicular hyperplasia is characterized by an increased number of lymphoid follicles with or without an increase in gland size. Patients with immunologic diseases such as myasthenia gravis, hyperthyroidism, collagen vascular diseases, or human immunodeficiency virus HIV infection are typically affected and CT may demonstrate normal thymus, thymic enlargement, or a focal soft tissue mass Rarely, thymic hyperplasia may appear as a heterogeneous mass with regions of low density due to deposition of fat between hyperplastic thymic tissue.

    Thymic hyperplasia may manifest on CT with a nodular or bulky configuration and mimic soft tissue neoplasms such as thymic epithelial tumors, lymphoma, and others. In such cases, patients should undergo further imaging to avoid unnecessary biopsies or surgeries. One option is for patients to be re-evaluated with CT in 3 months, during which time a decrease in thymic size is typically seen.

    Another option is further evaluation with chemical shift MR imaging utilizing techniques such as in-phase and out-of-phase gradient-echo sequences. Loss of signal on out-of-phase images due to the suppression of fat interspersed between hyperplastic thymic tissue is typically seen. In contrast, thymic epithelial neoplasms, lymphoma, and other soft tissue malignancies do not lose signal on out-of-phase images 25 - 27 Figure 7. Thymic epithelial neoplasms such as thymoma, thymic carcinoma, and thymic neuroendocrine tumors should be included in the differential diagnosis when a prevascular soft tissue mass is identified on CT.

    Lobulated or irregular contours, internal cystic or necrotic regions, and multifocal calcifications are more suggestive of invasive thymoma than noninvasive thymoma 51 , However, lymphadenopathy and distant metastases are much less common. Lymphoma should be considered in the differential diagnosis when a large soft tissue mass or a group of enlarged lymph nodes is present in the prevascular mediastinum on CT, the former of which suggests a primary mediastinal lymphoma and the latter of which suggests secondary disease Figure Primary mediastinal lymphomas include Hodgkin lymphoma and non-Hodgkin lymphomas such as diffuse large B-cell lymphoma, gray zone lymphoma, and T-cell lymphoblastic lymphoma.

    One imaging feature that can differentiate mediastinal lymphoma from other neoplasms such as thymic epithelial tumors and germ cell neoplasms is infiltrative growth that can encase or encircle vascular structures without evidence of invasion. Core needle biopsy combined with aspiration for flow cytometry or surgical biopsy are typically performed for diagnosis. It can also be used to guide biopsies and surgeries. They are particularly significant because of the difficulty in making a differential diagnosis, as they can simulate multiple lesions, both benign and malignant. Currently, with the improvement in non-invasive techniques, a pre-surgical diagnosis can be made in a high percentage of cases, although it is relatively common for surgery to be indicated to establish the definitive diagnosis.

    One controversial aspect of these tumours is the treatment to give, which ranges from observation to surgical resection, without there being a consensus on the best therapeutic option [1 , 2].

    The aim of this study is to analyse the utility of thoracic computed axial tomography CT in imaging diagnosis of the non-neoplastic mediastinal cysts and the results of surgery in these lesions. Between and , 20 patients underwent surgery in our department with a definitive anatomico-pathological diagnosis of benign mediastinal cyst, this being understood as one of the following: 1 bronchogenic cyst; 2 pleuro-pericardial cyst; and 3 enteric or duplication cyst. The cystic lymphangioma, cystic teratomas and cystic thymomas were excluded because they are neoplastic pathologies and occasionally can be cyst.

    The patients data were obtained from a retrospective review. The preoperative study of mediastinal cystic masses includes a complete blood test, chest radiography and, for the last 15 years, a thoracic CT scan. In the last 3 years the preoperative study has come to include, in cases with dubious diagnoses, nuclear magnetic resonance MRI.

    Of the 20 patients ten corresponded to bronchogenic cysts, seven to pleuro-pericardial cysts and three to enteric cysts two oesophageal duplication and one of a gastric origin. The most common symptom was thoracic pain in four cases followed by dyspnoea in three. Blood tests were normal in all the patients, except in one case, who presented with chronic anaemia which had been under treatment with iron for a year.

    Simple chest radiography revealed a mass effect in the anterior-superior mediastinum in nine cases. CT was done in five patients and revealed a cystic tumour in the anterior mediastinum. MRI was performed in the two most recent cases, which confirmed the presence of the cyst Table 1. All the patients underwent surgery, the approach being a thoracotomy in eight cases and a thoracoscopy in the rest. Only three patients had a preoperative diagnosis of benign mediastinal cyst. The main diagnostic doubt, especially in the pre-CT era, was with thymoma and cystic teratoma in six patients Table 2.

    All the cases had excision of the lesion. The mean size of cyst was 5. During the postoperative period three patients presented with mild complications one wound infection, one minimal haemothorax and one atelectasis. Four patients were asymptomatic, the cyst being a casual radiological finding. The other three presented with dyspnoea. Blood tests were normal in all the patients.

    Simple chest radiography in all cases revealed a right paracardial mediastinal tumour, which was confirmed by CT in the four cases in which it was done Table 1. All the patients underwent surgery: four with thoracotomy, one with sternotomy, and the rest with video-thoracoscopic surgery. In the four cases receiving CT the preoperative diagnosis was a pleuro-pericardial cyst; in the other three there was a diagnostic doubt with thymoma and cystic teratoma Table 2.

    The cyst was removed in all the cases. The mean size of cyst was 8. Two cases presented postoperative morbidity one pneumonia and wound infection; and one asymptomatic right phrenic paralysis. One patient was clinically asymptomatic, another presented with thoracic pain, and the third dysphagia.

    The blood test was normal in two cases, the other revealing microcytic chronic anaemia and hypochromia. A CT scan was done in the two more recent cases, which confirmed the cyst preoperatively Table 2. The third patient underwent surgery diagnosed with pulmonary neoplasia. The approach was thoracotomy in two patients and a midline laparotomy in the other.

    Introduction

    Excision of the cyst was performed in all cases. Pathological anatomy informed of oesophageal duplication cyst in two patients and a triple gastric duplication cyst and leiomyoma of the oesophagus in the third. The mean size of cyst was 4. The three patients are asymptomatic 2, 3 and 28 years after surgery Table 1.

    NNMCs are generally congenital and form around the 6th week of gestation, although pleuro-pericardial cysts can occasionally be acquired [2]. They may appear at any age, although they are more common in the 4th and 5th decades of life, generally detected in a routine radiological study [3] , as observed in our patients.

    There are no differences between sexes except for neuro-enteric cysts, which are more common in females [2]. They may be located in any part of the mediastinum, although bronchogenic cysts have a preference for the mid and superior mediastinum [3] , pleuro-pericardial cysts for the right anterior cardiophrenic angle [4] , and enteric cysts for the posterior mediastinum [2].

    NNMCs in adults usually begin as a incidental radiological finding in asymptomatic patients [5]. They may present a variety of symptoms, particularly coughing and chest pain, which are generally caused by the compression of neighbouring structures [2]. In the absence of complications, clinical features depend on the site of the cyst: paratracheal and carinal cysts may lead to tracheobronchial compression, causing coughing, wheezing, dyspnoea and stridor; para-oesophageal cysts can cause dysphagia, regurgitation and abdominal pain [1 , 2 , 5 , 6]. Neuro-enteric cysts with intraspinal spread can appear with neurological symptoms [7].

    The most serious complication, though fortunately rare, is malignant degeneration [5]. Chest radiography usually shows a well-delimited, homogeneous, spherical mediastinal image. Bronchogenic cysts are typically paratracheal or subcarinal, the pleuro-pericardial adhere to the heart and diaphragm, and the duplication are posterior [4 , 8]. When they are infected or communicate with an airway or digestive tract, an air bubble is produced inside the cyst [5] , as observed in some of our patients. CT has increased the diagnostic performance of non-invasive imaging techniques.

    It shows a well-defined spherical cystic lesion with a watery content of attenuated intensity and delimits its connection with neighbouring structures, especially the oesophagus and airway [2 , 8]. In the pleuro-pericardial cysts the wall is imperceptible and is located paracardially [2].

    When there is communication with the tracheobronchial tree, a gas-fluid level is seen in the cyst [9]. Currently MRI seems to provide a better definition of the cyst and its connection with neighbouring structures than CT, showing low-signal intensity images in T1 sequence and bright-signal intensity images in T2 [8 , 10]. Enteric cysts form during early embryogenesis when the anterior intestine and notochord are close, which is why abnormalities of the vertebral column are usually associated [2].

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    The Mediastinum: Radiologic Correlations With Anatomy And Pathology by E. Robert Heitzman

    For this reason MRI should be done in these patients to exclude the intraspinal spread of posterior mediastinal cysts. Other tests are useful for ruling out complications; for instance, gastro-intestinal endoscopy and bronchoscopy rule out communication of the cyst with the oesophagus or airway. Clinical features and radiology may give us a clue to diagnosis, but no exploratory technique or clinical manifestation is characteristic, as several pathologies can be simulated [2 , 3]. A differential diagnosis must always be made with other cystic pathologies, especially as there are cystic lesions of a benign appearance that can mask malignant neoplastic lesions.

    In our hospital the CT is a exploration routine in this lesions in the last 15—17 years. Actually the radiologic characterisation is important, because one well-defined cyst with a watery content of attenuated intensity in a typical localisation is very suggestive of these benign lesions.


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    • Definitive diagnostic confirmation is anatomico-pathological. The histological study shows an epithelium-coated cyst, varying according to cyst type, inside which bronchial components may be found [3 , 5]. The treatment of choice is complete excision of the cyst, even in asymptomatic patients, in order to prevent complications and establish diagnosis [3 , 5 , 11]. The prognosis after complete excision is excellent [2 , 7] , and the morbidity and mortality surgical rates are low.

      Occasionally a conservative attitude has been considered, with a clinical and radiological follow-up without surgery, especially with pleuro-pericardial cysts [2 , 9] ; however, this is a controversial subject. There are those who recommend conservative treatment as it avoids surgical morbidity and mortality, and there have even been reports of spontaneous resolution of the cyst [12]. Those against an expectant attitude show that resection carries little morbidity and mortality, and many of the patients that do not receive surgery at the time develop symptoms [13] related to cyst growth, which means that an operation then will involve a higher morbidity and mortality rate, together with a risk of malignancy and development of complications [14].

      What is indeed clear is that surgical excision is a must when some of the following criteria are met [5] : 1 symptomatic cyst; 2 suspected malignancy; 3 cyst infection; 4 tracheal compression; 5 progressive growth; 6 presence in children; or 7 atypical location or characteristics. This paper present a surgical series. We have not untreated mediastinal cysts.