[Pathology] Atlas of Reactive, Developmental, Inflammatory, and Tumorlike Conditions (part 1)

[Pathology] Atlas of Reactive, Developmental, Inflammatory, and Tumorlike Conditions (part 1)

A variety of reactive, developmental, inflammatory, and tumorlike conditions can occur in the mediastinum. Congenital and developmental cysts can present as a mass lesion in the mediastinum and may occur in a variety of settings. Congenital cysts usually occur in younger patients and are unilocular and small; they can arise anywhere along the anatomic course of embryonic descent of the thymus, including the neck. Developmental cysts can arise from displaced or ectopic remnants, such as foregut cysts and enteric duplication cysts. Acquired cysts can arise as a result of underlying infl ammatory processes and can grow to be quite large and multiloculated. Thymic hyperplasia is another reactive process that presents as enlargement of the thymus and clinically can simulate a malignancy 

Inflammatory conditions affecting the mediastinum include acute and chronic infl ammation (mediastinitis), granulomatous processes related to sarcoidosis (affecting primarily mediastinal lymph nodes) or fungal infection, and end-stage ibrosing inflammation resulting in idiopathic sclerosing mediastinitis. Other developmental abnormalities that occur with some frequency in the mediastinum are the presence of ectopic thyroid or parathyroid tissue, which may give rise to tumor growths secondary to hyperplasia, benign nodules, or development of malignancy. 

Finally, a variety of benign tumorlike conditions in this anatomic compartment can lead to tumor masses that may be confused with malignancy, including thymolipoma, thymofibrolipoma, and Castleman’s disease. 

Fig. 1.1 Congenital thymic cyst shows a distended, unilocular cavity

lined by thin translucent walls and filled with fl uid. Notice the portion

of normal thymus attached at the end of the cyst. This tumor developed

in a 15-year-old boy who presented with stridor and shortness of breath

owing to the large size of this particular cyst (approximately 10 cm in

greatest diameter)  

Fig. 1.2 Histologic appearance of a congenital thymic cyst, showing a

single layer of flattened cuboidal epithelium. The lining in congenital

thymic cysts can also contain stratified squamous epithelium and sometimes 

columnar ciliated epithelium. The contents of the cyst usually consist of clear, serous fluid. 

Note the absence of inflammation in the wall of the cyst  

Fig. 1.3 Another area in the wall of a congenital thymic cyst. Notice a

small island of involuting thymic remnants beneath the lining in the

wall of the cyst (arrows). This thymic remnant is composed of blandappearing 

spindle cells with dispersed nuclear chromatin and scant

cytoplasm reminiscent of the involuting thymus. Thymic remnants are

rarely identified in the walls of congenital thymic cysts

Fig. 1.4 Foregut cyst of the mediastinum. This 4-cm cyst was incidentally found 

in a 23-year-old woman during a routine chest X-ray and

showed a thickened, fi brous wall with a smooth and shiny outer surface.

It was located at the bifurcation of the trachea and main bronchi and

was easily detached and removed by blunt dissection. Foregut cysts

most commonly arise in the middle or posterior mediastinum. They can

occasionally communicate with the trachea or main bronchi and are

more common in young adults and children, although older individuals

also may be affected  

Fig. 1.5 Histologic examination of a foregut cyst shows a lining composed of 

columnar ciliated epithelium, with hyaline cartilage, smooth

muscle, and mucus glands in the walls of the cyst. Infection is a common 

complication and may lead to lung abscess formation in cases

associated with a tracheobronchial fistula

Fig. 1.6 At higher magnification, a portion of a foregut cyst shows

immature (spindled), stratified squamous epithelium with welldeveloped intercellular bridges showing partial maturation of the luminal surface toward ciliated columnar epithelium (arrow). The

identification of cartilage in the wall of the cyst is the most reliable way

to identify cysts that originate from the bronchi. Otherwise, the term

“foregut cyst” would be an appropriate designation 

Fig. 1.7 Foregut cyst showing a lining composed of columnar ciliated

epithelium in close proximity with clusters of residual involuting thymic epithelium in the wall of the cyst (arrowheads). The thymic epithelium in such instances can undergo hyperplastic changes, not to be confused with the development of thymoma

Fig. 1.8 Example of a foregut cyst of an enteric type presenting as a mass

in the posterior mediastinum in a 15-year-old boy with dysphagia. The

lining of the cyst is composed of gastric-type epithelium with a readily

identifiable muscularis propria. Occasionally, ciliated columnar epithelium can 

be identifi ed in these cysts when they arise from the esophagus.

Another term used for these cysts is “enteric duplication cyst.” The lining

can be of either a gastric or enteric type. Rarely, some foregut cysts show

admixtures of gastric, enteric, and bronchial epithelium

Fig. 1.9 Mesothelial (pericardial) cyst incidentally found at autopsy

(arrows). The cyst shows a smooth, translucent wall bulging from the

pericardium. The cyst was filled with clear, serous fl uid. Similar cysts

can occur higher in the anterior mediastinum and arise from the pleural

refl ection; these are designated as “pleural mesothelial cysts”  

Fig. 1.10 Histologic appearance of a mesothelial cyst of the anterior

pericardium shows a single layer of round to polygonal mesothelial

cells. Rarely, the mesothelium can show foci of papillary mesothelial

hyperplasia, not to be confused with malignant mesothelioma  

Continue part 2: [Pathology] Atlas of Reactive, Developmental, Inflammatory, and Tumorlike Conditions (part 2)

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