[Pathology] Atlas of Thymic Epithelial Neoplasms (part 1), Thymic Epithelial Neoplasms, Atlas of Mediastinal Pathology
Thymic epithelial neoplasms are the most common primary tumors of the anterior mediastinum. The terminology and classification of these tumors have been a topic of debate for many years and have continued to evolve as we have gained a better understanding of their pathogenesis and biologic behavior. Thymic epithelial neoplasms comprise a spectrum of lesions that can range from very low-grade, well- differentiated tumors to high-grade, poorly differentiated neoplasms, with a wide range of morphologies in-between. The most widely used classification is that proposed by the World Health Organization (WHO), which consists of several categories and utilizes mostly a combination of numbers and letters to designate the various types; however, a more simplified classifi cation scheme has been proposed by Suster and Moran that stratifies these tumors into three categories based on their degrees of differentiation and clinical behavior (Table 2.1). A confusing situation also exists with the persistent use of the terms “thymoma” and “thymic carcinoma” to refer to the ends of the morphologic spectrum in these tumors, implying that one is malignant and the other benign, when in reality all primary thymic epithelial neoplasms have the potential for aggressive or malignant behavior irrespective of their histologic appearance. For the time being, we will utilize the term “thymic carcinoma” in this chapter for tumors that are cytologically obviously malignant, following the standard and traditional nomenclature utilized by the majority of investigators and reserve the term “thymoma” for those tumors that fall mainly in the low-grade and intermediate range of the morphologic spectrum.
Well- and moderately differentiated (low- and intermediategrade) tumors are traditionally designated as thymomas. These tumors represent slow-growing, low- to intermediategrade neoplasms that retain at least some of the organotypical features of the normal thymus (e.g., lobulation, biphasic cell population composed of thymic epithelial cells admixed withlymphocytes, and dilated perivascular spaces). The tumors are often bulky and usually surrounded by a thick fibrous capsule that can contain calcifi cations. They usually manifest in the anterosuperior mediastinum, although rare cases can occur in ectopic locations, including the lung and perihilar, pleural, and posterior mediastinal locations. Thymomas are associated with paraneoplastic syndromes in up to 50 % of cases, particularly in myasthenia gravis but also in aplastic anemia, essential thrombocytosis, hemolytic anemia, hypogammaglobulinemia, mixed collagen-vascular diseases, myopathies, pure red-cell aplasia, systemic lupus erythematosus, and others. Thymomas can also undergo progression to malignancy and give rise to full-blown thymic carcinoma arising from a preexisting, more benign-appearing thymoma.
Poorly differentiated (high-grade) primary thymic epithelial neoplasms are designated by the conventional name of thymic carcinoma and are seldom associated with myasthenia gravis or other paraneoplastic syndromes. These tumors usually manifest a much more aggressive behavior and have
a generally poor prognosis despite aggressive therapy. Thymic carcinomas histologically resemble their counterparts in other organs (i.e., squamous cell, adeno-, clear cell, spindle-cell, and anaplastic carcinoma). A large number of histologic variants have been described, some of which have shown a tendency to be associated with secondary cystic changes such as basaloid and mucoepidermoid carcinoma.
The diagnosis of thymic carcinoma is, in general, a diagnosis of exclusion, since there is very little that is distinctive or pathognomonic about these tumors other than their location in the mediastinum. Morphologically, it can be very difficult to separate them from similar cancers occurring at other organs.
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Fig. 2.1 Thymoma at autopsy; incidental finding in a 54-year-old man
who died of complications of hypertension and pulmonary embolism
shows a large, fleshy, anterior mediastinal mass that is focally infiltrating adjacent pleura and lung parenchyma
Fig. 2.2 Cytologic appearance of the two basic types of thymoma in
the WHO classification: (a) WHO type A is composed of elongated
cells with spindle or oval nuclei, dispersed nuclear chromatin, and an
inconspicuous rim of cytoplasm. The cells are devoid of atypia.
(b) Thymomas WHO type B are composed of cells with round
vesicularnuclei with prominent, single eosinophilic nucleoli surrounded by an
ample rim of lightly eosinophilic or amphophilic cytoplasm. Notice the
abundance of small lymphocytes surrounding the larger epithelioid cells
Fig. 2.3 Gross appearance of spindle-cell thymoma (WHO type A) shows
a nodular fleshy appearance with homogeneous rubbery tissue. Notice that
the tumor is well circumscribed and surrounded by a thin fibrous capsule
Fig. 2.4 Typical histologic appearance of spindle-cell thymoma (WHO
type A) shows a fascicular proliferation of spindle cells arranged in
short fascicles with no discernible atypia. A few scant small lymphocytes are
seen scattered among the spindle cells
Fig. 2.5 Higher magnification of spindle-cell thymoma shows sheets
and fascicles of small, bland-appearing spindle cells containing scattered small lymphocytes
Fig. 2.6 Spindle-cell thymoma with storiform pattern. The tumor is
characterized by short storiform structures resembling the pattern of
growth of cutaneous dermatofibrosarcoma protuberans. Notice, however,
the bland appearance of the spindle cells with a sprinkling of small
lymphocytes. Tumors with these features can be mistaken for sarcomas
of fi brohistiocytic type or solitary fibrous tumors
Fig. 2.7 A spindle-cell thymoma with a whorling pattern shows discrete
rounded areas composed of concentrically aligned spindle cells
superficially resembling meningothelial whorls
Fig. 2.8 Variation in the theme of spindle-cell thymoma with whorling
pattern showing gradual sclerosis and hyalinization of the cellular
whorls, resulting in a giant rosette-like structure
Fig. 2.9 Higher magnification of the preceding image shows discrete,
rounded areas of stromal sclerosis containing a sparse population of
small round to oval cells surrounded by fascicles of spindle cells
To be continue: [Pathology] Atlas of Thymic Epithelial Neoplasms (part 2)
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