ANATOMY AND CLINICAL CONSIDERATIONS
The mediastinum is the median portion of the thoracic cavity, occupying the space between the lungs and containing all of the thoracic structures except the lungs. It is limited by the pleura of the right and left lungs laterally, by the sternum anteriorly, and by the vertebrae and ribs posteriorly. Despite its limited extent, the mediastinum is the site of a variety of inflammatory and neoplastic processes. Most can be recognized by percutaneous needle biopsy, as was initially demonstrated by Dahlgren and Nordenstrøm (1966)
and again in subsequent publications. Cytology of sputum and bronchial secretions, with rare exceptions, has no significant diagnostic role.
Many mediastinal lesions are asymptomatic and discovered as an incidental finding on chest roentgenograms. Topography is an important factor in making a diagnosis
because the different compartments of the mediastinum are likely to harbor different tumors or other lesions (Fig. 37-1
and Table 37-1
). Also of great importance is the age of the patient,
since many of the mediastinal lesions are age-related. The most important clinical questions to be answered by aspiration cytology are: (1) is the space-occupying lesion neoplastic, inflammatory, or a malformation; (2) if the lesion is neoplastic, is it benign or malignant; and (3) if it is malignant, is it best treated by surgical resection or by irradiation and chemotherapy? At all times the cytologic findings must be complemented by knowledge of the clinical, roentgenologic, and laboratory data.
The anterior and middle compartments are the most common sites of mediastinal tumors and benign, space-occupying lesions that include hyperplastic lymph nodes, granulomatous inflammation, and sclerosing mediastinitis.
Although space-occupying benign lesions and primary tumors of the mediastinum are less common than metastatic tumors, they usually present a much greater diagnostic challenge, and therefore are the most likely target of a transcutaneous needle aspiration biopsy. It is essential to accurately identify these lesions before appropriate therapy can be instituted. As mentioned above, close attention to the clinical findings, the location of a lesion in one of the compartments of the mediastinum (see Table 37-1
), and the age of the patient are important factors in reaching the correct diagnosis.
Histology of Normal Thymus
The normal thymus varies in size according to age. It is large in infants and children, increases slowly in size until puberty (but relatively more slowly than the growth of the child), and then undergoes progressive atrophy by apoptosis of component cells. In adults it is a vestigial structure, although it retains its basic histologic features.
The thymus is a lymphoid organ supported by a network of anastomizing large epithelial cells
) joined by desmosomes. Structures known as Hassall’s corpuscles,
which are similar to squamous pearls and are dispersed throughout the thymus, are formed by epithelial cells (Fig. 37-2D
). During the maturation in the thymus, T-lymphocytes
(so named because of their thymic origin) are located within spaces formed by the epithelial cells. The lymphocytes, enveloped by the epithelial cells, form so-called lymphoepithelial complexes. T-lymphocytes, unlike the B-lymphocytes, express both CD-4 and CD-8 antigens. In an interesting observation by Nerurkar and Krishnamurthy (2000)
, which has not yet been confirmed by others, penetration of lymphocytes into the epithelial cells (or emperipolesis
) was demonstrated in imprint cytology of the normal thymus.
Figure 37-1 Diagrammatic representation of mediastinal compartments.