Introduction
The thymus is an anterior/superior mediastinal organ, composed predominantly of T lymphocytes, scattered Hassall’s corpuscles, and epithelial cells. Thymic lesions include non-neoplastic (thymic cysts, thymic hyperplasia) and neoplastic lesions (thymoma, thymic carcinoma, neuroendocrine tumors, germ cell tumors, and stromal tumors).1 Fine needle aspiration (FNA) has proven to be a safe and reliable procedure to diagnose thymoma or thymic carcinoma. However, it can be challenging to diagnose and/or classify these lesions due to the overlapping morphologic and immunohistochemical features among the subtypes of thymoma, thymic carcinoma, and other neoplasms.
Thymoma
Thymoma is the most common thymic neoplasm, but its incidence is only 0.13 cases/100,000 population/year.2–5 Thymoma patients can be asymptomatic or present with local symptoms such as chest pain and superior vena cava syndrome, or paraneoplastic syndromes such as myasthenia gravis, hypogammaglobulinemia, and/or red blood cell aplasia.6–9
Thymoma consists of neoplastic epithelial cells and benign T lymphocytes.2–4 Based on the histologic morphology of neoplastic epithelial cells and the ratio of neoplastic epithelial cells to reactive lymphocytes, thymoma is categorized into type A, type AB, type B (B1, B2, and B3), and rare subtypes (metaplastic thymoma and micronodular thymoma with lymphoid stroma) by the World Health Organization classification system (Table 1).1,10
Table 1Cytologic features of different subtypes of thymoma
| WHO type | Cytologic features | Clinical outcome |
|---|
| A | Predominantly spindle neoplastic epithelial cells with scant lymphocytes | Benign |
| AB | Mixed spindle and round/polygonal neoplastic epithelial cells | Benign |
| B1 | Predominantly lymphocytes with scattered round or polygonal neoplastic epithelial cells | Low malignant potential |
| B2 | Mixed lymphocytes and round or polygonal neoplastic epithelial cells | Moderate malignant potential |
| B3 | Predominantly round or polygonal neoplastic epithelial cells with minimal lymphocytes | Moderate malignant potential |
Morphology
Histologically, type A thymoma is composed of predominantly spindled epithelial cells and rare lymphocytes. Similarly, individual cells or clusters of spindle cells with bland nuclei, minimal nucleoli, and smooth chromatin are the predominant cells by FNA cytology of type A thymoma, and small lymphocytes are scant (Fig. 1).
Type AB thymoma exhibits morphologic features of neoplastic epithelial cells of both type A (spindle) and type B (round/polygonal). A sharp transition between these two morphologies is often seen by histology. FNA of type AB thymoma shows mixed spindle and round or polygonal neoplastic epithelial cells with a variable number of small lymphocytes (Fig. 2).
Type B thymoma is divided into three subtypes: B1, B2, and B3 based on the ratio of neoplastic epithelial cells to small reactive lymphocytes. Although the ratio can be roughly evaluated by cytology, it is unclear if it can be used to accurately classify these subtypes.11–15
Histologically, type B1 thymoma shows abundant reactive small lymphocytes with medullary islands and few scattered round/polygonal epithelial cells. By FNA cytology, type B1 thymoma demonstrates cellular materials with abundant lymphocytes and very scant round/polygonal neoplastic epithelial cells. Sometimes, immunostaining of a cell block section becomes necessary to distinguish the epithelial cells because they are difficult to visualize on smear slides (Fig. 3).
Type B2 thymoma is also rich in lymphocytes but demonstrates more round/polygonal epithelial cells than type B1. FNA of type B2 shows both round/polygonal neoplastic epithelial cells and reactive lymphocytes (Fig. 4).
Histologically, type B3 is composed of abundant round/polygonal epithelial cells but scant lymphocytes. Similarly, by FNA cytology, type B3 is cellular with abundant round/polygonal neoplastic epithelial cells and scant lymphocytes. In addition, mild atypia can be seen (Fig. 5).
Thymoma also includes several rare subtypes: micronodular thymoma with lymphoid hyperplasia and metaplastic thymoma. Micronodular thymoma with lymphoid hyperplasia is composed of discrete nodules of neoplastic epithelial cells separated by lymphoid stroma. The neoplastic epithelial cells can be spindly or round/polygonal in shape. Metaplastic thymoma is composed of islands of neoplastic epithelial cells separated by fibroepithelial spindle cell stroma.1
Immunohistochemistry
The neoplastic epithelial cells of thymoma are positive for cytokeratins (AE1/3, CK5/6, etc.) and p63 (p40). The reactive T lymphocytes are positive for CD3. Moreover, CD1a and TdT are positive in thymocytes but may be absent in type A or AB thymoma. Some type A and type AB thymomas may show focal CD20 staining in neoplastic epithelial cells. The neoplastic epithelial cells are negative for CD117 and CD5, except for rare type B3 thymomas that exhibit focal CD117 staining.16,17 Polyclonal PAX8 has been shown to be positive in thymic epithelial neoplasms, including thymic carcinoma (77%), type A thymomas (100%), and type B thymomas (93%).18
Differential diagnosis
The differential diagnosis for type B1 thymoma contains lymphomas such as acute T-lymphoblastic lymphoma (leukemia) (T-ALL/lymphoblastic lymphoma), since both have abundant immature T lymphoid cells that exhibit a similar immunophenotype (TdT+, CD3+, and CD99+). T-ALL/lymphoblastic lymphoma occurs predominantly in children, while thymoma more often occurs in adults. The presence of neoplastic epithelial cells provides evidence to diagnose thymomas; however, it is often difficult to identify epithelial cells on aspirate smears. Immunohistochemistry for cytokeratins or others stains in a cell block may help to identify the presence of rare epithelial cells. T-ALL cells are larger with an irregular nuclear contour in comparison with small immature T lymphocytes in type B1 thymoma.
The most important differential diagnosis for type B3 thymoma (also known as atypical thymoma) includes primary thymic carcinoma and metastatic carcinoma because the neoplastic epithelioid cells from type B3 thymoma may resemble thymic carcinoma cells or metastatic carcinoma cells. Type B3 thymoma cells usually show mild-to-moderate pleomorphic nuclei. On the contrary, thymic carcinoma or metastatic carcinoma cells have irregular nuclei with prominent pleomorphism, distinct nucleoli, and coarse chromatin. Necrosis and mitosis are less common in type B3 thymoma. Immunophenotypically, type B3 thymoma epithelial cells are negative for CD5 and CD117, which are usually positive in thymic carcinoma. Metastatic carcinomas are usually positive for their specific site markers. For example, breast carcinoma is positive for GATA3, lung adenocarcinoma is positive for TTF1, gynecologic carcinoma is positive for PAX8, etc.
Type A thymoma may need to be differentiated from other spindle cell lesions such as a solitary fibrous tumor, schwannoma, and spindle neuroendocrine tumor, etc. Immunohistochemical studies can provide necessary information to differentiate type A thymoma from these entities.
Thymic carcinoma
Thymic carcinomas are rare and constitute approximately 20% of thymic epithelial neoplasms.19,20 Patients usually present with mass-related symptoms, weight loss, and anorexia, or they are asymptomatic. Paraneoplastic syndromes are uncommon in thymic carcinoma patients. Those with thymic carcinomas usually have poor clinical outcomes.19,20
Thymic carcinomas include multiple histological types, with squamous cell carcinoma accounting for 80% of cases. Nonsquamous thymic carcinomas include basaloid carcinoma; lymphoepithelioma-like carcinoma; mucoepidermoid carcinoma; adenocarcinoma, not otherwise specified; low-grade papillary adenocarcinoma; enteric-type adenocarcinoma; adenosquamous carcinoma; clear cell carcinoma; nuclear protein in the testis (NUT) carcinoma; sarcomatoid carcinoma; undifferentiated carcinoma; thymic carcinoma with adenoid cystic carcinoma-like features; and thymic carcinoma, not otherwise specified.21–23 These carcinomas morphologically resemble their counterparts in other anatomic sites.
Squamous cell carcinoma of the thymus can be keratinized or nonkeratinized and has a similar morphology as squamous cell carcinoma from other organ systems. It shows large malignant squamoid cells with round or ovoid nuclei and coarse chromatin. Necrosis and mitosis are common.
Lymphoepithelioma-like carcinoma shows irregular cords and islands of tumor cells separated by connective tissue stroma containing dense lymphoid infiltrates (lymphoepithelioma-like pattern) or minimal lymphoid infiltrates (desmoplastic pattern). The neoplastic epithelial cells are poorly differentiated with round or ovoid nuclei, vesicular chromatin, and prominent eosinophilic nucleoli. Pleomorphic and multinucleated tumor cells as well as comedonecrosis can be seen. Cytologically, the smears are cellular with predominant variable-sized round or ovoid epithelial cells and scattered small lymphocytes. The neoplastic epithelial cells have round-to-ovoid nuclei, vesicular chromatin, and prominent eosinophilic nucleoli. Lymphoepithelioma-like carcinoma cells are positive for cytokeratins, CD5, and CD117 (Fig. 6).
NUT midline carcinoma, also called NUT carcinoma, is a rare thymic carcinoma with a poor prognosis.24 NUT carcinoma is poorly differentiated with small-to-intermediate basaloid cells and abrupt squamous differentiation with keratinization. By FNA cytology, it shows discohesive monotonous cells that have a scant cytoplasm and small-to-intermediate round or ovoid nuclei. Dyskeratotic cells may be present on smears, but squamous differentiation and keratinization are not seen.25–27 A diagnosis of NUT carcinoma can be confirmed by nuclear staining of NUT by immunohistochemistry or NUT1 gene rearrangement by fluorescence in-situ hybridization.
Besides type B3 thymoma, thymic carcinomas must be differentiated from metastatic carcinomas, which are much more common. Morphological analysis is not very helpful due to significant overlapping. A history of malignancy in another site favors metastasis, while positive staining of CD5 and CD117 favors thymic carcinomas.
Conclusion
FNA has increasingly become an important technique to acquire tissue in order to diagnose thymic lesions of the mediastinum; however, a systemic approach should be considered, including the medical history, clinical and radiologic findings, cytomorphologic findings, and ancillary results. In the meantime, it is always important to obtain adequate materials for both diagnostic and therapeutic purposes using ancillary testing.
Abbreviations
- FNA:
fine needle aspiration
- NUT:
nuclear protein in the testis
Declarations
Funding
None.
Conflict of interest
The authors have no conflicts of interest related to this publication.
Authors’ contributions
Manuscript writing and critical revision (ZL and PW). All authors have made a significant contribution to this study and have approved the final manuscript.