| Abstract|| |
The mediastinum harbors a mixed bag of tumors, which may create significant diagnostic dilemmas. These tumors have widely variable therapeutic and prognostic implications. Correct pre-operative cytological diagnosis may obviate the need of surgical excision of many of these lesions. A stepwise algorithmic approach such as clinical history, radiological localization, salient cytomorphology and ancillary test helps in correct diagnosis of these tumors. This paper discusses the stepwise diagnostic algorithm for fine needle aspiration cytology diagnosis of mediastinal tumors.
Keywords: Cytology, diagnosis, fine needle aspiration cytology, mediastinum
|How to cite this article:|
Dey P. Diagnostic dilemma: Diagnostic algorithm in fine needle aspiration cytology of mediastinal tumors. Indian J Pathol Microbiol 2010;53:395-402
|How to cite this URL:|
Dey P. Diagnostic dilemma: Diagnostic algorithm in fine needle aspiration cytology of mediastinal tumors. Indian J Pathol Microbiol [serial online] 2010 [cited 2021 Aug 1];53:395-402. Available from: https://www.ijpmonline.org/text.asp?2010/53/3/395/68241
| Introduction|| |
The mediastinum shows a wide variety of tumors with variable cytomorphology and this may lead to diagnostic problems. ,,,,,,, The diagnosis of the mediastinal tumors by fine needle aspiration cytology (FNAC) requires a combination of clinical, radiological, biochemical, and cytomorphological information. This paper discusses the diagnostic algorithmic approach of FNAC of mediastinal tumors.
There are essentially four steps in the approach for diagnosis of mediastinal tumors [Figure 1].
| Step 1: Clinical History|| |
The first step in the diagnostic approach of mediastinal FNAC is procuring relevant clinical history. Young patients may have germ cell tumors, thymomas or lymphomas. On the other hand, older patients may have more chances of metastatic tumors. Certain symptoms help indicate involvement of certain organs  [Figure 2]. Clinical history of myasthenia gravis is usually associated with thymoma. Past history of malignancy or the presence of malignant tumors in other parts of the body may be important for diagnosis of metastatic malignant tumors. Presence of generalized lymphadenopathy or hepatosplenomegaly on physical examination may indicate lymphoma or leukemic deposits. In germ cell tumors there may be associated abdominal mass in the pelvic region.
| Step 2: Radiological Localization|| |
The mediastinum is arbitrarily divided into many imaginary compartments on the basis of known anatomical landmarks. The preferential locations of the mediastinal tumors in one particular anatomical compartment have been highlighted in [Figure 2]. 
| Step 3: Cytomorphology|| |
A thorough study of cytomorphology is extremely useful in reaching the correct diagnosis. The cytologic smears may be composed of predominantly round cells, spindle cells or pleomorphic bizarre cells. In certain situations no specific cell pattern may be noted [Figure 1]. We describe the algorithmic approach to diagnosis of mediastinal lesions on the basis of the predominant cell morphology below:
Predominant Round Cell Morphology
Smears with a predominant population of lymphoid cells
The cytology smears may show occasional groups of epithelial looking cells along with large number of discrete lymphoid cells. The differential diagnoses are thymoma, non-Hodgkin's lymphoma (NHL), Hodgkin's lymphoma (HL), and metastatic small cell carcinoma [Table 1].
|Table 1: Differential diagnosis of predominant round cell morphology in mediastinal mass|
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Cohesive clusters of cells are usually seen in thymoma and metastatic small cell carcinoma [Figure 1], whereas, the cells of lymphomas are always non-cohesive. FNAC smears of lymphocyte rich thymoma (Type B1) show abundant monomorphic population of small, mature lymphocytes with inconspicuous nucleoli. ,, In addition, clusters of epithelial cells surrounded by arborizing capillary networks are also noted. The epithelial cells have oval to plump shaped nuclei with pale homogenous chromatin, single prominent nucleoli and moderate amount of cytoplasm ,, [Figure 3].
|Figure 3: High power view shows epithelial cells with pale cytoplasm and oval to elongated nuclei in a case of thymoma (May Grunwald Giemsa stain, ×480)|
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Pure cytological diagnosis of malignant thymoma is difficult. The presence of nuclear pleomorphism, mitotic activities and necrosis may indicate the possibility of thymic carcinoma. In FNAC smears it is almost impossible to differentiate benign thymoma from malignant thymoma. , Occasionally, non-neoplastic thymic epithelial cells may be noted in FNAC smears in cases of thymic hyperplasia and these cases are difficult to diagnose on cytology alone. 
NHL is the most common neoplasm in the middle mediastinum and 15 to 25 % of peripheral lymphoma involves the mediastinal lymph node.  Dissociated population of immature lymphoid cells indicates the possibility of NHL. The presence of lympoglandular bodies in the background indicates the lymphoid origin of the cells. The dissociated lymphoid cells with convoluted nuclei, fine powdery chromatin and inconspicuous nucleoli are seen in lymphoblastic NHL. In large cell NHL, the smears show dissociated large lymphoid cells with fine chromatin and regular to convoluted nuclei [Figure 4].  The cells are usually B-NHL. Extensive sclerosis in mediastinal sclerosing B-NHL may prevent to yield good diagnostic material. In fact extensive sclerosis may distort the cells to such an extent that NHL may be mistaken as soft tissue sarcoma, thymoma or metastatic spindle cell malignancy. ,,
|Figure 4: Dissociated large lymphoid cells with fine chromatin and regular to convoluted nuclei in large cell non Hodgkin's lymphoma (May Grunwald Giemsa stain, ×480)|
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Nodular sclerosis Hodgkin's lymphomas may not yield good diagnostic material due to fibrosis and there may be lack of diagnostic Reed Sternberg cells.  Rarely the other uncommon lesions such as Castleman's disease or extra- medullary haemopoiesis may be seen as mass lesions in the mediastinum. , FNAC smears of the metastatic small cell carcinoma usually show dissociated cells with round to oval nuclei having minimal cytoplasm. Frequent crushing artifacts, nuclear molding and single rows of cells indicate the possibility of small cell carcinoma.
Predominant Population of Epithelial or Non-Lymphoid Cells
Predominant populations of non-lymphoid cells are present in thymoma, germ cell tumor, neuroblastoma, metastatic carcinomas, and nodular sclerosis Hodgkin's lymphoma. FNAC smears of germinoma show dual population of poorly cohesive cells consisting of malignant germ cells and reactive lymphoid cells in a tigroid (striped) background. Individual cells are large with prominent nuclei having clear cytoplasm due to glycogen accumulation. Small mature lymphocytes may be seen in the background.  Discrete large cells with prominent nucleoli, vacuolated foamy appearance of the background and fragile cells with chromatin threading may raise the suspicion of a germinoma. Single and clusters of moderately pleomorphic cells with multiple large prominent nucleoli along with background necrosis may raise the suspicion of embryonal carcinoma. Mediastinum is the most common extra gonadal site of primary germ cell tumor.  However, the possibility of metastasis must be excluded before the diagnosis of primary germ cell tumor in the mediastinum. It is important to note that the commonest primary germ cell tumor of the mediastinum is mature cystic teratoma.  FNAC of mature cystic teratoma yields pultaceous material and smears show discrete benign squamous cells on an acellular background.
FNAC smears of predominant epithelial subtypes of thymoma shows many clusters and also dissociated epithelial cells with plump round epithelial cells. Occasional cases of epithelial subtypes of thymoma may show atypical cytomorphology such as cellular overcrowding, dissociation and nuclear enlargement.  These cases should not be over diagnosed as thymic carcinoma.
At times it is extremely difficult to differentiate thymic carcinoma from metastatic carcinoma on basis of cytomorphology. Lung is the commonest primary source of metastatic carcinoma followed by breast and prostrate, testis and skin [Figure 5]. Predominantly dissociated and occasional clusters of round to oval cells along with rosette formation are noted in neuroblastoma. Nuclei of the cells are round with fine chromatin and inconspicuous nucleoli. Background of the smear shows the striking absence of lymphoglandular bodies.
|Figure 5: Round to oval cells with brownish melanin pigment in metastatic melanoma (H and E, ×480)|
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Predominant Spindle Cell Morphology
FNAC smears of predominant spindle cell morphology could be categorized as
- Lesions in which the spindle cell component represents reactive components such as the fibroblasts in solitary fibrous tumor of the mediastinum , fibromatosis  and idiopathic fibrosing mediastinitis. 
- Lesions in which the spindle cell components are the part of benign neoplasm, such as spindle cell thymoma, benign nerve sheath tumor etc.
- Lesions consisted of malignant spindle cells such as, malignant schwannoma, squamous cell carcinoma, diffuse large cell NHL with sclerosis, metastatic malignant melanoma, pleomorphic liposarcoma and myxoid liposarcoma [Table 2]. ,,,
|Table 2: Differential diagnosis of predominant spindle cell lesions in mediastinum|
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Oval to elongated spindle cells in small clusters along with mature lymphocytes in the background is usually noted in spindle cell variant of thymoma. The common neurogenic tumors in the mediastinum are neurofibromas, schwannomas, and malignant peripheral nerve sheath tumors.  The presence of large number of clusters and dissociated spindle cells with moderate nuclear pleomorphism and kinking nuclei indicate spindle cell tumors probably of nerve sheath origin. Cellular pleomorphism, bizarre cells and mitotic activities indicate the possibility of malignant tumor [Figure 6].
|Figure 6: Bunch of spindle cells with moderately pleomorphic nuclei and bizarre cell in a malignant peripheral nerve sheath tumors. (May Grunwald Giemsa stain, ×480)|
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In mediastinal sclerosing B-NHL the smears show discrete immature lymphoid cells with oval to spindle cells. Background shows lymphoglandular bodies. Extensive sclerosis in mediastinal sclerosing B-NHL may prevent yielding good diagnostic material. In fact extensive sclerosis may distort the cells to such an extent that NHL may be mistaken as soft tissue sarcoma, thymoma or metastatic malignancy with spindle cell morphology. ,,
Pleomorphic Cell Population
Predominant pleomorphic cell population may be noted in anaplastic large cell lymphoma (ALCL), lymphocytic depletion type of Hodgkin's lymphoma, metastatic poorly differentiated carcinoma and choriocarcinoma.
Large number of dissociated large pleomorphic cells, along with background lymphoglandular bodies, may raise the suspicion of anaplastic large cell lymphoma. In Hodgkin's lymphoma the large pleomorphic lobulated cells may be noted, however, the number of such cells is less. Metastatic poorly differentiated carcinoma may also show large number of pleomorphic cells. However, the malignant cells often show cohesive clusters. Choriocarcinoma often exhibits large pleomorphic bizarre cells and multinucleation.  These cells are admixed with smaller mononucleated cyto and intermediate trophoblasts. Background usually shows necrosis. The malignant syncytiotrophoblasts have abundant cytoplasm with frequent bi-nucleation.
No Specific Morphologic Pattern
There are many other lesions in the mediastinum which are difficult to put in a specific group. These lesions consist of intrathoracic goiter, parathyroid adenomas, lipoma, bronchogenic cyst, paravertebral abscess etc. ,,
| Step 4: Ancillary Techniques|| |
A battery of ancillary investigations may be needed for diagnosis of mediastinal tumors [Figure 7].
It could be done either on alcohol fixed smears or cell block sections. Cell block is preferable for immunocytochemistry. [Figure 7] highlights the significance of positivity of different antibodies in the diagnosis of mediastinal tumors. ,,,
Flow Cytometric Immunophenotyping
This may be helpful in diagnosis and classification of non Hodgkin's lymphomas.  It is advisable to have a panel of immunostain on suspected case of NHL.
Electron microscopy may be helpful in diagnosing the different mediastinal tumors.  It may be particularly helpful diagnosing metastatic squamous cell carcinoma, neuroblastoma or thymomas.
Raised level of alpha feto protein may be helpful in endodermal sinus tumors. Similarly, high level of HCG is helpful in diagnosis of choriocarcinomas.
A portion of the aspirate could be kept for microbial culture. Depending on the initial FNAC smear the sample should be processed.
| Discussions|| |
Mediastinum is involved by a variety of primary and metastatic tumors. Majority of the mediastinal tumors are not unique in this region and can be found in other parts of the body, with the exception of thymic lesions. Tumors in this region have variable therapeutic management and prognosis and hence it is very important to reach the correct diagnosis by FNAC of mediastinal lesions. Shabb et al.  obtained adequate material in a study of 42 cases of FNAC of mediastinal lesions and were able to reach in correct diagnosis in 86% of cases. There was only one false negative case. Singh et al,  reported only 12 (6%) discordant cases out of 189 cases of mediastinal FNAC. There was no false positive case in that study. The errors were mainly due to misdiagnosis of small cell carcinoma for lymphoma, differentiating primary germ cell tumors from metastatic carcinoma, and misinterpretation of large cell lymphomas as mesenchymal tumor because of distortion of cells by the mesenchymal tissue.
In a recent study, Desai et al.  report the sensitivity and specificity of diagnosis of mediastinal lesions by FNAC as 88 and 82% respectively. They considered FNAC to be a highly effective technique in diagnosing mediastinal tumors. In a two-year period a total of 80 cases of mediastinal FNAC were encountered by us. In all these cases FNAC was done either under the guidance of USG or CT scan. In a total of 58 cases (72.5%) a conclusive diagnosis was offered on FNAC material. The commonest tumor diagnosed by FNAC was metastatic carcinoma (23), followed by thymomas (8), NHL (8), primary malignant germ cell tumor (2), HL (2), nerve sheath tumor (1) and miscellaneous group with variable diagnosis (13). We did not encounter any complications due to FNAC.
The mediastinum is surrounded by many vital structures such as heart and great vessels. Therefore many precautions are needed to perform FNAC of the mediastinal lesions. In fact, a team of experts consisting of radiologists, cytologists, physicians and surgeons is required to perform mediastinal FNAC. Rarely complications such as surgical emphysema, mediastinitis, and hemorrhage have been reported. ,, Usually ultrasonologically guided, CT or fluoroscopic guided FNAC are performed. However, the other techniques such as transesophageal or transbronchial endoscopic guided mediastinal FNAC are also popular. These techniques yield equally good material with low risk compared to the conventional CT or USG guided FNAC. ,,
Initial assessment of clinical features and radiological data narrows down the probable diagnostic categories of mediastinal tumors. This may help the cytologist to determine the future course of action during the time of FNAC procedure. Aspirated material should be collected for cell block, flow cytometry, electron microscopy and microbial culture. Sections from the cell block could be used for immunocytochemistry and this is one of the corner stones in ancillary technique. Material collected for flow cytometry could be processed after the initial diagnosis of NHL is confirmed on FNAC smears. This stepwise logistic approach is very helpful in major diagnostic categorization of mediastinal tumors. However, it is not free from pitfalls. On cytology smears, it is impossible to differentiate thymoma from malignant thymoma.  Malignant thymoma may not show significant nuclear enlargement and pleomorphism, however on histology section distinct capsular invasion may be evident. The demonstration of DNA aneuploidy may be helpful in distinguishing malignant thymoma from its benign counterpart.  In contrary, FNAC of thymic carcinoma shows significant nuclear enlargement, nuclear pleomorphism and background necrosis , and therefore it is possible to suggest the diagnosis of thymic carcinoma on FNAC smears. Occasionally there may be difficulties in distinguishing thymic carcinoma from a metastatic poorly differentiated carcinoma purely on the basis of cytomorphology and immunocytochemistry.  Clinical history of primary malignancy is very helpful in this situation.
Sampling error is a major hurdle in the diagnosis of mediastinal lesions. , Tumors of the mediastinum may show necrosis, inflammation and fibrosis on FNAC smears. Needle may hit in the non-viable area and the quality of the material may be sub-optimal. Tumor heterogeneity particularly in case of mixed germ cell tumors may also pose diagnostic challenge. 
| Conclusions|| |
In summary, mediastinum is the site of a large variety of primary and metastatic tumors. Many of the lesions are unique with clinical features, radiological localization, and cytomorphology. Additional investigations such as immunocytochemistry, flow cytometric immunophenotyping, and tumor markers may help in further confirmation in diagnosis. Stepwise algorithmic approach provides more definitive diagnosis in majority of the cases.
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Department of Cytology, Post Graduate Institute of Medical Education and Research, Chandigarh-160 012
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2]