Clinicopathological Analysis of Mediastinal Masses : A Mixed Bag of Non-Neoplastic and Neoplastic Etiologies

The mediastinum extends antero-posteriorly from the sternum to the spine and from the thoracic inlet down to the diaphragm. The so-called Pandora’s Box has been divided into four compartments, i.e. superior, anterior, middle and posterior. while imaging studies suggest the location of the lesion and help in narrowing the differential diagnosis, morphological assessment by fine needle aspiration cytology (FNaC) or histopathological examination is imperative prior to therapeutic intervention. although FNaC is a cost effective tool for the establishment of pre-operative diagnosis of mediastinal masses, there is a complexity and technical difficulty associated with this technique owing to the narrow anatomic space of mediastinum. also, there are well known diagnostic pitfalls of this technique that limit its use (1).

with suspected mediastinal lesions, following which computed tomography (CT) was performed for further characterisation of the lesion.CT-guided fine needle aspiration (FNaC) was performed wherever possible.aspirates were smeared on clean glass slides, air dried and stained by May-Grünwald-Giemsa (MGG) stain.There were no cell block preparations studied.Following surgical excision, the excised specimen was fixed in 10% neutral buffered formalin and sent for histopathological evaluation.Diagnosis was confirmed on hematoxylin and eosin (H&e)-stained formalin-fixed paraffin-embedded sections.For cases in which final diagnosis could not be made on routine examination, a detailed immunohistochemical panel comprising of cytokeratin (CK), cluster of differentiation 45 (CD 45), CD 20, CD3, CD15, CD 30 and TdT was applied.Further, OCT-3/4, alpha feto protein (aFP) and beta-human chorionic gonadotropin (β-HCG) were applied to rule out a mixed component, if any, in a germ cell tumour.in brief, sections measuring 3-4 μm thick were cut, deparaffinized with xylene and brought to water through graded levels of alcohol.endogenous peroxidase activity was blocked by treating the slides with hydrogen peroxide for 30 min at room temperature.antigen retrieval was done by immersing the slides in citrate buffer using the pressure cooker method.The slides were then incubated overnight with the primary antibody (rabbit polyclonal) at 4 ˚C in a humidified chamber.The following day, secondary antibody was added.The sections were then incubated with di amino benzidine (DaB) for visualization of the peroxidase reaction.after being washed in water, the sections were counter stained with haematoxylin, dehydrated in alcohol, cleared in xylene and mounted.immunohistochemistry (iHC) was interpreted in a binary fashion as positive or negative.
The FNaC smears, H&e stained sections and immunohistochemistry slides were reviewed by two pathologists and correlated with the clinical findings including radiology.

RESULTS
Of the 65,089 cases received in our department, 60 patients with mediastinal masses were included in the study.among these, the majority of the patients presented with a mass in the anterior mediastinum (36.7%), followed by the middle mediastinum (33.3%) and posterior mediastinum (30%).The compartment-wise histopathological diagnosis of the mediastinal masses is described in Table i.The mean patient age at the time of presentation was 37.5 years (range 2-61 years).There were 43 males and 21 females with a male: female ratio of 2:1.
Retrospective analysis of the clinical and radiological findings of these patients was done.The most common presenting complaint was cough, seen in 54 patients (87.0%), followed by chest pain (50; 80.6%), weight loss (36; 58.0%), dyspnoea (26; 41.9%) and hoarseness (14; 22.5%).Myasthenic symptoms were seen in four patients with thymoma (Table ii).Two patients each of thymic cyst and mediastinal lipoma were diagnosed incidentally.Routine blood investigations including complete hemogram, liver function tests and kidney function tests were normal in all patients except four cases of tubercular lymphadenitis Mediastinal lymphadenopathy was seen in 18 cases.among 10 cases of lymphoma, there were 6 cases of Non-Hodgkin lymphoma (NHl) and four cases of classical Hodgkin lymphoma (Hl).There was no difficulty in FNaC diagnosis of these lesions except for three cases predominantly showing haemorrhage with few scattered lymphoid cells, inconclusive for an opinion.Histologically, most common type of NHl was diffuse large B cell lymphoma, seen in with raised erythrocyte sedimentation rate (eSR).Chest X-ray showed mediastinal widening in 40 patients.CT scan revealed the presence of mediastinal masses in their respective compartments in all cases.Mediastinal lymphadenopathy was noted in 18 cases on CT scan.CT guided FNaC was performed in 54 patients.
Histopathological examination revealed 73.33% benign and 26.67% malignant lesions.among the twelve cases of thymoma, the most common variant was the aB subtype (9 cases) followed by two cases of B1 subtype and one case of thymic carcinoma.Thymic tumours were classified as per the world Health Organisation (wHO) classification.Six patients with anterior mediastinal masses showed a biphasic population of spindle-shaped epithelial cells interspersed with lymphoid cells on FNa smears.a provisional diagnosis of thymic hyperplasia versus thymoma was considered.
The excision biopsy specimen showed features suggestive of thymoma, aB subtype.On the other hand, FNa smears from three patients predominantly showed presence of haemorrhage along with few scattered lymphoid cells and were inconclusive.Subsequently they were diagnosed as thymoma, aB subtype on histopathological examination (Figure 1a-C).Two female patients suffering with  while there was no difficulty in the FNa diagnosis of seminoma, the yolk sac component could not be identified cytologically.
eighteen cases of neurogenic tumours were identified in the posterior mediastinum.Of these, schwannoma was the most common and seen in 8 patients (Figure 7a), followed by six cases of neurofibroma (Figure 7B) and two cases of ganglioneuroma.FNa smears in the majority of these cases were moderately cellular with presence of spindle-shaped cells either in clusters or singly scattered.The individual cells were spindle-shaped with scant to moderate cytoplasm and hyperchromatic spindle-shaped nucleus.No Verocay bodies, nuclear atypia or stromal fragments were seen.a provisional FNa diagnosis of spindle cell tumour was given in ten of these cases, while the remaining cases showed paucicellular smears and were labelled inconclusive for opinion.Two cases of neuroblastoma (Figure 8a,B) were seen in a 2-and 4-year-old male child respectively, FNa smears of which were inadequate for diagnosis.
Cystic lesions of the mediastinum consisted of two cases each of bronchogenic cyst, thymic cyst and retrosternal goitre with cystic change respectively, diagnosed on    histopathological examination of the excised specimen.FNaC was not performed in these cases owing to the cystic nature of the lesion.One case each of mediastinal lipoma was readily diagnosed on cytology and confirmed on histopathology.also, a 22-year-old female patient showed a large fat-containing anterior mediastinal mass extending along the right pericardium on CT scan (Figure 9a).FNa showed features suggestive of lipoma.On histopathology, a well-encapsulated tumour was seen showing intermixed thymic tissue within mature adipocytes, suggestive of thymolipoma (Figure 9B).

DISCUSSION
Primary mediastinal tumours are rare, accounting for 3% of tumours occurring within the chest (3).we studied 60 cases of mediastinal lesions received over a period of 5 years.The mean patient age was 37.5 years (range 2-61 years) with a male preponderance.Our findings were in concordance with the previously published literature (2,4).
Nearly 50% of all mediastinal masses involve the anterior compartment, including thymoma, teratoma, lymphoma, thyroid and parathyroid lesions.Congenital cysts and neurogenic tumours are most commonly observed in the middle and posterior mediastinum respectively (3). in our study, the anterior compartment (36.7%) was maximally involved followed by the middle mediastinum (33.3%) and posterior mediastinum (30%).Thymoma was the most common tumour of the anterior compartment followed by germ cell tumours.The middle mediastinum mainly nodal involvement with lymphoma most commonly followed by tubercular lymphadenitis.Neurogenic tumours were seen in the posterior mediastinum only.Studies conducted in the past showed nearly similar trends (5)(6)(7).
in the present study, 96.8% of the subjects were symptomatic at presentation.Dubashi et al. ( 5) reported a similar incidence of 97% with other studies reporting a range of 60-88% (2,4).
in 1987 however, Davis et al. (4) reported a much higher incidence of asymptomatic cases (38%).This difference may be attributed to the fact that most of the patients in our setting visit the hospital for their symptoms rather than for routine examination.The most common symptoms reported in this study was cough followed by chest pain, dyspnoea and weight loss which was similar to the previously reported incidence (8,9).
Pre-operative diagnosis of mediastinal masses is a challenging task owing to the non-specific clinicoradiological signs and technical difficulty in performing FNaC.The investigation of choice for the detection of mediastinal masses is CT scan (10).Magnetic resonance imaging (MRi) is of help in further categorisation of the lesions (11). in our study, CT scan identified mediastinal masses in all cases but was unable to categorise the lesion in any.CT scan thus helps in compartment localisation of the mediastinal mass.This further helps in narrowing the probable differential diagnosis.
CT-guided FNaC is a safe, useful and cost-effective method for the diagnosis of mediastinal masses.However, close proximity to vital organs can be a hindrance for this technique.in our study, FNaC was performed in 54 cases.There were 7 cases which revealed inadequate material.Thymic lesions often pose a diagnostic challenge for the reporting cytologist.while the lymphocyte-rich variant of thymoma (B1 subtype) can easily be misdiagnosed as lymphoma (11), epithelial rich variant (a subtype) on the other hand is a close mimicker for carcinoma (12).Similar to previous studies, our study also highlights the difficulty encountered in the FNa diagnosis of thymic lesions especially two cases of B1 subtype where the possibility of a lymphoma could not be ruled out and six cases of aB subtype in which thymic hyperplasia versus thymoma could not be accurately diagnosed on FNaC.Thymic carcinoma on the other hand was readily diagnosed due to the frank malignant features.
apart from the thymic lesions, neurogenic tumours are difficult to categorise on cytology.Schwannomas are usually difficult to differentiate from neurofibroma unless characteristic Verocay bodies are seen, while ganglioneuroma cannot be diagnosed till ganglion cells are aspirated.Both these findings were not seen in our cases and a final diagnosis of spindle cell tumour was rendered on FNaC.Further, some degree of cellular and nuclear atypia is commonly seen in neurogenic tumours.This warrants a careful search for mitotic figures and excision biopsy for proper histopathological evaluation.Thus characterisation of benign versus malignant on FNaC is a diagnostic difficulty which is then overcome on histopathology.On the other hand, germ cell tumours cannot readily be diagnosed on FNaC since tigroid background is not entirely specific for seminoma and when lymphocytes/ lymphoglandular bodies are present, a confusion with malignant lymphoma is possible, as seen in two of our cases.Furthermore, smears showing discohesive pattern on cytology should be differentiated from poorly differentiated adenocarcinoma, melanoma and lymphoma.in our study, the component of yolk sac tumour was missed in a single case of mixed germ cell tumour.This can be attributed to selective sampling which is a well-known hurdle in aspiration cytology.as reported earlier by Chhieng et al. particularly in case of mixed germ cell tumours may also pose diagnostic challenge (13).Similarly, fat fragments were selectively sampled from an anterior mediastinal mass in a 22-yearold female which was diagnosed later as thymolipoma on histopathological evaluation.
a rather rare case encountered in our study which deserves special mention was lennert's lymphoma in a 33-year-old male patient with mediastinal lymphadenopathy involving the middle compartment.lennert's lymphoma is a variant of peripheral T-Cell lymphoma, not otherwise specified in the wHO classification of 2008 (14).its rarity and the lack of strict diagnostic criteria explain why only a few reports on lennert's lymphoma have been published so far.Our case showed presence of RS like cells mimicking Hodgkin's lymphoma and posed the same diagnostic dilemma as the previously published cases. in 2013, Parimal et al. (15) reported a series of 5 cases of lennert's lymphoma which constituted 0.71% of all peripheral T-cell lymphomas in their institution.They concluded that classical Hodgkin's lymphoma is a close mimicker of lennert's lymphoma

Figure 8 :
Figure 8: A) CT scan showing large heterogeneous mass with internal calcifications in the right hemithorax replacing the right lung.B) Diffuse sheets of small round blue cells with formation of rosettes (inset) suggestive of neuroblastoma (H&e; x200).

Table I :
Histopathological diagnosis of the mediastinal masses

Table II :
Clinical features of patients with mediastinal tumours