the ewing 's sarcoma / primitive neuroectodermal tumour ( es / pnet ) family of tumours is part of a rare group of malignant neoplasms with small round - cell morphology . es / pnet is an uncommon primary malignancy of the bone primarily affecting children and young adults with more than 80% of patients being younger than 20 years of age at the time of presentation . less frequently , es / pnet arises from soft tissues and then the neoplasm is classified as extraskeletal es / pnet . involvement of the mediastinum as the primary site of origin is extremely uncommon in the literature , only four cases were reported in a span of 20 years in a large case series from a tertiary care centre in india . a rare case of primary pnet tumour of the anterior mediastinum mimicking massive pleural effusion in a young male is reported here . a 30-year - old male , carpenter by profession , presented with dry cough and progressive shortness of breath for two months , right - sided severe chest pain for one and half month and low - grade fever for one month . the patient was a non - smoker , non - alcoholic and non - diabetic . clubbing was absent and there was no peripheral lymphadenopathy . on examination of the respiratory system features of right hemithoracic volume enlargement were present . percussion note was dull all over the right hemithorax with presence of tenderness on superficial percussion . on auscultation , a tubular breath sound was heard near the right infraclavicular area parasternally , in all other areas on the right side vesicular breath sound was markedly diminished . examination of other systems including the lymphoreticular system and testicular examination were all within normal limits . chest x - ray postero - anterior view revealed right - sided homogenous opacity involving all three zones with loss of costophrenic and cardiophrenic angles along with contralaterally shifted mediastinum [ figure 1 ] , suggesting right - sided massive pleural effusion ; but on thoracentesis , only 50 ml hemorrhagic fluid could be aspirated from the right infrascapular area after repeated attempts . pleural fluid study yielded reddish - coloured , predominantly lymphocytic , exudative fluid with high adenosine de - aminase ( ada ) [ total cell count 170/cmm , lymphocyte- 80% , neutrophil-15% , ada-122 iu , protein-3.7g / dl , sugar- 86 mg / dl ] and pleural fluid for pap stain was negative . subsequently contrast - enhanced ct ( cect ) scan of the thorax was done and showed a huge non - homogenous mass of heterogeneous density with multiple foci of necrosis occupying the entire right hemithorax , possibly arising from the anterior mediastinum , causing shifting of the mediastinum to the left and compression of the superior venacava ( svc ) , there was minimal pleural effusion [ figure 2 ] . chest x - ray pa view showing homogenous opacity on the right side involving all zones with obliteration of the costophrenic angle and contralateral mediastinal shift ct scan of the thorax ( cect ) showing a large mass of heterogeneous density with foci of necrosis occupying the entire right hemithorax ct - guided fine needle aspiration cytology ( fnac ) was performed and the pathologist opined it as small round - cell tumour , possibilities : non - hodgkin 's lymphoma or small - cell carcinoma of lung [ figure 3 ] . ct - guided tru - cut biopsy of the lesion was performed next and it revealed a tumour composed of round , oval or fusiform cells with hyperchromatic nuclei and scanty cytoplasm , mitotic figures were present [ figure 4 ] and it was reported again as small round - cell tumour , possibly small - cell carcinoma of the lung . subsequently , fibreoptic bronchoscopy was done and it showed narrowing of the right bronchial tree possibly due to external compression but no intraluminal growth was noted . on immunohistochemistry of the histology block of tru - cut biopsy , tumour cells expressed mic-2 and were immunonegative for cytokeratin , synaptophysin , chromoganin a , cd 20 , cd 3 , tdt , and the findings were consistent with primitive neuroectodermal tumours ( pnets ) . cytology of ct - guided fnac slide under high - power field showing highly cellular smear with small round cells with hyperchromatic nuclei and scanty cytoplasm ( hematoxylin - eosin , 400 ) histopathology of ct - guided tru - cut biopsy slide under high - power field showing sheets of small round or oval cells with hyperchromatic nuclei and variable amount of cytoplasm , tumour cells showing rosette formation at places ( hematoxylin - eosin , 400 ) finally , a diagnosis of pnet of the mediastinum was made . so it was an extremely rare case of peripheral pnet arising from the anterior mediastinum . in 1973 , hart and earle first coined the term primitive neuroectodermal tumour ( pnet ) , which drew attention to the primitive nature of the tumour rather than histogenesis . this entity is primarily a disease of childhood and young adults with a median age at diagnosis being nine years , and 80% of the cases are under 20 years of age . pnets are highly malignant , undifferentiated neoplasms of the es / pnet family , arising from the germinal matrix cells of the primitive neural tube . pnets are a variety of small round blue cell tumors ( srbct ) and may involve more than one site at presentation . when located in extraosseous sites , supratentorial brain , spinal canal , and soft tissues of the thigh are the usual sites ; primary involvement of extracranial sites like the mediastinum , chest wall , kidney is very rare . in a study of extraskeletal es by ahmad et al . primary mediastinal pnets are mostly located in the posterior mediastinum like other neurogenic tumours , primary involvement of the anterior mediastinum is extremely uncommon . these neoplasms are usually seen in young males who predominantly present with chest pain as in our case . radiologically , it is very difficult to differentiate these neoplasms from other common anterior mediastinal tumours , ct scan of the thorax reveals an ill - defined , heterogeneous mass with necrotic foci within , calcification is usually absent . smears are generally highly cellular and are composed of both single cells and groups of loosely cohesive cells with a high nuclear / cytoplasmic ratio , hyperchromatic nuclei without prominent nucleoli , distinctively smooth nuclear membrane contour , finely granular chromatin , one or two small nucleoli and scanty but almost always present perinuclear clear cytoplasm suggesting epithelial differentiation . in atypical es and pnet the cellular and nuclear atypia is more marked than in conventional es , rosette- like structures are more common , the distinction between large light and small dark cells less obvious and cells with thin cytoplasmic processes are present . in atypical cases diagnosis on fnac is quite challenging and the extraskeletal variant of es / pnet is more often difficult to diagnose on cytopathology , and a histology with immunohistochemistry can only clinch the diagnosis in these cases.the classic histological pattern of es / pnet consists of solid sheets of small uniform  primitive  cells with round nuclei and scanty cytoplasm that lack significant differentiation . in more differentiated es / pnet , homer - wright rosettes may be identified . due to the lack of characteristic morphologic features , es / pnet is difficult to distinguish from histologically similar small round - cell tumours including rhabdomyosarcoma , desmoplastic small round - cell tumour , poorly differentiated synovial sarcoma , mesenchymal chondrosarcoma , neuroblastoma and lymphoma . immunohistochemical expression of the mic2 gene product ( cd99 ) in es / pnet is helpful in separating this entity from other small round - cell tumours . however , cd99 expression is not specific for es / pnet , as it can be expressed by other sarcomas , including poorly differentiated synovial sarcoma and mesenchymal chondrosarcoma . other ancillary techniques , including cytogenetic analysis , reverse transcriptase polymerase chain reaction and fluorescence in situ hybridization may provide further aid in confirming the diagnosis of es / pnet . demonstration of the t(11;22)(q24;q12 ) chromosomal translocation ( ews - fli1 gene rearrangement ) is highly specific for es / pnet as it is encountered in more than 90% of the neoplasms . management is usually multimodal : surgery followed by ifosfamide or doxorubicin - based chemotherapy and radiotherapy . overall , pnets are aggressive neoplasms that have less than 20% five - year survival , but the overall prognosis of extraskeletal es / pnet seems to be better . a retrospective study of 24 patients with extraskeletal es showed an overall five - year survival rate of 61% . wide tumor - free resection margins in conjunction with multi - agent chemotherapy are necessary for good clinical outcomes . although , a rare entity , extraskeletal es / pnet should be considered in the differential diagnosis of primary mediastinal neoplasms , especially in children or young adults and if a primitive small round - cell tumour is encountered in cytology or histology . histopathology with immunohistochemistry , cytogenetic analysis and other molecular tests to identify chromosomal translocations , are invaluable to establish specific diagnosis .