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benign peripheral nerve sheath tumours with epithelioid morphology , are known to be rare cutaneous lesions of neural origin , closely mimicking malignant epithelioid soft tissue tumours .
benign peripheral nerve sheath tumors with epithelioid morphology ( bepnst ) are uncommon entities ; a cutaneous location and epithelioid morphology often leading to misdiagnosis .
many tumors fall under this umbrella term , with some researchers separating out epithelioid schwannoma into a distinct entity .
a few epithelioid lesions have overlapping features of neurofibroma and schwannoma , while some lesions have features typical of neither .
non neural , epithelioid benign and malignant soft tissue tumors are close mimics and would fall into the differential diagnoses of this lesion .
the epithelioid variant of malignant peripheral nerve sheath tumor ( empnst ) is a well categorised lesion with distinct features .
the benign counterpart though is rarely reported , and often under various terms like  cutaneous epithelioid schwannoma  benign epithelioid schwannoma depending on dominant histological features supporting schwannoma , co - existence of conventional neurofibroma , or relationship with a nerve .
few tumors reported have features of schwannoma / neurofibroma or fall into a  grey zone in between .
 bepnst by laskin and colleagues in their landmark study , which elucidated features classic to this category .
a male patient aged 47 years , gives history of sudden increase in the size of a nodular lesion over the knee , following a minor trauma .
the swelling had been present for the last 35 years and was slowly increasing in size .
a history of trauma sustained to the site at the age of 12 was elicited , which healed with a small bleb sized lesion over the lateral end of scar .
it was associated with pain following pressure at that site with full range of movements of the knee joint .
the surgeon noted a nodular cutaneous lesion over the lateral aspect of the knee joint measuring 2.5 cm across , which was freely movable over the underlying structures .
specimen received in the pathology department was a single skin covered nodular tissue measuring 2  1 cm .
cut section showed a single cyst with luminal areas of haemorrhage and thick wall with solid white areas .
histopathological examination ( hpe ) showed a mid to deep dermal lesion with central cystic degeneration and coagulative necrosis .
short fascicles of spindle and epithelioid cells were seen [ figures 2 and 3 ] , with moderate eosinophilic cytoplasm , vesicular fusiform nucleus , prominent eosinophilic nucleolus and an occasional mitotic figure , vaguely resembling antoni a areas [ figure 4 ] . areas of haemorrhage rimmed by hemosiderrin laden macrophages , stromal hyalinisation and chronic inflammation were noted .
tumor cells were s100 positive diffusely [ inset [ figure 3 ] , and sma and cd68 negative .
single , skin covered nodule over knee joint ; skin is stretched over the lesion photomicrograph of lesion . ( a ) epithelioid and spindle cells ( h and e , 400 ) , ( b ) epithelioid cells ( h and e , 400 ) photomicrograph of spindle cells ( h and e , 100 ) inset : s100 positive tumor cells photomicrograph of antoni a areas ( h and e , 50 )
the cutaneous benign peripheral epithelioid nerve sheath tumor ( bepnst ) , as endorsed by laskin and colleagues , is an umbrella term reserved for morphological entities of neural origin with a predominance of epithelioid cells . falling into this category
include lesions with partial resemblance to schwannoma , neurofibroma , features of both or neither , with ihc being positive for neural markers .
a tumor found in various sites and not restricted to any age group , it 's characteristic clinical features include a benign behavior , superficial location and small size ( < 3 cm ) .
usually solitary , multiple lesions with schwannoma like morphology have been described in association with carney 's syndrome and neurofibromatosis .
the lesions have a wide distribution , with the lower limb being the preferred site .
laskin and colleagues in their landmark study described the histological features characterising this lesion . with a predominant dermal / subcutaneous location
tumors are surrounded by thick connective tissue mantle , and comprise one or more closely apposed nodules of uniform population of epithelioid 
nevus like cells with moderate to abundant eosinophilic cytoplasm , vesicular nucleus , small nucleolus , low mitotic activity ( 0 - 6/50 hpf ) forming nests , cords , trabeculae , clusters in a myxohyaline matrix .
a minor spindle cell component in clusters / fascicles is seen merging with the epithelioid cells .
the tumor cells demonstrate immunohistochemical positivity for s100 , gfap , laminin , collagen iv .
permeative intraneural growth and rosette like structures , are additional features supportive of neural origin . a collagen rich variant exhibiting hyalinised collagen
the present case was a dermal circumscribed , nonencapsulated lesion with epithelioid cell predominance , minor spindle cell component , with other features and ihc helping us decide in favor of bepnst .
follow up of the patient revealed a benign clinical nature . clinically not different from other benign cutaneous lesion associated with pain , like angioleiomyoma , hpe remains the mainstay for diagnosis in such cases .
histological differential diagnoses include the rare empnst , myopericytoma , leiomyoma and benign fibrous histiocytoma ( bfh ) , the latter two with epithelioid morphology . similar to bepnst on microscopy and ihc to some extent , features that favor empnst include hyperchromasia , large nucleoli , increased mitoses ( often atypical ) , and necrosis .
ihc an integral part of current pathological diagnoses offers some succor , with s100 and gfap being negative in these tumors , pointing towards their non - neural origin .
myopericytoma shows cells closely apposed to numerous blood vessels besides being sma positive , as also a leiomyoma .
this case represents a rare lesion of neural origin with features partially resembling it 's more common soft tissue counterparts . the benign clinical nature and the strong resemblance to the malignant counterpart with epithelioid morphology ,
necessitates the need for the dermatologist / dermatopathlogist to consider this lesion in the differential diagnoses of benign cutaneous painful lesions exhibiting epithelioid morphology . a rare neural lesion with epithelioid cells on morphology , this case of bepnst presented with a history of post traumatic occurrence over a prolonged period .
benign epithelioid peripheral nerve sheath tumor , a rare entity is an umbrella term describing benign , neural origin tumors with epithelioid morphology . clinically indistinguishable from other benign cutaneous lesions 
 , histopathology offers the only source of accurate diagnosis . 
 morphologic mimics include many benign and malignant soft tissue lesions . 
 besides a predominant epithelioid component , the lesion can also show a fair share of spindle cells . 
 a circumscribed nodular tumour of low mitotic activity , it often exhibits areas resembling schwannoma or neurofibroma . 
 an awareness of this entity and its varied morphological aspects helps to arrive at the correct diagnosis and hence avoid unnecessary extensive surgical procedures . 
 this case presents features of this benign tumor which occurred in a 47 years old man .