malignant peripheral nerve sheath tumor ( mpnst ) accounts for about 5 - 10% of all soft tissue sarcomas . the fact that the presence of this tumor in the buttock region is extremely rare has prompted the authors to report this case . this is an interesting tumor not encountered very much in the neuro - oncology literature . it can occur in sporadic form or over a setting of neurofibromatosis-1 ( nf-1 ) . when mtt develops over nf-1 , the diagnosis can be confirmed based on morphologic grounds supported by an immunostain such as s-100 protein . a 40-year - old male presented with a large mass in the gluteal region gradually increasing in size over 6 months . ten years back , operation of a nodule in the same area had revealed histopathological diagnosis of neurofibroma . on further questioning , the patient revealed having two small painful nodules over his back since the presence of the operated nodule . his family history suggested that his father was a diagnosed case of von recklinghausen disease ( nf-1 ) . the resected mass consisted of a large globular mass measuring 10  7  6 cm . the cut section showed solid grey - white and hemorrhagic areas ( fig . 1 ) . multiple sections studied revealed alternate hypocellular and hypercellular areas composed of spindle cells with wavy nucleus and nuclear palisading . thick - walled blood vessels were seen with perivascular accentuation of tumor cells in the hypocellular areas ( fig . mitotic figures were brisk in the cellular areas , 11/10 high power fields with few abnormal mitoses . the tumor also showed a good number of scattered large cells both rounded and elongated with abundant deep eosinophilic cytoplasm and clearly visible cross striations ( fig . the patient was subjected to radiotherapy on being referred to the oncology department . during a follow - up period of 8 months , slightly more than half of the cases of mtt have been reported in conjunction with nf-1 . in our case , when it occurs in sporadic form , other spindle cell sarcomas like fibrosarcoma , malignant fibrous histiocytoma and rhabdomyosarcoma can come as differential diagnosis . the tumor develops after a long latent period of 10 - 20 years . in our case there is an agreement that the diagnosis of mpnst can be on morphologic grounds supported by positivity for s-100 protein . the morphologic features are ( a ) alternating hypocellular and hypercellular regions , ( b ) the appearance of thin wavy comma - shaped / bullet - shaped nucleus in the hypocellular areas , ( c ) presence of nuclear palisading , ( d ) presence of nerve whorls or tactoid bodies resembling wagner - meissner corpuscles , ( e ) prominent thick - walled vasculature , and ( f ) presence of heterologous elements like rhabdomyoblasts , cartilage and bone . such tumors show focal positivity for s-100 protein in 50 - 90% of cases , suggesting a nerve sheath origin . . the 5-year survival rate of mtts is only 5 - 15% in contrast to mpnsts where it is 50 - 60% . integrated positron emission tomography and computed tomography have been used to assess remission and response to therapy . the prognosis of mtt depends on the location , grade and completeness of surgical margins . it is good for the head , neck , extremities and worse for other sites including the buttock . in a study it was observed that mtt in association with nf-1 has a poor prognosis compared to sporadic forms .