Bombay Hospital Journal, Vol. 53, No. 4, 2011 Abstract Malignant peripheral nerve sheath tumour [MPNST] is often associated with Von 1 Reckling Hausen's disease with an incidence ranging from 2-29 %. It is a fact that 3- 6% of von Reckling Hausen's neurofibromas[NF] transform spontaneously before the th 1,3 4 decade of life. MPNST developing in patients with NFI behaves more aggressively 1,2,3 than denovo tumour and metastasises within 2 yrs of diagnosis. Here we present a case of NFI who presented with localised gigantism of the right hand which on histopathology proved to be MPNST with areas of primitive neuro ectodermal tumour[PNET] differentiation arising from a preexisting neurofibroma. Department of Pathology, Meenakshi Medical College, Enathur, Kanchipuram. Tamil Nadu 631552. A Rare Presentation of MPNST with PNET Differentiation Rajeswari K, Irfan A Ansari, Rajib K Gupta, Geetha Prakash Introduction eurofibromatosis, also known as Von NReckling Hausen's disease is an autosomal dominant multifocal neurocutaneous disorder with an estimated birth incidence of 1 in 2500 and a prevalence of 1 in 5000. Both, NF1 and NF2 have high denovo mutation rate and carry a high risk of tumour formation, although NFI carries risk of high grade 3,4,5 malignancy. Peripheral nerve sheath tumours are contributors to the morbidity and mortality of NFI. MPNST is a rare and aggressive sarcoma that arises from the nerve sheath or show features of nerve sheath differentiation with an incidence of 0.001% in the general population and 0.16% in patients with NFI. Though very rare, the exact incidence of MPNST showing PNET differentiation is not 5,6 known. Case History A 23 yr old young man came with c/o progressive enlargement of his right hand since childhood. It started as a small papule, but gradually increased to involve the entire hand and was absolutely painless. There was no family history of similar complaints. On examination, there was localised gigantism of his right hand involving little and ring fingers with hyperpigmentation of the overlying skin. Physical examination revealed multiple nodules of varying sizes ranging from 0.5 to 1.5 cms in the trunk. Multiple café au lait spots were present over the trunk, axillae and groins (Fig.1). Fig. 1 café au lait spots over the trunk and axillae 789