malignant peripheral nerve sheath tumors ( mpnsts ) are very rare high - grade sarcomas with a poor prognosis at childhood and adolescence , and only 10%20% of mpnsts are seen in these periods.14 mpnsts often present with pain associated with mass lesions or neuronal involvement in the head  neck , abdomen , chest wall , and other anatomic regions , but extremity involvement is uncommon , especially in children or adolescents.3,5,6 the most important risk factor for mpnst development is the presence of neurofibromatosis type 1 ( nf1 ) , and half of the mpnst cases occur in patients with nf1 history.2,7,8 mpnsts may be confused with neurofibromas and/or plexiform neurofibromas in patients with nf1.7,9,10 we present a case of giant mpnst located at the right thigh of a patient with nf1 taking into account the available literature . in our study , electronic searches for reports of similar cases and other rare manifestations using the keywords  neurofibroma  ,  neural sheath tumors  , soft tissue sarcomas in children with neurofibromatosis  ,  neurofibroma and adolescents  , and  neurofibromatosis and malignancy  were conducted in google , pubmed , springerlink , embase , elsevier , and ovid databases for all english articles with abstracts . only 30 reports focusing on neurofibroma and mpnsts in the patient  with or without neurofibromatosis  or those that provided important theoretical contributions are referenced in the current work . a 15-year - old male presented to our clinic with a swelling and light pain at the right thigh for a year . on physical examination , a mass with indeterminate borders was palpated at the anteromedial area of the right thigh . there were typical nf1 skin findings such as caf au lait spots and peripheral neurofibromas all over the body ( figure 1 ) . magnetic resonance imaging ( mri ) revealed a 139 cm mass at the anteromedial thigh with posterior extension , which was isointense in t1 weighted ( t1w ) images , heterogeneously iso- to hyperintense in t2 weighted ( t2w ) images , and diffusely contrast enhancing except the central cystic areas in the postcontrast images ( figure 2 ) . with a suspicion of malignancy , histopathological evaluation led to an initial diagnosis of benign neurofibroma , and total surgical excision was planned . ethical approval for this study was not taken , but we informed both the patient and the patient s parents about this study and obtained written informed consent . the mass was excised under general anesthesia . during surgery , the mass was observed to have a hard , fixed , and large capsule . histopathological evaluation revealed a smooth - surfaced , grayish - brown 1399 cm solid mass weighing 610 g , which appears nodular and heterogeneous ( figure 3 ) . a malign mesenchymal tumor involving diffuse necrotic areas was observed . there were occasional atypical mesenchymal cells with clear cytoplasms and vesicular nuclei , as well as diffuse pleomorphic multinucleated giant cells . diffuse atypical mitosis ( 42 mitoses per 10 hpf [ high - power field ] ) was striking . immunohistochemical analysis revealed strong ( + ) staining with s-100 , vimentin , cd-56 , cd-99 , and p-53 , as well as 80% positivity for ki-67 staining ( figure 4 ) . considering these data , the lesion was diagnosed as mpnst ( grade-3 ; federation nationale des centres de lutte contre le cancer ) . they may behave aggressively and make up 5%10% of all soft tissue sarcomas.2,4,7,1113 mpnsts may arise from all peripheral nerves and frequently reported localizations are the extremities , trunk , head , and neck.1,2,5,9,11,13,14 in patients with neurofibromatosis , mpnst most frequently occurs at ages of 3040 years . when compared with benign tendon sheath tumors , mpnsts more frequently occur at extremities.15 mpnsts are rare in childhood and have an poor prognosis.5,10 although mpnst can be found in any part of the body , extremity involvement is observed less frequently in pediatric cases.3,5,6,16 patients with mpnst frequently are admitted with complaints of a palpable mass , pain , and/or other neurological symptoms due to neuronal involvement.11 solitary and/or plexiform neurofibromas in teenagers or children can be confused with mpnsts , but signs of atypia or significant mitotic activity are not present.1721 malignant transformation and recurrence of the solitary neurofibromas without neurofibromatosis are unusual.20 other cases of solitary and/or plexiform neurofibroma in pediatric patients without neurofibromatosis have been reported in body regions with slightly less frequency . these include the mons pubis,17 eyelid dermis,18 intrascrotal and extratesticular sites,19 abdominal wall,20 and parotid region.17,21 mpnsts can resemble benign tumors , both histologically and radiologically.4 demir et al5 suggested that the diagnosis is easily made in patients without nf1 who present with a palpable mass and pain , but diagnosis is frequently delayed in patients with nf1 due to confusion of these lesions as neurofibroma and/or plexiform neurofibroma . to make an early diagnosis of mpnst , mri evaluation and biopsy should be performed immediately when malignancy is suspected.22,23 however , both plexiform neurofibroma and mpnsts have similar mri findings , which demonstrates areas of low - to - intermediate signal intensity on t1w sequence and high signal intensity on t2w sequence with heterogeneous enhancement.21 wasa et al24 reported that patients with mpnst generally have giant masses , and presence of peripheral enhancement pattern , perilesional edema - like zone , and intratumoral cystic lesions are observed more frequently . moreover , they reported that heterogeneity in t1-weighted images support mpnst in patients with nf1 . in addition , presence of markedly increased nodularity also suggests malignancy.25 recent studies suggest that positron emission tomography  computed tomography ( ct ) scanning is especially helpful to differentiate mpnst from neurofibroma and/or plexiform neurofibroma.16,2225 hemorrhagic or necrotic heterogeneity in mri or ct may suggest malignancy , but this finding may also be observed in benign peripheral nerve sheath tumors.10,21 garg et al17 reported that radiological investigations and fine - needle - aspiration cytology may not be very helpful for diagnosis . the rarity of positive cytology may be secondary to the adhesive nature of cells in such tumors . microscopy reveals proliferation of all elements of nerves , including axons , schwann cells , and fibroblasts . immunoreactivity for s-100 protein , nse , and vim is characteristic.17 our case had findings of nf1 . after mri , adequate sample was obtained with fine - needle - aspiration biopsy due to suspicion of malignancy . histopathological evaluation was consistent with benign neurofibroma , and therefore total surgical excision was planned . but histopathological evaluation of the excised material was interpreted as mpnst with malignant cells at the surgical border . we suggest that malignancy should be suspected in nf1 cases that show masses with heterogeneous contrast involvement and occasional cystic and necrotic places . histopathological evaluation of the needle biopsy material may yield a false diagnosis , but larger samples that are obtained after surgery show marked nuclear polymorphism , mitosis , vascular invasion , necrosis , hypercellularity , a high proliferative index ( immunopositivity to ki67 ) , and many more atypical spindle cells.3,12,25,26 the minimum amount of histological examination to establish diagnosis includes immunohistochemical stains for des , myog , vim , s-100 , proliferative activity marker mib-1 , potential expression of p53 , cerbb2 , p27 , p16 oncogenes , or tumor suppressor genes . expression of p53 plays an important role in the evolution of mpnst.24,27,28 our case had histopathological and immunochemical findings similar to the earlier findings , and diagnosis of mpnst ( grade-3 ; fnclcc ) was made . despite the fact that mpnsts have high local recurrence rates and are generally associated with poor prognosis , little is known about prognostic factors or effective clinical management for this tumor type.4,10,13,16 the most significant factors affecting the prognosis of mpnst in the pediatric age group are surgical resectability , local invasiveness , tumor size ( > 5 cm ) , extent of surgery , intergroup rhabdomyosarcoma study grade , tumor location ( central vs peripheral ) , presence of nf1 , and adjuvant treatment.2,5,7,9,15,22 recurrence is seen in 75% of the cases , and 5-year survival ranges between 16% and 53% . poor prognosis is associated with a tumor size > 5 cm , subtotal resection of tumor , and preexisting nf1.7,29,30 because mpnst is an aggressive tumor , complete surgical resection is necessary for a successful treatment . however , complete surgical resection is rarely possible , and the most important reason of treatment failure is the difficulty in achieving local control . information about clinical management of this tumor is sparse because there are very few reports in the literature , and this tumor is rarely seen in the pediatric population.2 although response to radiotherapy and chemotherapy is weak , adjuvant radiotherapy and chemotherapy are frequently required for mpnst patients.2,4,14,22 carli et al2 reported that postoperative radiotherapy may help to achieve local control in patients with minimal residual tumors . in addition , they reported that chemotherapy may be effective in tumors thought to be unresectable.2,12 because mpnst has greater chance for recurrence than other sarcomas , current treatment is radical surgical resection . recurrence may occur years after the surgery ; therefore , long period of follow - up is warranted.2,3,7,25 in conclusion , mpnst is a rare , soft tissue sarcoma of childhood and adolescence , which may be misdiagnosed as neurofibroma or plexiform neurofibroma in patients with nf1 . especially , a giant mass and mri features of heterogeneous involvement , along with a cystic and nodular mass , should suggest malignancy . the main target of surgical treatment is complete surgical excision . in young patients with mpnst , the most important risk factor is the presence of nf , and in such cases , total excision of the mass is very difficult .