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metanephric stromal tumour ( mst ) , first described in 2000 by argani et al . , is a rare , purely stromal specific renal neoplasm that mainly affects children .
its characteristic microscopic appearance and immunohistochemical profile helps to differentiate it from congenital mesoblastic nephroma ( cmn ) and clear cell clear cell sarcoma of kidney ( ccsk ) .
since then , there have been only five case reports of this tumour in the english literature .
its recognition is essential as it spares the patient from unnecessary potentially toxic adjuvant chemotherapy .
a case of paediatric metanephric stromal tumour is described and the clinical presentation , radiological and pathological findings , differential diagnoses and biological behaviour are discussed .
he had one episode of hematuria at two years of age but no other complaints such as abdominal pain , fever , loss of appetite or weight loss . on systemic examination
as part of the initial work up , an ultrasonography of abdomen was performed , this showed a partly exophytic hypo - echoic mass arising from the interpolar region , measuring 4.7  4.8  5.3 cm .
ultrasound guided biopsy of the renal mass was performed , which on microscopic examination showed a tumour composed of sheets of spindle shaped cells with oval to elongated nuclei and moderate amounts of cytoplasm .
sectioning revealed a firm white tumour , 7.5  5.5  4.5 cm replacing most of the kidney .
the neoplasm appeared to abut the pelvis but there was no infiltration into pelvic or perirenal fat [ figure 1 ] .
microscopically the tumour had a slightly nodular architecture due to varying cellularity and was composed of spindle cells with thin tapered bland nuclei an indistinct cytoplasm [ figure 2 ] .
focally , a concentric arrangement around entrapped tubules and vessels , imparting  the onion skin morphology , was noted [ figure 3 ] .
the tumour was well demarcated from the adjacent kidney , but without a definite capsule [ figure 4 ] .
some blood vessels showed hypercellularity around the endothelium with thickened walls but there were no features of dysplasia [ figure 5 ] .
there was no evidence of capsular or vascular invasion , heterologous differentiation or neoplastic epithelium .
on immunohistochemistry , the cells were positive for vimentin , focally positive for cd-34 [ figure 6 ] and negative for wt-1 , desmin and smooth muscle actin .
gross pathological findings : cut surface of the kidney shows that the tumour was firm grey white , 7.5  5.5  4.5 cm in size , replacing most of the kidney microphotograph of neoplasm showing slightly nodular architecture due to varying cellularity and was composed of spindle cells with thin tapered bland nuclei an indistinct cytoplasm ( hematoxylin and eosin stain , 100 ) microphotograph of neoplasm showing concentric arrangement around entrapped tubules , imparting  onion skin appearance ( hematoxylin and eosin stain , 200 ) microphotograph of neoplasm showing well demarcation from the adjacent kidney but without a definite capsule ( hematoxylin and eosin stain , 200 ) microphotograph of neoplasm showing blood vessels having hypercellularity around endothelium with thickened walls but there were no features of dysplasia ( h and e stain , 200 ) immunohistochemical staining for cd 34 showing focal positivity ( 200 )
the origin of mst is not entirely clear , but it is thought to be related to wilm 's tumour , unlike other renal stromal tumours such as cmn , ccsk and rhabdoid tumour of kidney ( rtk ) .
beckwith has postulated that mst may represent the result of maturation of intralobar nephrogenic rests ( inr ) with the loss of any active blastemal component .
characteristic histological features of mst include ( a ) alternating cellularity that imparts a nodular low power appearance ( b ) onion skin cuffing around entrapped renal tubules ( c ) heterologous differentiation and vascular alterations ( d ) juxta glomerular cell hyperplasia and ( e ) patchy positivity of tumour cells for cd 34 , though variable .
the last three features are not present in all the cases.[14 ] our case did not show heterologous differentiation , angiodysplasia of the vessel wall or juxta glomerular cell hyperplasia . however , the other histological features and patchy positivity for cd34 and negativity for desmin helped to exclude other neoplasms in the differential diagnosis .
mst is differentiated from cmn by its scalloped , subtly infiltrative border , in contrast with the deeply invasive nature of most cmn .
also mst stains for cd34 , while cmn is reported to be positive for desmin .
clinically , patients of mst are older than the known upper limit for those with cmn ( cmn is usually a tumour of infancy ) .
mst can be differentiated from ccsk by the regular branching capillary vascular pattern which is characteristic of ccsk but absent in mst .
the distinction between mst and metanephric adenofibroma ( maf ) was possible due to the absence of discrete epithelial nodules , more characteristic of maf .
is an abdominal mass followed by hematuria , recurrent urinary tract infection ( uti ) , incontinence , fever , anemia and hypertension .
our patient presented with the complaint of dribbling of urine . as part of the initial work up , ultrasound of the abdomen
mst is a benign tumour that is centered in medulla , but , in one case continous extension through bladder into the prostatic urethra has been reported .
only three cases , out of the thirty one cases described by argani et al . had a maximum dimension greater than 7.5 cm , the largest in his series measuring 10 cm .
none of the five cases reported thereafter [ table 1 ] were larger than 5.5 cm in diameter .
most msts were previously classified as congenital mesoblastic nephroma until recently , when beckwith and others re - characterized this neoplasm after reviewing the files of the national wilm 's tumour study pathology centre .
an accurate radiological diagnosis is impossible and the role of partial nephrectomy is unclear at this time . at present , all clinical mst should be fully excised in order to establish a definitive diagnosis . once the diagnosis is confirmed , no further adjuvant therapy is required .
summary of the 5 cases of metanephric stromal tumour described in the literature after the report of argani et al.:[26 ]
a case of incidentally detected metanephric stromal tumour ( mst ) is reported here . 
 this is a rare , recently recognized pediatric benign stromal specific renal neoplasm . 
 a review of the english literature revealed only five cases after its original description by argani et al . recognition of this entity can spare a child from potentially toxic adjuvant chemotherapy that might be used to treat malignant lesions which are part of the differential diagnosis , particularly clear cell sarcoma of kidney ( ccsk ) .