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infantile myofibromatosis is a benign fibrous tumor of infancy and was first described by stout in 1954 . in most cases , it is present at birth , and in 90% of cases , the tumor appears within the first 2 years of life [ 2 , 3 ] .
the prognosis is excellent in the solitary type , which is limited in the skin , muscle , and subcutaneous lesions [ 2 , 3 , 4 ] .
in contrast , the multicentric form of infantile myofibromatosis , which has visceral involvement , can be life - threatening [ 4 , 5 ] .
therefore , surgical excision is recommended [ 2 , 6 ] . we present a case of a 3-week - old girl showing features of infantile myofibromatosis ( solitary type )
a 3-week - old , otherwise healthy japanese girl had a solid , red - colored , cutaneous nodule on left shoulder .
the nodule had a central concavity with a crust on the surface and measured 20  21 mm in diameter ( fig .
1 ) . physical examination and ct imaging of the head , chest , abdomen and pelvis revealed no additional lesions .
no infiltration of the tumor into the muscle was identified by mri imaging ( fig .
the nodules were composed of spindle cells with round or oval nuclei and eosinophilic cytoplasm .
a diagnosis of infantile myofibromatosis , leiomyoma , leiomyosarcoma , histiocytoma , or other sarcoma was suggested .
3c ) , but not caldesmon , desmin or s100 protein ( not shown ) .
the mitotic figures were very infrequent [ 6 mitoses per 10 low - power images ( 40 ) ] .
a slight male predominance among patients with both the solitary and multicentric variants was noted by wiswell et al . .
most cases of infantile myofibromatosis are solitary nodules , accounting for up to 70% of cases in one study , and up to 80% in another series .
the prognosis is excellent in the solitary type [ 2 , 3 , 4 ] . in the case of solitary - type infantile myofibromatosis
in contrast , a quarter of the cases with the multicentric form may have visceral involvement and can be life - threatening [ 2 , 4 , 5 ] .
the solitary type of infantile myofibromatosis is usually benign and is typically found in the dermis , subcutis , or deep soft tissues .
the distribution is predominantly on the head , neck , and trunk like our case .
solitary infantile myofibromatosis on an upper extremity accounted for only 13.3% in one study of 45 cases .
the histologic hallmark of infantile myofibromatosis is an un - encapsulated , well - circumscribed lobule of peripheral spindle cells , which bear a close resemblance to smooth muscle [ 9 , 10 ] .
often there is a central area of hemangiopericytoma - like small rounded cells surrounding blood vessels [ 11 , 12 ] .
this combination of features gives infantile myofibromatosis its recognizable biphasic appearance , though the hemangiopericytoma - like appearance was not detected in this case .
the presence of smooth muscle actin in the spindle cells indicates the diagnosis of infantile myofibromatosis or leiomyosarcoma .
considering the difference in prognosis , it was necessary to rule out leiomyosarcoma   in this case . while at least 1 mitotic cell per field in high - power ( 200 or 400 ) fields is detected in leiomyosarcoma ,
very infrequent mitotic figures [ 6 mitotic cells per 10 low - power ( 40 ) fields ] were observed , which definitively indicated infantile myofibromatosis in this case .
previously , radical excision had been advocated as the treatment of choice , because it had been believed that the solitary form gave rise to multiple nodules with potential visceral involvement by metastases .
however , it is now more probable that the solitary and multicentric forms are distinct entities and that the solitary form remains localized and can regress .
therefore , a wait - and - see approach has been suggested more recently as a treatment option .
however , in our patient , the decision was ultimately made to treat with surgical removal to exclude a diagnosis of leiomyosarcoma , which would have had a poor prognosis .
the course was consistent with previous reports of solitary - type infantile myofibromatosis [ 2 , 4 , 15 ] , and supports our histological diagnosis .
infantile myofibromatosis is a rare fibrous tumor of infancy . 
 the cutaneous solitary type has typically an excellent prognosis . however , histologically , it is important to rule out leiomyosarcoma , which has a poor prognosis . 
 the low frequency of mitosis was definitive for a diagnosis of infantile myofibromatosis . 
 we present a cutaneous solitary - type case of infantile myofibromatosis . 
 following incisional biopsy , the tumor remitted spontaneously .