Discussion:
Infantile myofibromatosis is one of the most common tumors of infancy. It may also occur in adults. It manifests as solitary or multicentric nodules in the dermis or subcutis. A solitary nodule is commonly seen in the region of the head, neck, and trunk. Multicentric lesions occur in muscle, internal organs and the skeleton, in addition to the dermis and subcutis.

Histologically, infantile myofibromatosis shows a biphasic pattern or zoning phenomenon. A staining area contains plump myoid spindle cells with eosinophilic cytoplasm arranged in nodules, short fascicles or whorls, with or without hyalinization. This pattern may however, be distributed haphazardly throughout the lesion. A central or dark staining area contains round or polygonal cells with slightly pleomorphic hyperchromatic nuclei or small spindle cells typically arranged around a distinct hemangiopericytoma-like vascular pattern. Myofibromatosis shows a positive immunoreaction with smooth muscle actin and vimentin, but is negative for S100. Prognosis depends on the extent of visceral involvement and it may be fatal.

There may be focal hemorrhage, cystic degeneration or coagulative necrosis and these may lead to a misdiagnosis of sarcoma. In addition, about 20% of the cases have intravascular growth , but this does not affect prognosis.

The differential diagnosis includes neurofibroma, nodular fasciitis, infantile fibromatosis, fibrous histiocytoma, fibrosarcoma.

Nodular fasciitis: Uncommon in infants; no hemangiopericyma-like pattern; occasionally has extravasated red blood cells. However, there is zonation, and vimentin and SMA are positive

Neurofibroma: Usually affects older children; evidence of café au lait spots

Fibrous histiocytoma: Storiform pattern; no zonation. Usually express factor XIIIa, but not SMA.

Infantile fibromatosis: More uniform spindle cell pattern; no zonation; no central hemangiopericytoma-like vascular pattern

Fibrosarcoma: Distictive herringbone pattern; commonly involves the extremities; spindle cells with hyperchromatic nuclei; prominent mitotic activity; usually has hemorrhage and necrosis with infiltrative borders.

Reference:

Fletcher, CBM, Unni, KK, Mertens, F (Eds.): World Health Organization Classification of Tumors. Pathology and Genetics of Tumors, Soft Tissue and Bone. IARC Press: Lyon 2002.