Infantile myofibromatosis: report of two cases.

Two cases of solitary type infantile myofibromatosis (IM) were presented. Case 1 was a 6-month-old male infant with a firm, dark red, fixed tumor on his right knee. A computerized tomographic scan revealed that the tumor was attached to the underlying muscle. Case 2 was a 1-month-old male infant with a tumor on his right knee, similar to that in case 1. In neither case was there any clinical evidence of visceral involvement. Histological and immunohistochemical findings were similar. The lesions appeared histologically as non-encapsulated nodules composed of whorled fascicles of spindle-shaped cells and a vascular element with a hemangiopericytoma-like appearance. The tumor cells were positively stained with PTAH. They were positive for alpha-smooth muscle actin and vimentin, but negative for desmin. These findings support the myofibroblastic nature of IM. In case 2, the tumor regressed spontaneously at the age of 12 months. Unlike the multicentric form, spontaneous regression of the solitary form of IM has not previously been reported.