Infantile Myofibromatosis (IMF) was first described by Stout in 1954 as 'congenital generalized fibromatosis' and was renamed as infantile myofibromatosis by Chung and Enzinger in 1981 after recognition of the myofibroblastic nature of the lesion.
(1)Solitary myofibromatosis: Characterized by the presence of one nodule in the skin, muscle, bone or subcutaneous tissue; and
(2)Multicentric type which can be divided into two sub-types :(2a)Multicentric lesions but without visceral involvement. (2b)Visceral involvement is present.
(3) Adult-type myofibroma
Note: The term myopericytoma was first proposed in 1996 by Requena as an alternative designation for ' solitary myofibroma' . It was later adopted in 1998 to describe a spectrum of tumours with striking concentric perivascular proliferation of spindle cells (perivascular myoid cells). [This spectrum includes - infantile-type myofibromatosis ; solitary myofibroma ; benign myopericytoma; infantile hemangiopericytoma ; glomangiopericytoma]
IMF is the most common fibrous tumour of infancy and must be considered when evaluating children who present with either solitary or multiple tumours, particularly during the neonatal period.
In Solitary myofibromatosis skin, subcutaneous tissue and skeletal muscle are affected most frequently, usually in the head, neck and trunk. The most frequently affected bones are the skull, vertebrae, ribs, femur and tibia. Solitary lesions involving the viscera are rare. Solitary or multicentric lesions confined to skin, soft tissues or bone has a good prognosis. These are usually cured by simple excision.
In the generalized form, the most common locations are the lung, heart, gastrointestinal tract and pancreas, as well as rarely the central nervous system., Infants with generalized visceral lesions have the worst prognosis. Death in these cases often occurs due to cardio-pulmonary or gastrointestinal complications.
Solitary Myofibroma - Adult lesions are often solitary and superficial in nature. The tumour usually presents as a cutaneous nodule in head and neck region. Visceral and osseous lesions have not been reported.
Gross features: The lesion may present as a firm scar or a superficially located well circumscribed but uncapsulated nodule. The deeper lesions are more infiltrative and diffuse in nature.
Microscopically, each nodule has a central and peripheral area.
The peripheral area consists of spindle cells (myofibroblasts) with eosinophilic cytoplasm and ovoid nuclei arranged in well-demarcated short bundles and fascicles resembling smooth muscle.
The central portion consists of less differentiated rounder cells with pale cytoplasm and basophilic, small round nuclei, arranged in a hemangiopericytoma-like pattern.
Hemangiopericytomatous and myofibroblastic components occur in variable proportions.
More primitive areas show necrosis. Focal areas of calcification may be present. Mild to moderate nuclear pleomorphism can be found in most lesions. Normal mitotic figures may be quite common. Abnormal mitotic figures are usually not present.
Vascular invasion (subendothelial proliferation of perivascular spindle cells) is common, but does not affect the prognosis.
Cells are vimentin and alpha-smooth muscle actin - Positive. Desmin- shows inconstant expression. S100 protein, epithelial membrane antigen & keratin - Negative
Infantile hemangiopericytoma- Infantile hemangiopericytoma versus infantile myofibromatosis. Study of a series suggesting a continuous spectrum of infantile myofibroblastic lesions. Am J Surg Pathol 1994 Sep;18(9):922-30
Sections from the central cellular portion may show features mimicking small round cell sarcomas with a hemangiopericytoma-like vasculature: Eg- PNET , mesenchymal chondrosarcoma, poorly differentiated synovial sarcoma
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