Primitive Myxoid Mesenchymal Tumor of Infancy, PMMTI

Definition

A newly proposed primitive fibroblastic neoplasm of at least borderline malignancy occurring in infants to be distinguished from congenital-infantile fibrosarcoma. There has been one paper reporting six cases1. Prior reports of congenital-infantile fibrosarcoma and undifferentiated sarcoma may have included cases of primitive myxoid mesenchymal tumour of infancy.

Epidemiology

Clinical features

Radiology

Macroscopic appearances

The tumour is multinodular, unencapsulated but focally infiltrative.

Histopathology

There is a diffuse proliferation of primitive spindle, polygonal or round cells within a myxoid stroma. The cell cytoplasm varies from pale eosinophilic to clear and vacuolated. Cellularity is low to moderate. The cells may form nodules with greater cellularity towards the margins, where a herringbone patttern may be apparent. Cells may cluster around arterioles and venules. The mitotic may be up to 10 per 10 HPF but atypical mitoses are lacking.

Recurrent tumours tend to be more cellular.

Immunohistochemistry

	Vimentin	5/51
	SMA	1/5(focal staining in one case)1
	MSA	0/61
	Desmin	0/61
	Myoglobin	0/21
	Myogenin	0/21
	Myosin	0/11
	S-100	0/61
	Leu7	0/51
	Synaptophysin	0/21
	NSE	0/51
	Cytokeratin	0/51
	EMA	0/61
	CD34	0/51
	CD99	0/31
	Thyroglobulin	0/11
	CD1a	0/11
	CD45	0/11
	CD68	0/11
	Mast cell tryptase	0/11

Ultrastructure

The tumour cells are poorly differentiated fibroblasts lacking intermediate filaments. The matrix contains collagen.

Molecular genetics

The ETV6-NTRK3 gene fusion product is absent by PCR.

Differential diagnosis

Congenital-infantile fibrosarcoma: is not so myxoid, lacks vacuolated cells and shows a t(12;15)(p13;q25) translocation resulting in the ETV6-NTRK3 gene fusion product.
Undifferentiated sarcoma
Infantile fibromatosis: the diffuse mesenchymal type resembles PMMTI but differs in showing interspersed adipose cells, peripheral lymphocytes, a distinct bundled and fascicular architecture, higher cellularity, and greater mitotic activity.
Low-grade fibromyxoid sarcoma: usually occurs after the age of two years. There are both fibrous and myxoid zones and a curvilinear vasculature is apparent. A t(7;16)(q32-34;p11) translocation results in a chimeric FUS/CREB3L2 gene.
Myofibrosarcoma is very rare in infancy. A myxoid background is lacking. There is positivity for desmin, smooth muscle actin, and muscle specific actin.
Myxofibrosarcoma: occurs in the elderly.

Management

Surgical excision.

Prognosis

The tumours show a long clinical course with local recurrences. The behaviour is more aggressive than for congenital-infantile fibrosarcoma.

References

1 Alaggio R, Ninfo V, Rosolen A, et al. Primitive myxoid mesenchymal tumor of infancy: a clinicopathologic report of 6 cases. Am J Surg Pathol 2006; 30:388-94

Sheng WQ, Hisaoka M, Okamoto S, et al. Congenital-infantile fibrosarcoma. A clinicopathologic study of 10 cases and molecular detection of the ETV6-NTRK3 fusion transcripts using paraffin-embedded tissues. Am J Clin Pathol 2001; 115:348-55

This page last revised 15.4.2006.