OBJECTIVETo study the clinicopathologic characteristics, immunophenotypes and differential diagnosis of primitive myxoid mesenchymal tumor of infancy (PMMTI).

METHODSThe clinical data, histological features and immunohistochemic results of 3 cases of PMMTI were reviewed.

RESULTSThere were 2 males and 1 female aged 4 years, 2 days and 3 months respectively. The tumor occurred in the head and neck (n = 2), and lumbar regions (n = 1).Histologically, they were composed of ovoid, short spindled to polygonal mesenchymal cells with less eosinophilic cytoplasm, or vacuolated cytoplasm. There was mild nuclear atypia with mitotic activity of 0-2/10 HPF.In most areas, the neoplastic cells showed a diffuse growth pattern, whereas in some areas, they formed a vaguely nodular pattern with peripheral collagenized stroma. They were embedded in a myxoid stroma that contained a rich delicate vascular network. Besides, small cyst-like spaces were also present in one case. The tumor cells expressed vimentin, but not alpha smooth muscle actin, desmin, myogenin, S-100 protein, CD34 and cytokeratin. The patients underwent surgery.One patient had local recurrences twice and died 2 years later. Compared with the primary tumor, the recurrent lesions exhibited increased cellularity, marked cellular atypia and mitotic activity (10/10 HPF). The other two patients remained well with no evidence of disease at last during follow-up.

CONCLUSIONSPMMTI is a rare soft tissue tumor of infancy, composed of primitive mesenchymal cells and myxoid stroma.It occurs mainly in the somatic soft tissues of the trunk, head and neck region, and the extremities, and is characterized by a high rate of local recurrence if incompletely excised. Metastasis and tumor related death may occur, albeit very rarely.Increased awareness of this novel entity will help avoid misinterpreting the lesion as a variety of other infantile mesenchymal neoplasms, including congenital fibrosarcoma and lipoblastoma.