Synopsis

 infants or neonates (50%)
 localization: trunk, extremities, and head and neck
 nonencapsulated tumors
 multinodular appearance
 focal infiltrative growth
 white fleshy cut surface
 tumor diameter ranging from 2 to 15 cm
 diffuse growth
 primitive spindle cells or polygonal cells or round cells
 myxoid background
 vaguely nodular pattern
 peripheral collagenized stroma
 higher cellularity at the periphery
 delicate vascular network in the background

Immunohistochemistry

 diffuse reactivity for vimentin
 no reactivity for smooth muscle actin, muscle specific actin, desmin, S-100 protein, or myogenin

Electron microscopy

 poorly differentiated fibroblastic proliferation

Cytogenetics

 Chr.Y rearrangements
 Chr.9 rearrangements
 Chr.3 rearrangements

Differential diagnosis

 soft tissue sarcomas

• embryonal rhabdomyosarcoma
• Ewing sarcoma/primitive neuroectodermal tumor
• congenital infantile fibrosarcoma
• primitive sarcomas
  • undifferentiated sarcoma

Complications

 possible recurrences
 possible metastasis

References

 Alaggio R, Ninfo V, Rosolen A, Coffin CM. Primitive myxoid mesenchymal tumor of infancy: a clinicopathologic report of 6 cases. Am J Surg Pathol. 2006 Mar;30(3):388-94. PMID: #16538060#