LAUSR

Undifferentiated sarcoma (UNDS) of the soft tissue is an exceedingly rare disease. Its diagnosis depends mainly on molecular and immunohistochemical analyses to exclude other soft-tissue sarcomas. It is difficult to confirm a positive diagnosis by imaging pathological features because of their rarity and similarity with other conditions. Since 2013, only 13 cases of undifferentiated soft-tissue sarcoma, mostly diagnosed through imaging of solid tumors in infant and children, have been reported. The authors present a rare case of a 3-month-old Chinese boy with UNDS primarily in the left lower extremity and characterized by a cystic and solid growth pattern.