= true mesenchymal benign neoplasm

Histo: bland spindle shaped cells embedded in hypovascular abundantly myxoid stroma ± hypercellular areas with increased vascularity

Age: 4^th–6^th decade ; M<F

Location: head & neck (scalene muscle, masticator space and posterior paraspinal muscles)

Site: intramuscular myxoma (82%), intermuscular, subfascial

Associated with: Mazabraud syndrome McCune-Albright syndrome

CT:

• well-defined ovoid mass with attenuation similar to water

MR:

• well-defined ovoid lesion with SI characteristic of fluid ← high mucin + low collagen content
• thin peritumoral rim of perilesional fat on T1WI (70%):
• T2 hyperintense ← fatty atrophy of muscle
• fine inner linear stranding on T2WI ← fibrous septa
• internal enhancement:
  1. peripheral ← pseudocapsule
  2. peripheral + internal linear stranding
  3. peripheral + internal central focal patchy areas

Dx: core-needle / surgical biopsy

DDx: myxoid peripheral nerve sheath tumor, myxofibrosarcoma, myxoid liposarcoma, extraskeletal myxoid chondrosarcoma, undifferentiated pleiomorphic sarcoma