Low-grade myofibroblastic sarcoma

Last revised by Joachim Feger on 15 Sep 2022

Low-grade myofibroblastic sarcomas, also known as myofibrosarcoma are locally aggressive rarely metastasizing fibroblastic tumors.

Low-grade myofibroblastic sarcomas most commonly occur in adults in the 4th and 5th decade of life with a minor male predominance and are rarely found in children 1.

Typical manifestation is a painless growing mass, rarely associated with pain or symptoms related to organ displacement 1.

The diagnosis of low-grade myofibroblastic sarcoma is based on histology and immunohistochemistry 1.

Diagnostic criteria according to the WHO classification of tumors: soft tissue and bone (5th edition) 1:

  • diffuse infiltrative growth pattern often affecting skeletal muscle
  • cellular bundles composed of spindle cells with pale eosinophilic cytoplasm
  • at the least moderate focal nuclear atypia
  • variable positivity for desmin and/or smooth muscle actin (SMA)

Low-grade myofibroblastic sarcomas are aggressive tumors with diffusely infiltrative growth characterized by fascicles of eosinophilic spindle cells with at least focal nuclear atypia 1.

Low-grade myofibroblastic sarcomas feature a wide anatomic distribution. Common locations include the extremities and the head and neck region including the tongue and oral cavity. The skin and gastrointestinal tract are rarely affected 1-3.  

Macroscopically low-grade myofibroblastic sarcomas are firm, lobulated tumors with a pale appearance. Their margins are usually infiltrative and less commonly well-circumscribed 1.

The microscopic spectrum of low-grade myofibroblastic sarcomas includes the following 1-3:

  • diffuse infiltrative growth pattern
  • sometimes between skeletal muscle fibers
  • composed of spindle tumor cells in cellular fascicles with pale eosinophilic cytoplasm
  • at least focally nuclear atypia
  • sometimes collagenous hyalinized background

Immunohistochemistry stains should express smooth muscle actin and/or desmin 1.

There are only varying descriptions of imaging appearances in low-grade myofibroblastic sarcomas. They seem to appear as unspecific soft tissue mass and vary significantly also with the site of the tumor 2,4.

Some low-grade myofibroblastic sarcomas might show calcifications or ossification. Lesions with osseous involvement have been described as osteolytic, destructive masses 2.

Imaging findings have been described as diverse and seem to vary with the site of involvement 2,5.

Signal characteristics were described as follows:

  • T1: mostly hypo- to isointense to muscle
  • T2: heterogeneous high signal intensity
  • T1 C+ (Gd): ranges from homogenous to heterogeneous predominantly peripheral enhancement

The radiological report should include a description of the following:

  • form, location and size
  • tumor margins
  • relation to adjacent bones and muscular fasciae
  • relationship to local nerves and vessels
  • relationship to other organs

Similar to other mesenchymal tumors, surgery is the mainstay. Local recurrence is common often happening after a prolonged time. Metastases occur rarely 1.

Low-grade myofibroblastic sarcomas have been defined as a separate entity by the German pathologist Thomas Mentzel and colleagues as a specific entity in 1998 3-6.

Conditions that can mimic the presentation and/or the appearance of low-grade myofibroblastic sarcomas include 1-4:

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