Soft Tissue
Fibroblastic / myofibroblastic tumors
Low-grade myofibroblastic sarcoma

Author: Raul Gonzalez, M.D. (see Authors page)
Editor: Jerad M. Gardner, M.D.

Revised: 26 August 2016, last major update April 2013

Copyright: (c) 2002-2016,, Inc.

PubMed Search: Low-grade myofibroblastic sarcoma [title]

Cite this page: Low-grade myofibroblastic sarcoma. website. Accessed October 28th, 2016.
Definition / General
  • Rare tumor composed predominantly of malignant myofibroblasts
  • Also called "myofibrosarcoma"
  • Deep intramuscular tumor of head and neck (tongue and oral cavity) and extremities (1.5 - 17 cm), rarely abdominopelvic (J Clin Pathol 2008;61:301)
  • Commonly recurs, only rarely metastasizes; recurrences may be higher grade
  • No corresponding "high-grade myofibroblastic sarcoma"; such a tumor would fit under the designation of undifferentiated pleomorphic sarcoma
  • No uniformly accepted criteria for diagnosis; some authors suggest EM appearance is the gold standard (Int J Surg Pathol 2013;21:29)
  • Any age, but predominantly patients in their 30s or 40s; slight male predominance
Case Reports
  • Complete excision
Gross Description
  • Firm, pale, fibrous cut surface, ill-defined margins
Micro Description
  • Circumscribed to diffusely infiltrative with fascicles or storiform growth of spindled tumor cells
  • Cells have ill-defined pale eosinophilic cytoplasm, fusiform nuclei that are elongated or wavy with evenly distributed chromatin or round and vesicular with indentations and small nucleoli
  • At least focal moderate nuclear atypia with hyperchromasia and irregular nuclear membranes
  • Collagenous matrix with prominent hyalinization
  • May have numerous thin-walled capillaries
  • 1 - 6 mitoses / 10 HPF
  • No histiocytic giant cells or prominent inflammation
Micro Images

Images hosted on PathOut servers:

Courtesy of Mark R. Wick, M.D.



Case of the Week #310

Images hosted on other servers:

SMA+, vimemtin+, fibronectin+, type IV collagen-

Femur, hypocellular area with a proliferation of spindle cells, SMA

Positive Stains
Negative Stains
Electron Microscopy Description
  • Myofibroblastic features of indented and clefted nuclei, variable rough endoplasmic reticulum, discontinuous basal lamina, fibronexus junctions (Ultrastruct Pathol 2008;32:97)
Electron Microscopy Images

Images hosted on PathOut servers:

Courtesy of Mark R. Wick, M.D.

Molecular / Cytogenetics Description
  • Gains at 1p11→p36.3 (66%), 12p12.2→p13.2 (45%), 5p13.2→p15.3 (31%), +22 (28%), loss at 15q25→q26.2 (24%) (Am J Clin Pathol 2009;131:701)
Differential Diagnosis
  • Fibromatosis: minimal nuclear atypia, usually negative for myogenic markers (but may sometimes express them focally)
  • Fibrosarcoma: may have focal (not diffuse) myofibroblastic differentiation; "herringbone" fascicular pattern, a diagnosis of exclusion (Histopathology 2006;49:152) that should be made with much hesitation
  • Inflammatory myofibroblastic tumor: tumor of myofibroblasts with prominent lymphoplasmacytic infiltrate scattered amongst tumor cells, usually less atypia than myofibroblastic sarcoma, often ALK+ (Hum Pathol 2008;39:846)
  • Leiomyosarcoma: alternating fascicular pattern, more eosinophilic cytoplasm, more strong and diffuse staining for myogenic makers, usually dense cytoplasmic actin staining rather than "tram track" pattern
  • Myofibroma / myofibromatosis: often multiple and often in children (although solitary lesions and lesions in adults are both common as well), usually zonated with peripheral myoid nodules with intervening central cellular spindle areas with ectatic "hemangiopericytic" vessels; more diffuse or atypical forms of myofibroma may be very difficult to distinguish from myofibroblastic sarcoma (zonation is helpful feature)
  • Nodular fasciitis: not infiltrative, not deep, < 3 cm usually, lacks chromosomal anomalies seen in myofibroblastic sarcoma (although recently a recurrent translocation resulting in MYH9-USP6 gene fusion has been described in many cases of nodular fasciitis (Lab Invest 2011;91:1427)
  • Solitary fibrous tumor (SFT): diffusely positive for CD34, usually "patternless" haphazard arrangement of cells, usually no atypia (except in malignant SFT)