Stanford School of Medicine
Surgical Pathology Criteria
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Retroperitoneal and Pelvic Fibromatosis


  • Cytologically bland, at most moderately cellular, deep infiltrative fibroproliferative process primarily involving retroperitoneum or pelvis

Alternate / Historical Names

  • Intra-abdominal fibromatosis

Diagnostic Criteria

  • Over age 5 years by definition
  • Essentially the same as abdominal desmoid fibromatosis except for location and lack of association with pregnancy or polyposis
  • Primarily based in retroperitoneum or pelvis by definition
    • Extremely rare outside the pelvic retroperitoneum
    • If retroperitoneum involved by extension from mesentery, lesion should be considered mesenteric fibromatosis
  • Low to moderate cellularity
  • Bland spindle cells in fascicles or haphazard
    • Nuclei small, dark to slightly enlarged and vesicular
    • Cytoplasm scant
  • Stroma may be densely collagenous or myxoid
    • May have keloid like fibers
    • Rare metaplastic cartilage and bone
  • Mitotic figures infrequent
    • Rarely >5 per HPF
    • Never atypical
  • Inflammation not prominent
    • May be seen focally, usually peripherally
  • Gross circumscription may be deceptive
  • Nearly always over 3 cm
    • Usually 3-7 cm
  • Focal hemorrhage may be seen
    • Hemosiderin rare

    Richard L Kempson MD
    Robert V Rouse MD
    Department of Pathology
    Stanford University School of Medicine
    Stanford CA 94305-5342

    Original posting: October 15, 2007
    Updates: January 26, 2008; February 2, 2009; February 8, 2009

Supplemental studies


Actin 50%
Desmin usually negative, occasionally focal
Beta-catenin 90%
CD117 CD117 frequently negative, variable reports of focal/weak staining
CD34 negative
S100 negative
Keratin negative
Estrogen receptor 0-10%
Progesterone receptor 0-25%
Response to hormone therapy is not related to receptor status
Reports of frequent strong reaction for CD117 appear to be due to a cross-reacting antibody (Miettinen 2001)


Differential diagnosis

Fibromatosis, Abdominal Desmoid, Extra-abdominal Desmoid, Mesenteric, Retroperitoneal and Pelvic Scar
Uniform Multiple stages of organization
Hemosiderin rare Hemosiderin common
Rare under 3 cm Rare over 3 cm
Inflammation not prominent Inflammation may be prominent
Usually infiltrates muscle Infiltration of muscle uncommon


Fibromatosis, Mesenteric, Retroperitoneal and Pelvic Sclerosing Mesenteritis
Scant inflammation Prominent inflammatory component
Fibrous mass with peripheral infiltration Surrounds and entraps fat
Fat necrosis infrequent Fat necrosis frequent
Beta-catenin positive (nuclear) Beta-catenin negative


Fibromatosis, Mesenteric, Retroperitoneal and Pelvic Retroperitoneal Fibrosis
Scant inflammation Prominent inflammatory component
Forms a mass Diffuse, usually no mass lesion
Rarely involves both ureters Medial deviation of both ureters
Stroma diffusely collagenous Broad bands of hyalinized collagen


Fibromatosis, Mesenteric or Retroperitoneal and Pelvic Gastrointestinal Stromal Tumor
CD34 negative CD34 60-70% positive
CD117 frequently negative, variable reports of focal/weak staining CD117 74-95% positive
DOG1 negative DOG1 87-94%
Beta-catenin positive 90% (nuclear) Beta-catenin negative
Low to moderate cellularity Moderate to high cellularity
Cytologically bland May be cytologically atypical
Prominent thin walled dilated veins Lacks prominent veins
Infiltrative margin Usually circumscribed, pushing margin
No cystic degeneration or necrosis May have cystic degeneration or necrosis
GIST with epithelioid or abundant spindled cytoplasm can be distinguished morphologically


Fibromatosis, Abdominal Desmoid, Extra-abdominal Desmoid, Mesenteric, Retroperitoneal and Pelvic Inflammatory Myofibroblastic Tumor
Rare <12 years of age Age usually <14, rare >30
Inflammation not prominent Prominent inflammation, particularly plasma cells
Alk1 negative Alk1 frequently positive
Beta catenin 80-90% Beta catenin negative


Fibromatosis, Mesenteric, Retroperitoneal and Pelvic Adult Fibrosarcoma
Intra-abdominal Very rare in abdominal cavity
Mild to moderate cellularity Usually more cellular
Fine chromatin Usually clumped chromatin
Rarely >5 mitotic figures / hpf Usually >5 mitotic figures / hpf
Rarely necrotic Frequent necrosis
Distinction between grade I fibrosarcoma and fibromatosis may not always be clear


Fibromatosis, Abdominal Desmoid, Extra-abdominal Desmoid, Mesenteric, Retroperitoneal and Pelvic Myxofibrosarcoma
No pleomorphism Cytologic pleomorphism present
Usually dense stroma but focally may be myxoid No dense stroma
Vessels lack arching pattern Arching vessels


  • Over age 5 years by definition
    • Nearly always over age 12
    • Most occur in women 20-30 years old
  • Not associated with pregnancy or familial polyposis
  • Frequent recurrence
    • 20-30% overall
    • Recurrences may be locally aggressive and destructive
  • Deep fibromatoses differ primarily in their clinical association
      Abdominal Extra-abdominal Mesenteric Pelvic / Retroperitoneal Infantile
    Pregnancy + - - - -
    Familial polyposis rare - 10-15% - -




  • Kempson RL, Fletcher CDM, Evans HL, Henrickson MR, Sibley RS. Tumors of the Soft Tissues, Atlas of Tumor Pathology, AFIP Third Series, Fascicle 30, 2001
  • Fletcher CDM, Unni KK, Mertens F. Pathology and Genetics of Tumours of Soft Tissue and Bone, World Health Organization Classification of Tumours 2002
  • Weiss SW, Goldblum JR. Enzinger and Weiss's Soft Tissue Tumors, 4th edition, 2001
  • De Wever I, Dal Cin P, Fletcher CD, Mandahl N, Mertens F, Mitelman F, Rosai J, Rydholm A, Sciot R, Tallini G, Van Den Berghe H, Vanni R, Willen H.  Cytogenetic, clinical, and morphologic correlations in 78 cases of fibromatosis: a report from the CHAMP Study Group. CHromosomes And Morphology.  Mod Pathol 2000 Oct;13(10):1080-5
  • Yantiss RK, Spiro IJ, Compton CC, Rosenberg AE. Gastrointestinal stromal tumor versus intra-abdominal fibromatosis of the bowel wall: a clinically important differential diagnosis. Am J Surg Pathol. 2000 Jul;24(7):947-57.
  • Miettinen M. Are desmoid tumors kit positive? Am J Surg Pathol. 2001 Apr;25(4):549-50.
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