Breast - nonmalignant
Benign tumors / changes
Fibromatosis of breast

Author: Monika Roychowdhury, M.D. (see Authors page)

Revised: 7 August 2017, last major update June 2017

Copyright: (c) 2002-2017,, Inc.

PubMed Search: Fibromatosis [title] breast [title]
Cite this page: Fibromatosis of breast. website. Accessed October 19th, 2018.
Definition / general
  • Rare benign entity with clonal proliferation of fibroblasts and myofibroblasts similar to counterparts elsewhere
Essential features
  • Rare benign entity characterized by spindle cell proliferation of fibroblasts and myofibroblasts as well as varying amounts of collagen
  • No metastatic potential, however, it can have local recurrences
  • Radiologically and grossly appears as a mass lesion
  • Histology shows low grade myofibroblastic proliferation
  • Treatment by wide local excision
  • Also referred to as desmoid tumor or low grade fibrosarcoma
  • May arise in mammary gland or in chest wall musculoaponeurotic tissue and extend into breast
Clinical features
Radiology description
  • Irregularly shaped, high density with spiculated margin on mammography
  • Ultrasound shows a poorly defined hypoechoic mass with posterior acoustic shadowing and an echogenic rim, closely mimicking cancer (AJR Am J Roentgenol 2005;185:488)
Radiology images

Images hosted on Pathout server:
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Mammogram, contributed by Dr. Mark R. Wick

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Associated distortion and tenting of the underlying pectoralis muscle

Case reports
  • Wide local excision with careful attention to negative margins although may recur even with negative margins (Ann Surg Oncol 2008;15:274)
Gross description
  • Ill defined, white tan gray fibrous tissue
Gross images

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Solid ill defined mass

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Underlying chest wall
musculature, ribs and
parietal pleura

Microscopic (histologic) description
  • Irregular, nonencapsulated proliferation of spindle cells forming interlacing fascicles with variable collagen deposition and cellularity
  • Usually extends into surrounding fat and glandular parenchyma
  • May have focal lymphoid aggregates at periphery
  • May contain eosinophilic inclusions similar to those of infantile digital fibromatosis
  • No / rare mitotic figures
  • No epithelial component
Microscopic (histologic) images

Images hosted on Pathout server:

Contributed by Dr. Mark R. Wick

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Neoplastic infiltrate surrounds ducts and lobules

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Sparsely cellular collagenous area with lymphocytes

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Spindle cells with uniform nuclei and no mitotic activity

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Keloidal collagen surrounds atrophic duct

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Spindle cells in myxoid stroma

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Focally, cells have large hyperchromatic nuclei

Images hosted on other servers:

Various images

Bland spindle cells and collagen

Mammary ductal epithelium

Myofibroblastic spindle cells

Low grade myofibroblastic proliferation

Invading muscle

Bland fibroblasts of moderate cellularity


Fig A: Infiltration into adjacent breast tissue
Fig B: Infiltration into skeletal muscle
Fig C: Monotonous, bland spindle cells with abundant
extracellular collagen, occasional small nucleoli

Cytology description
  • Hypocellular spindle cells with no / rare glandular elements
Positive stains
Electron microscopy description
  • Fibroblasts and myofibroblasts
Molecular / cytogenetics description
  • Nuclear accumulation of beta-catenin in stromal tumor cells (82%), somatic alterations of APC / beta-catenin pathway (79%, Hum Pathol 2002;33:39)
Differential diagnosis
Board review question #1
A 39 year old woman presents with a 3 cm left breast mass with skin retraction. Histological examination of the needle core biopsy shows a predominantly benign spindle cell proliferation intermixed with varying amounts of collagen. A rare focus shows benign glandular elements. SMA is positive; ER, PR is negative. What would be your recommendation for this patient?

  1. Benign lesion, no further treatment needed
  2. Phyllodes tumor, needs lumpectomy
  3. Tumor reduction by chemoradiation prior to surgery
  4. Wide local excision
Board review answer #1
    D. Wide local excision - fibromatosis should be treated by wide local excision to prevent local recurrences. No other therapy is needed unless the lesion is unresectable due to its extension into surrounding structures.