Case 38

A 40-year-old female with a 5.0 cm subcutaneous abdominal mass.

1. What stain will you order to work up and what's the likely diagnosis?

   
   

IHC

• Pan-keratin
• SMA
• Desmin
• MUC4

See All Options

You can vote for more than one answer.

Answer

This is a case of desmoid fibromatosis and it would be ideally nuclear positive for B-catenin (at least 80% of cases). Here is a table to compare fibromatosis versus nodular fasciitis. The core feature is long, sweeping fascicles of bland fibroblasts and myofibroblasts without substantial cytological atypia; infiltrative growth.

Feature

Desmoid Fibromatosis

Nodular Fasciitis

Main Differential Diagnoses

- Fibrosarcoma  - Scar tissue  - Myofibroblastic tumors  - Low-grade fibromyxoid sarcoma (LGFMS)  - Solitary fibrous tumor (SFT)

- Fibrosarcoma  - Myxofibrosarcoma  - Reactive spindle cell lesions  - Fibroma  - Low-grade myofibroblastic sarcoma

Growth Pattern

Infiltrative, extends into adjacent soft tissues

Well-circumscribed but can be infiltrative at edges

Cellular Morphology

Uniform, elongated spindle cells arranged in long sweeping fascicles

Loosely arranged, plump myofibroblasts in a "tissue culture-like" pattern

Stroma

Collagenous, often dense with minimal myxoid changes

Myxoid to collagenous, often gelatinous

Nuclear Features

Bland, elongated nuclei with minimal atypia

Vesicular nuclei, occasional mitotic figures but no atypia

Mitotic Activity

Low mitotic rate

High mitotic rate but lacks atypical mitoses

Necrosis

Absent

Absent

Vascularity

Poorly vascularized

Highly vascularized with extravasated RBCs

Immunohistochemistry (IHC)

- β-catenin: Positive (nuclear)  - SMA: Focal  - Desmin: Variable  - S100, CD34: Negative

- SMA: Positive  - β-catenin: Negative  - Desmin, S100, CD34: Negative

Genetics

- CTNNB1 mutations (sporadic cases)  - APC mutations (associated with familial adenomatous polyposis)

- USP6 gene rearrangement in some cases

Prognosis

High recurrence rate (~30-50%) but no metastasis

Self-limited, often regresses spontaneously

Treatment

Surgery (with negative margins), watchful waiting, targeted therapy (e.g., imatinib, sorafenib for unresectable cases)

Conservative management; excision rarely needed

References: https://tumourclassification.iarc.who.int/chaptercontent/33/40

Case credit: UCSD Pathology

Author: Wangpan Jackson Shi