Definition: Nodular fasciitis is a benign proliferation of fibroblasts in the subcutaneous tissues, commonly associated with the deep fascia.

Synopsis

 small and solitary subcutaneous nodule
 solid lesion, 5 or 20 mm in diameter
 rapid growth (less than a month in 50%)
 localization:

• upper extremities (volvar aspect of the forearm), lower extremity, trunk in adults
• head and neck region of infants and children
• any superficial soft tissue of the body, including the breast, mucosal surfaces, bladder, and parotid gland.

 benign proliferation of fibroblasts and myofibroblasts in the subcutaneous tissues
 haphazardly arranged bundles of fibroblasts (medium cellularity)
 myxoid or mucoid background
 fine capillary network arranged in a radial pattern around a larger central vessel or vessels
 fibroblasts are typically large and plump, similar to those of granulation tissue
 pleomorphic fibroblasts
 common mitoses
 spindle-shaped fibroblasts arranged in long fascicles slightly curved, whorled or S-shaped

• extension along the fibrous septa of submucosal fatty tissues.
   variable amount of collagen and acid mucopolysaccharide in intercellular matrix
   scattered chronic inflammatory cells
   foamy histiocytes and osteoclast-like multinucleated giant cells in long-standing lesions
   small slit-like spaces often separate the fibroblasts
   extravasation of erythrocytes
   occasional microcysts
   not often encapsulated but usually well demarcated from surrounding tissues

Ultrastructure

 myofibroblasts

• basic fibroblastic appearance with peripherally located bundles of myofilaments with dense patches similar to those of smooth muscle cells

Immunochemistry

 vimentin + (VIM+)
 smooth muscle actin + (SMA+)
 muscle-specific actin +
 desmin - (DES-)

Cytogenetics

 3q21 rearrangements (#9797787#)

• t(3;15)(q21;q22) (#7923068#)

 Ch.15 rearrangements (15q22-15q25)

• t(3;15)(q21;q22) (#7923068#)
• t(2;add(15)(p11.2) (#12550774#)
• t(15;15)(q13;q25) (#12606136#)

 t(16;16)(p13.3;p11.2) (#12550774#) (16p13.3 and 16p11.2)

Differential diagnosis

 fibrous tumors

• proliferative myositis
• proliferative fasciitis

 fibroblastic tumors

• fibrosarcoma

 fibrohistiocytic tumors

• fibrous histiocytoma

 others

• liposarcoma

 small foci of metaplastic bone or cartilage

• ossifying fasciitis
• fasciitis ossificans15)(q31;q26) (#7621414#)
• t(15;15)(q13;q25) (#12606136#)
• parosteal fasciitis

Variants

 intravascular nodular fasciitis (in blood vessels)
 fascial nodular fasciitis (in deep fascia)
 intramuscular nodular fasciitis

• muscle is completely replaced by the fibrovascular proliferation (unlike proliferative myositis, which infiltrates between muscle fibers)

Localization

 soft tissues
 intraarticular nodular fasciitis (#16434899#)

Prognosis

 These lesions rarely recur, do not develop metastases, and are readily cured by local excision.
 Despite its often aggressive microscopic appearance, nodular fasciitis is a self-limiting lesion which is readily treated by simple local excision.
 Deeper lesions tend to be somewhat larger and less well demarcated, hence, require a wider local excision.
 Recurrence rates vary from 1-6% with this treatment and some lesions have been reported to regress and disappear without treatment.