Markedly mxyoid areas can be seen, especially in early lesions. The region here is paucicellular -- fibroblastic cells are embedded in a loosely textured myxoid matrix.
Reactive appearing stellate cells arranged in short fascicles embedded in a loose stroma confers a so-called feathery pattern.
This focus shows more hyalinization and plentiful extravasated RBCs in the stroma.
Tapered myofibroblastic cells with elongated cytoplasm are seen, resembling a cell culture. The cells are oriented in the same direction, but lack distinct fascicular bundling.
A different case showing an area with moderate cellularity and early stromal fibrosis. The nuclei are rather plump here.
Other areas of this lesion demonstrate the classic loose myxoid areas and RBC extravasation.
Extravasated RBCs can be focally numerous. The nuclei of the fibroblastic cells are vesicular with visible nucleoli. There should not be prominent atypia.
Here is an older area of sclerosis showing a decrease in cellularity. Older lesions tend to be more fibrotic and earlier lesions are more mxyoid.
Yet another case where the stroma is more loosely textured.
Nodular fasciitis (NF) is a pseudosarcomatous reactive lesion composed of fibroblasts and myofibroblasts (Folpe). It appears as a solitary painful mass arising most often in the forearm of young adults. The second most common site is the head and neck region, which is the most common site for infants and young children.
Most nodules are subcutaneous, although about 10% are intramuscular. Some lesions arise in the periosteum (parosteal fasciitis), within the skin (intradermal fasciitis), or within vessels (intravascular fasciitis) (Fletcher).
Historically, NF has been categorized as a benign lesion, although cytogenetic studies have found a clonal proliferation. Grossly, the lesion is well circumscribed but not encapsulated. It is usually small (less then 2 or 3 cm).
The histology depends somewhat on the age of the lesion: early lesions are more myxoid and older lesions more fibrotic and less cellular. Fascicles of fibroblasts and/or myofibroblasts are set in a loose, collagenous, and variably myxoid stroma. The short intersecting fascicles combined with a loose stroma imparts a so-called feathery appearance ("cell culture" appearance). The cells are plump with fibrillary cytoplasm, reminiscent of a fibroblast cell culture. The nuclei are vesicular with prominent nucleoli. Scattered inflammatory cells (mainly lymphocytes) and mitotic figures are common. Microcystic change and extravasated RBCs can be present as well. NF can be mitotically active, but there should never be atypical mitotic figures. 10% of cases have scattered multinucleated giant cells.
The myofibroblasts are positive for muscle specific actin, smooth muscle actin, calponin and negative for desmin. Since NF can be myxoid, hyalinized, cellular proliferations, the most important differential diagnostic consideration is that of various sarcomas. In general, sarcomas which exhibit more prominent pleomorphism, contain areas of necrosis, are larger in size and deeper seated in location (Folpe).
Affects young to middle age adults (20-50). There is no sex predilection. Presents as a rapidly developing tender mass and a history of trauma is present in 10-20% of cases (Folpe). The preferred anatomic site is the forearm, although anywhere on the body can be affected.
Benign. If left untreated, most will regress spontaneously. Only 2% of cases recur, usually due to incomplete excision (Fletcher).
→Rapid growth, small size and subcutaneous locations are features to help distinguish from other entities such as sarcomas.
→Fibroblasts and myofibroblasts are embedded in a loosely textured myxoid stroma, giving a so-called "cell culture" appearance. Mitotic activity and cellularity can be present, but not hyperchromatic atypical nuclei.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 1539-40.
Folpe AL, Inwards CY. Bone and Soft Tissue Pathology: Foundations in Diagnostic Pathology Philadelphia, PA: Elsevier; 2010: 43-5.
Rosai, J. Rosai and Ackerman's Surgical Pathology. 9th Ed. Philadelphia, PA: Elsevier; 2004: 2244-5.