Desmoid tumors consists of a monotonous proliferation of spindled (myo)fibroblastic cells in a variably collagenous or hyalinzed background. The nuclei are bland and vesicular. Pleomorphism and tumor necrosis are not features of desmoid tumors.
At the tumor periphery, entrapment of muscle can occur. Note the atrophic skeletal muscle with bizarre sarcolemmal nuclei at 12:00. Often times, there may also be lymphoid aggregates along the tumor edge, but that is not seen here.
Case 2 - This different case demonstrates more tapered and wavier fibroblasts and myofibroblasts.
Scattered thin-walled vessels are present. There is a varying number of blood vessels in desmoid tumors -- sometimes, they can be thickened with perivascular hyalinization (not seen here).
Beta-catenin staining can be helpful. The APC gene mutations act to upregulate B-catenin. B-catenin positivity can seen in the cytoplasm and nucleus, but only abnormal nuclear accumulation is considered specific (Folpe, Fletcher).
At higher power, abnormal nuclear accumulation of beta-catenin can be appreciated. The spindle cells are cut en face (lower) and on edge (upper).
Deep fibromatosis, also known as desmoid tumors, are deep-seated infiltrative growths that often recur. They are classically divded by anatomic location: extra-abdominal (60%), abdominal wall (25%) and intra-abdominal (15%)(Folpe, Fletcher).
Desmoids can arise sporadically or in association with familial adenomatous polyposis (Gardner syndrome). Mutations in the APC gene on chromosome 5 underlie the pathogenesis of both FAP-associated and sporadic desmoids. Sporadic lesions tend to be extra-abdominal with the most common sites being the shoulder girdle and promixal extremities. FAP-associated desmoid tumors (Gardner syndrome) tend to develop in the abdomen, especially the mesentery, pelvis or retroperitoneum (Folpe, Fletcher).
Pregnant women tend to develop desmoids in the anterior abdominal wall. Desmoids that arise from a scar are termed cicatricial fibromatosis i.e. arising from a caesarean section scar. Children tend to develop desmoid tumors in the head and neck, and they are more cellular and locally aggressive growths (Folpe, Fletcher).
Grossly, desmoid tumors are firm, poorly circumscribed, fairly large (usually greater 5 cm) masses with a pale, whorled, trabecular cut surface. The margins are irregular and infiltrative. Histologically, it is a monotonous proliferation of fibroblasts/myofibroblasts with tapered or plump vesicular nuclei and somewhat fibrillary cytoplasm. The stroma is variably collagenous, hyalinized or only focally myxoid.
Thick-walled blood vessels may be seen and some authors believe that vascularity may correlated with risk of recurrence (Fletcher). At the tumor edge, lymphoid aggregates and degenerate skeletal muscle fibers with bizarre sarcolemmal nuclei may be present.
Desmoid tumors are most likely true neoplasms rather than a reactive process due to growing evidence of clonality and the presence of APC mutations. In some cases, the APC mutation act through upregulation of B-catenin and nuclear and cytoplasmic accumulation of B-catenin on IHC can be helpful in confirming the diagnosis. One-third of desmoids show trisomies of 8 or 20. Most of these lesions are KIT negative (Folpe, Fletcher).
Differential diagnostic considerations would include nodular fasciitis (stellate cells, loose myxoid stroma, hemorrhage, few blood vessels, smaller size and more superficial location), scar tissue (also highly collagenized) and desmoplastic fibroblastoma (less cellular, more myxoid stroma, lack a fascicular growth pattern, smaller size, S-100 positivity). More cellular desmoids may raise concern for fibrosarcoma or monophasic synovial sarcoma, however, desmoids do not exhibit prominent nuclear atypia or necrosis (Folpe).
Age range is wide, but peak incidence occurs in the second to fourth decade. There is a strong female predilection (F:M ratio of 2:1). The finding of a desmoid tumor in a patient under 30 should prompt a search for other manifestations of Gardner syndrome (GI polyps and soft tissue tumors including osteoma, epidermoid cysts and desmoids).
Excision is recommended for symptomatic tumors. A subset appear to respond to estrogen antagonists (tamoxifen), NSAIDs or low-dose chemotherapy (methotrexate or doxorubicin). Some growth can even be inhibited by imatinib (Gleevec) although the reason for this is unclear as these lesions do not exhibit KIT mutations (Fletcher).
A benign tumor with no metastasis. However, due to infiltrative growth, recurrence is common. In some cases, the tumor may compress surrounding structures such as nerves or erode underlying bone.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 1551-3.
Folpe AL, Inwards CY. Bone and Soft Tissue Pathology: Foundations in Diagnostic Pathology Philadelphia, PA: Elsevier; 2010: 54-8.