Introduction

Desmoplastic fibroma (DF) is a primary benign fibrous tumor of bone, histologically resembling the soft tissue desmoid tumors. It was first described as a distinct entity by Jaffe [₁]. It is a very rare tumor and accounts for 0,1 to 0,3% of all benign bone tumors [₂]. Due to its local inflitrative pattern, agressive biologic behaviour and high risk of recurrence, it is assumed as a borderline or semi-malignant tumor. There is no age predilection but most of the affected patients are younger than 30 years of age. Clinical signs are nonspecific. Pain and swelling are the predominant symptoms, but some patients may be asymptomatic and the tumor may be an incidental finding on plain radiographs. Other patients may present with a pathological fracture. The mandible is the most common site of involvement followed by femur, pelvis and long bones in decreasing ratios respectively. Etiology is still uncertain. Radiological features are usually inconclusive with diagnosis and histopathologic examination determines the definitive diagnosis. Although there is no evidence based treatment strategy at present, wide excision is recommended to prevent local recurrence [₃, ₄, ₅].

Case Report

An 18-year-old man was admitted to our clinic with a complaint of pain and growing lump in his left knee, lasting for six months. Physical examination revealed a hard, tender and fixed 3X4 cm mass over the posterolateral aspect of his left knee on fibular head. Knee movements could be done without pain in normal range. Neurovascular examination was normal. Past medical history was unremarkable. Plain radiographs showed an osteolytic expansile lesion in the proximal fibula with cortical thinning. The lesion had a soap bubble appearance, due to the presence of multiple internal bony ridges within the tumor. There was lack of matrix calcification (Fig.1).

Figure 1

Figure 1: Direct radiographs of the patient at initial admission

Skeletal scintigraphy showed a massive hypervascularization of the proximal third of the fibula in the vascular phase and increased activity at the fibular head in the bone phase (Fig.2).

Figure 2

Figure 2: Bone Scintigraphy

Computed tomography (CT) demonstrated a purely medullar lytic lesion but endosteal scalloping of the posterior cortices had resulted in focal thinning and disruption of the posterior cortex, with subtle associated soft tissue. No fluid levels were seen (Fig. 3).

Figure 3

Figure 3: Axial CT imaging of the lesion

These radiological findings suggested that the lesion had a benign nature but local infiltration alerted us for malignancy. We planned open biopsy and frozen section for histopathological diagnosis, and curettage and grafting if the diagnosis was eligible. At operation a pale yellow colored tumor with rubbery consistency was found (Fig. 4).

Figure 4

Figure 4: Intra-operative appearence.

Frozen section of the soft tissue component was reported as “benign lesion with desmoid tumor”. The tumor was curetted through an oval cortical window from intramedullary cavity of fibula. The walls of the cavity were further curetted with high speed burr and the cavity was filled with allogenous bone graft (Fig. 5).

Figure 5

Figure 5: Immediate post-operative radiographs

Post-operative period was uneventfull and patient was discharged after three days. Histologically, the diagnosis of desmoplastic fibroma was established. The tumor showed a homogeneous pattern of small, spindled, fibroblastic cells embedded in a dense and abundant stroma of fine collagen bundles. The cells had small ovoid to elongated nuclei. The chromatin was fine and evenly distributed. Nucleoli were not seen and there were no mitotic figures (Fig. 6).

Figure 6

Figure 6: Microscopic appearence of the tumor. (Stain: hematoxylin and eosin; original magnification, X 100)

At the final follow-up, 3 years after surgery, the patient was free of symptoms, but MRI showed local recurrence.

Discussion

Radiological characteristics of DF are not specific and differential diagnosis is a matter of debate in most cases. DF is a purely ostolytic expansile lesion usually with central or eccentric settlement without matrix calcification. Cortical thinning and soap bubble appearance accompany the lesion on plain radiographs. A narrow distinct transition zone could be distinguished without sclerosis. Excluding calvarium, it usually arises in the metaphysis of long bones sometimes with extension to epiphysis and subchondral bone [₄]. Computed tomography (CT) is useful to delineate the whole circumferential cortices. Cortical destruction, extraosseous extension and soft tissue invasion could easily be detected [₆]. MRI seems to add very little to the differential diagnosis since the signal characteristics are non-spesific but it is beneficial in surgical decision making. DF shows low to intermediate signal intensity on T2-weighted sections with heterogenous enhancement after intravenous administration of gadolinium contrast. Protrusion through cortical defects into soft tissues is also visible [₇].

The radiological differential diagnosis includes unicameral bone cyst, fibrous metaphyseal defect, giant cell tumor, aneurysmal bone cysts, hemangioma, non-ossifying fibroma, chondromyxoid fibroma, fibrous dysplasia and eosinophilic granuloma. When cortical destruction and a soft tissue mass are identified, particularly fibrosarcoma and low grade intraosseous osteosarcoma should also be suspected. Plain x-rays, CT and MRI together with the patient's age, site of involvement should be evaluated as a whole to narrow the differential diagnosis. Although ?t is a rare tumor, DF of bone should be included in differential diagnosis of osteolytic expansile lesions [₄, ₇].

Definitive diagnosis could only be established with histologic examination. Histologically, desmoplastic fibroma of bone resembles the counterpart of the soft tissue desmoid tumor. Macroscopically, the tumor is grayish white and of a rubbery, tough consistency. It is characterized by its collagen-producing fibroblasts. The fibroblasts are evenly distributed in the stroma, and have ovoid, plump to slender nuclei with pale staining and evenly distributed chromatin. Nucleoli are hardly seen, mostly absent. Mitoses are very rare. The collagenous fibers are tightly packed, very broad and are laid down in parallel bundles. On immunohistochemistry, the cells characteristicly express vimentin, which is a mesechymal marker [₃,₅].

Various options for the treatment of DF are reported in the literature. Currettage and bone grafting is most commonly employed treatment method but this procedure has a high risk of recurrence. Agressive currettage of the walls with high speed burr is advocated as an efficious method as an alternative without recurrence [₅]. Radiotherapy is an other altervative treatment method that may be used where surgical removal is technically difficult and has high morbidity [₈]. Performing an en bloc resection with a wide margin of normal tissue is the widely accepted treatment method, and it may be curative [₃]. Reconstruction after wide marginal resection may be a problem especially where the lesion is close to joints [₉].

To conclude, DF is a rare bone tumor with unspecific radiological features. It should be kept in mind in differential diagnosis of local agresive osteolytic lesions. Histologic examination is necessary for definitive diagnosis. Wide marginal resection is the treatment of choice to prevent recurrences.