Fibrous Dysplasia With Degenerative Atypia: A Benign Lesion Potentially Mistaken for Sarcoma

Archives of Pathology & Laboratory Medicine,  Jul 2004  by Bertoni, Franco,  Arias, Luis Fernando,  Alberghini, Marco,  Bacchini, Patrizia

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Fibrous dysplasia is a benign disorder of bone in which proliferating fibrous tissue replaces the bony spongiosa. Cytologic atypia is generally not considered a feature of this proliferating tissue. We present a case of fibrous dysplasia with marked atypical nuclear changes consistent with degenerative or regressive changes. A 42-year-old man presented with an osteolytic lesion of the right iliac bone. Histologic study demonstrated a fibro-osseous lesion with woven bone trabeculae and bland-looking fibrous tissue. Several areas showed atypical cells with enlarged pleomorphic nuclei and bizarre features. There was no change in the nuclear-cytoplasmic ratio nor were mitotic figures identified. The differential diagnosis is discussed. When radiographic and other histologic findings suggest fibrous dysplasia, the atypical nuclear changes should not, by themselves, alter the diagnosis.

(Arch Pathol Lab Med. 2004;128:794-796)

Fibrous dysplasia (FD) is a common benign disorder of bone characterized by a tumorlike proliferation; the mass is almost always intramedullary and composed of fibrous tissue with an irregular pattern of osseous metaplasia. It may involve a single bone or multiple bones.1 Although FD is sometimes considered a true neoplasm of bone (in recognition of its presentation as a mass lesion), it is not a neoplasm; hamartomatous proliferation and localized failure of bone to mature from the woven to the lamellar form are theories that may account for FD.1-3

Although, when asymptomatic, FD can be discovered at any age, it usually begins prior to puberty, grows slowly, and tends to become quiescent after puberty.3 It commonly presents in radiographs as an incidental lucent lesion replacing the bony spongiosa, with well-defined and occasionally sclerotic margins. Histologically, it appears as a proliferation of thin wavy spicules of woven bone found in a bland, slightly to moderately cellular fibrous background. Bony trabeculae are usually not surrounded by osteoblasts. Unusual histologic features include foci of osteoblastic rimming, scattered trabeculae of lamellar bone, cystic degeneration, and highly cellular fibrous stroma. Cytologie atypia and more than just occasional mitoses are not features of FD.1-3

This article presents an unusual case of FD of bone with scattered foci of nuclear atypia with degenerative nuclear changes, which led to diagnostic problems.

REPORT OF A CASE

In June 1990, a 42-year-old man presented with a 2-year history of lower back pain. He had no history of trauma and the pain was not relieved by rest. Radiography of the pelvis showed a poorly circumscribed, osteolytic lesion in the anterior upper aspect of the right iliac bone. It was intramedullary and showed thinning and focal erosion of the overlying cortex (Figure 1, a). Computed tomography demonstrated cortical thinning and slight bony expansion without soft tissue extension (Figure 1, b). The radiographic findings were not specific. Results of a computed tomography of the thorax and other investigations were essentially normal. Open biopsy was performed. During surgery, a firm yellow-white to pink tissue was noted. This tissue was completely contained between the 2 cortices of the ilium. Because of diagnostic uncertainty, a definitive resection of the tumor with wide margins was performed. Stability of the right sacroiliac and hip joints was preserved. No further treatment was used. The patient recovered with minimal disability and has been well with no evidence of recurrence or metastatic disease for more than 12 years.

MATERIALS AND METHODS

The whole lesion, including its interface with the adjacent bone, was sectioned for histologic study. For light microscopy, 3- to 5-µm paraffin sections of formalin-fixed tissue were stained with hematoxylin-eosin. Retrospectively, 400 cases of fibrous dysplasia of bone in the files at Istituto Rizzoli (Bologna, Italy) were reviewed. Only cases that had been treated surgically and were histologically proven were included. The aim of this review was to investigate, using a large series of FD cases, the true incidence of this phenomenon, which may be overlooked in otherwise diagnostically obvious cases.

PATHOLOGIC FINDINGS

Grossly, the lesion was well defined from the surrounding bone and was tan with reddish hemorrhagic areas. Microscopic examination showed a paucicellular fibro-osseous lesion with randomly arranged, woven bone trabeculae and bland-looking, loosely textured, intervening fibrous tissue with thin capillary vessels (Figure 2, a). The delicate bone trabeculae were not surrounded by osteoblasts for the most part, but scattered foci of osteoblastic rimming were observed. Several areas showed atypical cells with enlarged, pleomorphic, hyperchromatic nuclei, sometimes multilobed, scattered singly or in clusters in the fibrous stroma and in the trabeculae; several of these nuclei had a bizarre aspect (Figure 2, b). The nuclei commonly showed vacuolization similar to that seen in bizarre nuclei of other benign lesions.4-6 There was no change in the nuclear-cytoplasmic ratio, because the cytoplasmic volume also increased. No mitoses, typical or atypical, could be identified in the numerous histologic slides (Figure 2, c and d). In summary, the histologic findings were those of FD with pseudomalignant cytologic features. A histopathologic diagnosis of FD with degenerative or regressive nuclear changes was made.