Teaching Case

A 72-year-old man with a history of right lower limb deformity since the age of six years presented to the department due to increasing pelvic pains over the past two weeks. Physical examination revealed a shortened right lower limb with palpable right pubic mass upon which he was sent for imaging.

The CT scan of the Chest, abdomen, and pelvis done for tumor screening showed bone expansion with cystic changes and ground glass matrix in the pelvic girdle (Figure 1), proximal right femur, humerus, and scapula with coarse trabeculation (Figure 2). The right femur demonstrated the shepherd crook’s deformity (Figure 3). Also noted was the focal ground-glass area with cystic changes in the left scapula and humerus. An ill-defined heterogeneously enhancing soft tissue density mass was seen arising from the right superior pubic ramus with cortical destruction [Lodwick-Madewell classification 3B] (Figure 4).

The biopsy of resected neoplasm showed areas of diffuse population of pleomorphic cells with numerous mitosis and foci of necrosis suggestive of high-grade pleomorphic malignancy (Figure 5). Based on Immunohistochemistry (IHC), Vimentin turned out to be positive, while Desmin, CD34, S-100, HMB45, and CD 45 were all negative. 

Fibrous dysplasia (FD) is a benign fibro-osseous lesion that typically occurs within the medullary space of bones in both pediatric and adult patients [1]. FD mostly occurs as a single lesion (monostotic) that is usually diagnosed in the first three decades of life and, less often multiple sites (polyostotic). Polyostotic FD is typically diagnosed in the first ten years of life and tends to involve the skeleton diffusely or on one side of the body. In both forms of FD, the long bones mainly the proximal femur and tibia, jaw, skull, and ribs are the most commonly affected sites [1]. Many patients of FD are asymptomatic, while others can present with pain, deformity, asymmetry, or pathologic fracture. 

FD lesions are typically intramedullary, well-circumscribed, and often expansile lesions of variable densities. FD shows varying degrees of ground-glass density and may appear almost entirely lucent to almost completely sclerotic [2]. In the long bones, lesions are usually diaphyseal or meta-diaphyseal and rarely involve the epiphysis. They may be eccentric or centrally located. Although endosteal scalloping and cortical thinning can be seen, the outer cortex remains smooth and intact in benign diseases [2]. Expansile remodelling can cause “bulging or undulating” contours [3]. Because FD weakens the bone, weight-bearing bones can become deformed and bowed, resulting in the “shepherd’s crook” deformity. Although there have been numerous case reports, as well as a few small series describing malignant degeneration of FD, many cases are from the era before cross-sectional imaging was routinely available and descriptions of the imaging findings remain limited [4, 5, 6, 7,8]. 

Microscopically, FD is composed of characteristic curvilinear trabeculae of woven bone in C, S, or Y shapes and bland fibroblastic cells. Histologically, the malignant transformation of FD most commonly occurs as high-grade osteosarcoma, chondrosarcoma, and fibrosarcoma [4]. Vimentin expression may occur in pleomorphic sarcoma and osteosarcoma [9].

A detailed review of the clinical information revealed a history of polyostotic FD. Imaging findings with pathological correlation were compatible with the malignant transformation of FD.

Even though the malignant transformation of the FD is rare, clinical symptoms such as increasing pain with radiological findings such as bony destruction with or without soft tissue mass should raise the suspicion for malignant transformation.

This case underlines the importance of correlation of clinical, radiologic, and pathological findings.

Written informed patient consent for publication has been obtained.