Teaching Case

A 43-year-old man with a known history of fibrous dysplasia came to the emergency clinic due to excruciating pain in the left scapular region. Clinical examination revealed hard palpable, non-mobile mass in the inferior aspect of the left scapula, which prompted further evaluation by imaging.

CT scans of the thorax showed expansive lesions with ground glass matrix and cystic changes involving the left first, fourth and ninth rib (figure 1), the largest of the lesions involving the ninth rib measured approximately 8 x 14 cm with cortical destruction medially (figure 2). Also visualized was a small lytic lesion with a sclerotic margin in the T4 vertebra (figure 3).

The CT-guided trucut biopsy of the ninth rib lesion was done (figure 4).

The specimen sections (figure 5) showed cores of a cellular lesion formed of fascicles of spindle-shaped cells with elongated fusiform nuclei in a collagenous stroma.

The cells also showed mild anisonucleosis. Mitosis was sparse. Also seen were the pink osteoid matrix. No curvilinear trabeculae were demonstrated.

Fibrous dysplasia (FD) is a benign bone condition in which fibrous connective tissue and poorly formed trabecular bone develop in the place of normal bone. Two major types of fibrous dysplasia exist, namely monostotic and polyostotic forms. The malignant transformation of FD is extremely rare [1].

FD occurs mostly as solitary (monostotic) or multifocal (Polyostotic) lesions. Though malignant degeneration in fibrous dysplasia is very rare, somehow, it seems to occur more in the polyostotic form with an approximate frequency of 0.4-1% among all forms [2, 3].

Most often the sites involved in a malignant degeneration are the femur, tibia, maxilla, and mandible [1–4]. The ribs are a very unusual location of fibrous dysplasia and malignant degeneration to osteosarcoma has only been described in two different case reports [2, 5,7].

Few cases of polyostotic fibrous dysplasia with multicentric osteosarcoma have been described before, but are exceptional [4].

The general prognosis of sarcomas developing in fibrous dysplasia is very poor [3, 4]. Metastatic lung disease is the most common cause of death [3]. Patients with fibrous dysplasia are usually followed up regularly with CT examinations but there are no specific guidelines that exist.

Other imaging modalities like conventional radiographs and MRI can be used to evaluate FD-related malignancies. Radiographs, often the first investigation demonstrates moth-eaten, mineralized or poorly marginalized osteolytic lesions with cortical destruction. However, there is an overlap of findings noted in a benign FD lesion. An MRI in patients with malignant transformation is more valuable in evaluating soft tissue component and show variable signal intensity from a mixed hypointense and intermediate signal on T1- weighted images to hyperintense signal intensity on T2-weighted images. Additionally, heterogenous post-contrast enhancement is typical.

In conclusion, malignant degeneration in fibrous dysplasia is extremely rare. A high index of suspicion is necessary to detect sarcomatous transformation at an early stage.

Radical surgical excision remains the most valid treatment option.

Besides the fact that malignant transformation of the FD is very rare, clinical symptoms such as excruciating pains with imaging findings such as bony remodeling with cortical destruction and soft tissue infiltrating mass should raise the suspicion for malignant degeneration.

This case emphasizes the importance of a regular follow-up in a known case of FD where clinical, radiological, and pathological correlation revealed a malignant transformation of the benign bone condition.

Written informed patient consent for publication has been obtained.