Teaching Case

A 74-year-old asymptomatic woman presented to our hospital to undergo an abdominal contrast-enhanced CT for follow-up imaging after surgical resection of an angiomyolipoma of the right kidney diagnosed with ultrasound and MRI.

As a collateral finding, CT revealed a bone lesion on the right iliac wing, next to the sacral articular surface. The lesion had a mixed density, predominantly sclerotic and determined slight bony enlargement; the cortical profile was intact, and no periosteal reaction was identified (Figures 1a and 1b). No other bone lesions were detected at CT. The patient underwent a Technetium-99m hydroxydiphosphonate bone scintigraphy, revealing a marked increased radioisotope uptake in the right sacroiliac joint. A 6-month follow-up CT was scheduled, and the patient was still asymptomatic; the CT scan revealed an increase in the size of the lesion, a larger osteolytic component with some areas of cortical disruption (Figure 2). Bone and pulmonary metastases were detected. Contrast-enhanced MR examination of the pelvis showed a bone lesion centred in the right sacroiliac joint with extra-osseous enhancing soft tissue component (Figures 3a, 3b, 3c and 3d).

Paget’s disease is the second most common chronic skeletal disorder after osteoporosis, with an incidence of 3–4% in people over 40 years of age [1]. Paget’s disease causes abnormal and excessive remodelling of bone that can lead to pathological fractures and, in rare cases, to sarcomatous degeneration [2]. The incidence of malignant transformation is about 1% [3]. The most frequent forms of malignant transformations of Paget’s disease are osteosarcoma (22–90%), fibrous histiocytoma (26%), fibrosarcoma (3–25%) and chondrosarcoma (1–15%) [3].

Paget’s disease usually affects the skull, spine, pelvis, and lower extremities. The structural changes may cause bone weakening and symptoms including pain, stiffness and reduced mobility [4]. Nevertheless, the majority of patients are asymptomatic, the diagnosis being an incidental finding on x-ray, CT or MRI [5]. Malignant transformation is more frequently observed in males between 61–70 years of age [6]. Clinically, it may be characterised by worsening of pain, pathological fractures, night sweats and weight loss; an elevation of serum alkaline phosphatase may be observed [7].

The primary radiographic feature of Paget’s disease is the widening of the bone, accompanied by other findings such as cortical thickening, osteolysis, and osteosclerosis [2]. The radiological findings of sarcomatous changes in Paget’s disease can include intensive sclerotic reaction, cortical destruction, bony spiculation, a non-healing fracture or the presence of a soft-tissue mass. CT and MRI have an important role in the diagnosis of sarcomatous transformation, allowing the identification of bony disruption as well as extra-osseous tissue.

In our patient, Paget’s disease of the bone was initially suspected based on CT and scintigraphy results. Follow-up CT and MR findings at 6-month follow-up were consistent with primary bone sarcoma, and an incisional biopsy was performed. The histological examination suggested the diagnosis of a secondary chondrosarcoma growing on abnormal bone in the setting of an underlying Paget’s disease. Due to the lack of conditions of surgery or chemotherapy, the patient underwent immunotherapy. Three-month follow-up CT examination revealed disease progression.

Malignant transformation represents a rare complication of Paget’s disease of the bone and can be suspected in patients with worsening symptoms or pathological fractures, although it can be asymptomatic. Follow-up imaging in Paget’s disease of the bone is needed, and radiologists play a pivotal role in the diagnosis and identification of complications. A definitive diagnosis of malignancy requires bone biopsy.

All patient data have been completely anonymised throughout the entire manuscript and related files.