Ossifying metaplasia of urothelial metastases: original case with review of the literature

Abstract

Background: Ossifying metaplasia is an unusual feature of urothelial carcinoma, with only a few cases reported. The largest series included 17 cases and was published in 1991. The mechanism of ossification is unknown and hypotheses of osteogenic precursor cells, inducing bone formation, are proposed.

Case presentation: A 75 year-old patient was treated for a high grade transitional cell carcinoma of the bladder by surgery, chemotherapy and radiotherapy. Histology showed foci of bone metaplasia, both at the periphery of the tumor, and in a lymph node metastasis. 1 year later, a heterotopic bone formation was discovered in the right retroperitoneal space, near the lumbar spine, increasing rapidly in size during follow-up. Several imaging exams were performed (2 CT, 1 MRI, 1 Pet-CT), but in the absence of typical features of sarcoma, diagnosis remained unclear. Histology of a CT-guided percutaneous biopsy showed urothelial carcinoma and mature lamellar bone. Integration of these findings with the radiological description of extraosseous localization was consistent with a diagnosis of osseous metaplasia of an urothelial carcinoma metastasis. The absence of bone atypia in both the primary and metastases argues against sarcomatoid urothelial carcinoma with osteosarcomatous differentiation.

Conclusion: Osseous metaplasia of an urothelial carcinoma metastasis is unusual, and difficult to distinguish from radiotherapy induced sarcoma, or from sarcomatoid carcinoma. Rapid progression, sheathing of adjacent structures such as vessels (like inferior vena cava in our case) and nerves and bony feature of lymph node metastases necessitate histological confirmation and rapid treatment. Our case illustrates this disease and evaluates the imaging features. In addition we discuss the differential diagnosis of osseous retroperitoneal masses.

Fig. 1

Histological findings at the time of the primary tumor resection (Hematoxylin & Eosin, H&E): The primary tumor consisted of a high grade urothelial carcinoma infiltrating the bladder wall; at the periphery of the tumor, a small focus of osteomedullary bone intimately admixed with the urothelial carcinoma was composed of small and non atypical osteocytes within mature-appearing trabeculae of lamellar bone (a, Original magnification 20× and b, 200×). c: a focus of bone metaplasia is seen at the hilum of a metastatic lymph node (100×)

Fig. 2

Follow-up CT performed 1 year after surgery (a) shows a small calcified nodule (white arrow) in the right retroperitoneal space. 6 months later (b) the calcified mass has increased in size. On another control 6 (c) months later the ossification progressed in size and into the adjacent vertebra (black arrow)

Fig. 3

Coronal reformat of a CT shows the multifocal ossifications along the retroperitoneal space (black arrowheads) as well as the intravertebral extension (black arrow). A calcified nodule is seen in the left inguinal area (white arrow)

Fig. 4

PET (a) and PET-CT fused axial slices (b) demonstrate the moderately hypermetabolic (SUV max 5.1) lesion of the retroperitoneum

Fig. 5

Axial MRI shows the lesions as hypointense on T2 (a) and T1 (b), with peripheral enhancement (arrow) on T1 fat sat after injection of contrast (c)

Fig. 6

Axial CT slices (a, b) depict the sheating of the vessels by the ossified mass (arrow) as well as the sheating of the lumbar nerves (arrowheads)

Fig. 7

A percutaneous biopsy was performed under CT guidance using a Bonopty 16G biopsy set

Fig. 8

Percutaneous biopsy of the retroperitoneal mass showed non atypical lamellar bone trabeculae (H&E, 400×) associated with p63 positive malignant epithelial cells consistent with metastatic urothelial carcinoma (Inset, p63, 600×)