Diagnosis: Paget’s disease
Discussion: Paget’s disease is a chronic condition characterized by abnormal osteoblastic and osteoclastic activity contributing to exuberant bone remodeling. The disease can be divided into an active phase, an inactive phase, and a mixed pattern. During the active phase, oseolysis predominates, leading to increased vascularity, aggressive resorption, and replacement of hematopoietic tissue by fibrous tissue. The inactive phase leads to sclerosis, coarsening of trabecuale, and loss of vascularity. A mixed pattern results in a combination of these two patterns. The newly formed bone is soft and deformed, with a disorganized pattern.
The typical onset is insidious and patients can be asymptomatic for many years. Characteristic clinical features may not be present for several decades. Paget’s disease is typically seen in patients between 55 and 85 years old, with a male-to-female ratio of 2:1. Presenting symptoms may consist of fatigue, enlarged bones, erythema over affected bones, and neurologic disorders, as well as elevated serum alkaline phosphatase and urine hydroxyproline levels.
The etiology is unknown, but some have suggested parvomyxovirus or other viral infection as a possible inciting event. However, regional differences suggest environmental influences may also be present.
Most characteristic radiographic imaging features are enlarged bones, with a coarse, thickened trabecular pattern. Typical locations of involvement such as the pelvis, spine, skull, and proximal long bones are helpful clues. The disease can be polyostotic or monostotic. In the pelvis, one may see thickening of the iliopectineal line or acteabular protrusion. A “cotton wool” appearance due to the mixed blastic/lytic activity can be seen. Long bones may demonstrate “candle flame” lysis due to a v-shaped lytic defect. Later stages of long bone involvement demonstrate bowing.
On MRI, low-signal-intense sclerotic, cortically based foci can be seen and the marrow replacement is non-mass like in character. During the active phase, high signal intensity can be seen on T2-weighted images. Marrow enhancement can sometimes be seen in the active phase. A bone scan will show uptake of radiotracer in the lytic phase and be normal during the sclerotic, burned-out phase.
In this case, the imaging appearance rules out other entities such as osteoblastic metastases, fibrous dysplasia, myelofibrosis, and multiple myeloma. Characteristic features of Paget’s disease such as bony expansion, primarily unilateral involvement, preferential cortical involvement, and lack of bone destruction exclude those other diagnoses. Typically, metastases will demonstrate greater involvement of the medullary space rather than the preferential cortical involvement shown here. Lymphoma would show cortical bone erosion, with destruction. Myeloma would show endosteal scalloping and well-defined lytic lesions. Fibrous dysplasia would have a “ground glass” appearance that is not present.
