MRI is recommended when degeneration into osteosarcoma is present—indicated by permeative lesions with cortical breakthrough and a soft-tissue mass. MRI is helpful to further characterize the lesion. Absence of the normal fatty marrow on T1-weighted images would be concerning for tumor involvement.
Bone scintigraphy is used to determine the extent of disease. It will show increased uptake when the lesions are active.
Laboratory testing. Serum alkaline phosphatase (sAP) is frequently elevated in patients with Paget disease (normal range, 20-140 IU/L) and reflects the extent and activity of disease. However, this correlation is not always reliable; it depends on monostotic vs polyostotic involvement, as well as which bones are involved. For example, sAP levels may be markedly elevated when the skull is involved but normal when other bones are involved.11 In patients with elevated sAP, serum calcium and 25-hydroxyvitamin D measurements should be obtained in anticipation of bisphosphonate treatment.
Biopsy. If the radiographic findings are typical for Paget disease, bone biopsy is not indicated. However, the main competing diagnosis to consider is malignancy; in atypical cases when imaging is unable to elucidate an underlying tumor, biopsy would be warranted.
Differentiating Paget disease from sclerotic metastasis is important. In metastasis, there will be no trabecular coarsening or enlargement of the bone.
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