A Case Report of Progressive Paget Disease Mimicking a Malignancy: A Second Episode 21 Years Later.

Paget osteosarcoma is a rare but serious complication of Paget disease requiring immediate management before further malignant transformation. This case report examines the progression of a previously reported case of Paget disease with atypical pseudotumor manifestation, mimicking osteosarcoma over a 21-year time lapse. After presenting with substantial pain and elevated alkaline phosphatase levels, imaging proved extensive bony expansion of the lesion with high-grade trabecular and cortical thickening and extraosseous soft-tissue extension, prompting the need for biopsy to rule out Paget sarcoma. The atypical features of the pseudotumor's development helps distinguish key radiographic and clinical criteria for malignant development.