MON-807 - Distinguishing Benign from Malignant Changes in Paget'S Disease: A Case of Suspected Pagetic Osteosarcoma

Paget’s disease of bone frequently involves the skull, occurring in approximately 25%-65% of cases. Malignant transformation to osteosarcoma is rare, affecting an estimated 0.1%-1% of patients, however, represents the most serious complication with an exceptionally poor prognosis. Differentiating malignant transformation from benign pagetic changes is challenging due to overlapping clinical and radiologic features.

An 86-year-old male with long standing Paget’s disease on Bisphosphonate therapy presented with extensive cranial involvement and hearing loss. He had received prolonged oral Bisphosphonate therapy followed by intravenous Zoledronic acid, which he tolerated well. Prior to Zoledronic acid infusion, serum Alkaline phosphatase (ALP) exceeded 1500 u/l and declined to 200 u/l on follow-up, indicating an excellent biochemical response. The patient remained asymptomatic without bone pain, headaches, or neurological deficits.
Incidental head imaging during hospitalization for an unrelated Deep Vein Thrombosis revealed diffuse Pagetic changes and a newly identified 11-mm focal cortical defect of the left parietal bone. This finding raised concern for possible malignant transformation versus metastatic prostate cancer. However, Prostate specific antigen remained low at 0.6 ng/ml and the patient lacked typical features of either pagetic osteosarcoma or progressive metastatic disease (escalating pain, focal swelling, neurological changes, or treatment resistance). Whole-body bone scintigraphy showed intense uptake in pagetic sites with no lesions suggesting osteoblastic malignancy or metastases. The cortical defect demonstrated no periosteal reaction, soft-tissue mass or mass effect. Given the excellent biochemical response to bisphosphonate therapy, clinical stability, and non-aggressive imaging features, a multidisciplinary team recommended close surveillance rather than invasive biopsy.

This case illustrates the diagnostic challenge of evaluating a focal skull lesion in a patient with both extensive cranial Paget’s disease and remote malignancy. The differential diagnosis included benign pagetic remodeling, pagetic Osteosarcoma, and metastatic prostate cancer. Integration of biochemical response, clinical symptoms, and imaging characteristics can distinguish benign from malignant processes, guiding appropriate management decisions and avoiding unnecessary invasive procedures in stable patients.

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