Undifferentiated Pleomorphic Sarcoma with Dual Smooth-Muscle and Osteogenic Differentiation Mimicking Plasma Cell Dyscrasia: A Rare Diagnostic Pitfall.

INTRODUCTION

Pleomorphic sarcoma (PS) is a high-grade soft-tissue malignancy with marked morphologic variability. Divergent differentiation is recognized, but dual smooth-muscle and osteogenic lineage differentiation is exceedingly rare and may mimic other malignancies.

CASE REPORT: A 52-year-old male presented with progressive low-back pain, weight loss, and an enlarging iliac mass. Magnetic resonance imaging showed multiple lytic lesions suggestive of plasma-cell dyscrasia; however, serum electrophoresis was negative. Histopathology revealed high-grade PS, and immunohistochemistry demonstrated simultaneous positivity for smooth-muscle actin and osteocalcin, confirming dual smooth-muscle and osteosarcomatous differentiation. The patient was referred to oncology for multidisciplinary management.

CONCLUSION

PS with dual divergent differentiation represents a rare diagnostic challenge. Clinical and imaging features may falsely suggest hematologic malignancy, underscoring the necessity of correlating radiology, histopathology, and immunohistochemistry.