Purpose or Case Report: Kaposiform hemangioendothelioma (KHE) is a rare, locally aggressive vascular tumor of intermediate malignancy, predominantly affecting infants and young children. Bone involvement and extension into adjacent soft tissues are uncommon and pose significant diagnostic and therapeutic challenges.

We report a 6-year-old boy diagnosed with KHE involving the left hip joint region, iliac bone, and proximal femur. The initial presentation in late 2020 included left lower limb pain, mild swelling, and limited mobility. Imaging studies suggested inflammatory arthritis, but subsequent MRI and histopathological examinations excluded Ewing sarcoma and revealed a spindle-cell vascular proliferation consistent with KHE. Disease progression led to osteolytic changes resembling Gorham–Stout disease.

Following multidisciplinary consultations, systemic therapy with Sirolimus was initiated (starting dose 0.7–0.8 mg daily, adjusted to therapeutic levels). Supportive hydration and dietary management were continued. The treatment was well tolerated, with no significant laboratory abnormalities. Serial MRI follow-ups demonstrated a gradual regression of bone and soft-tissue enhancement and no new lesions, indicating partial radiologic remission. Clinically, pain subsided, mobility improved, and no functional impairment was observed at the latest follow-up in October 2025.

This case illustrates the clinical efficacy of Sirolimus in achieving disease stabilization and partial regression in bone-involved KHE. Early recognition, accurate histopathological diagnosis, and interdisciplinary management remain critical for optimizing outcomes in pediatric vascular tumors with osseous extension.
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