Parosteal osteosarcoma with high-grade transformation and lung metastasis after 12 years: a rare presentation.

Parosteal osteosarcoma represents a low-grade, well-differentiated type of cortical surface osteosarcoma with a relatively better prognosis. Systemic metastasis is rare overall and occurs in 10-15% of patients only. Delayed recurrences (>5 years) of parosteal osteosarcoma are even rarer and only few isolated case reports of local recurrence with systemic metastasis occurring beyond 10 years exist in the literature. We present here a case of femoral parosteal osteosarcoma, successfully treated previously, that recurred with high-grade transformation locally and with lung metastasis after 12 years. The patient was managed by lung wedge resection followed by type A1 rotationplasty. Histopathology report confirmed evidence of high-grade transformation (de-differentiation) both locally and at the metastatic site, in view of strong CDK4 and MDM2 nuclear immuno-expression. The present report highlights the importance of long-term follow-up, even for a seemingly less aggressive osteosarcoma variant with identification of a novel pattern of lung metastasis.