Delayed Aggressive Local Recurrence of Chondroblastic Osteosarcoma 3 Years After an Excellent Pathological Response: A Case Report.

Late recurrence of osteosarcoma after a prolonged disease-free interval is an uncommon but significant clinical challenge, representing a failure of primary therapy to eradicate microscopic residual disease. We present the case of a 19-year-old male diagnosed with chondroblastic osteosarcoma of the proximal tibia who was treated with standard multimodal therapy, including neoadjuvant chemotherapy and limb-salvage surgery. He achieved wide (R0) surgical margins and an excellent (Huvos Grade IV, > 95% necrosis) pathological response. After a 3-year disease-free interval, he presented with an aggressive and extensive local recurrence in the residual tibia, which ultimately necessitated a below-knee amputation for local control. This case demonstrates that an excellent initial pathological response and wide surgical margins do not preclude the risk of a delayed and highly aggressive relapse in osteosarcoma. It serves as a critical reminder that a "cure" is provisional and underscores the absolute necessity of indefinite, long-term oncologic surveillance for all survivors. The phenomenon of late recurrence highlights the urgent need for research into tumor dormancy and the development of surveillance tools capable of detecting minimal residual disease.