Spontaneous osteosarcoma transformation of a chondroblastoma
Published in the Indian Journal of Orthopaedics, the authors report a case of benign chondroblastoma which subsequently manifested as high-grade conventional osteosarcoma. According to the authors this is a first of such case ever reported in the literature.

Furthermore, the authors discuss the three different types of aggressive chondroblastoma and additionally also discuss regarding a variant of osteosarcoma known as chondroblastoma-like osteosarcoma.

A 29-year-old male presented with a painful swelling over his right knee for 4 months duration in 2011. The plain radiograph showed an eccentric lytic lesion over the epiphysis of right proximal tibia. Open biopsy confirmed chondroblastoma extended curettage and bone using Bone cement [Polymethyl methacrylate (PMMA)] was performed. The final diagnosis remained as chondroblastoma.

He was followed up monthly for the first 3 months, then 3 monthly for a year and 6 monthly until he developed local recurrence. At each followup, clinical palpation and X-ray were performed to look for local recurrence. He was symptom free for the next 4 years.

In 2015, he presented to the followup clinic with pain and swelling over the right knee. Plain radiograph revealed a lytic lesion just adjacent to previous PMMA cement in keeping with local recurrence. MRI revealed a homogenous lesion adjacent to the previous bone cement. Chest X-ray at this point was clear with no evidence of lung metastasis. Alkaline phosphatase and lactate dehydrogenase were within normal limit. Repeat curettage with bone cementation was done for the lesion.

Within a few weeks, his knee pain and swelling worsened. He was in pain, especially while weight bearing and the pain did disturb his sleep. His knee range of motion was restricted from 15° to 110° of flexion. The histopathology report from the curettage unfortunately came back as conventional osteosarcoma. Further systemic staging showed multiple small lung nodules in keeping with distance metastasis. The bone scan, however, did not pick up any skeletal metastasis.

He further underwent proximal tibia wide resection with endoprosthesis reconstruction and medial gastrocnemius flap followed by adjuvant chemotherapy. Postoperative, his wound healing was uncomplicated, and he started regaining good function of his right lower limb with physiotherapy. The resected specimen was send for histopathology and margins and revealed a high-grade conventional osteosarcoma with close margins.

The histology was reviewed and confirmed by two bone and soft-tissue pathologist as the local pathologist was well aware of the rarity of this case. The patient was started on adjuvant chemotherapy consisting of doxorubicin 25 mg/m2 and cisplatin 50 mg/m2. Sadly, he succumb to lung metastasis after his 3rd cycle of chemotherapy.

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