Giant Cell Tumor with Chondroid Differentiation: A Case Report on This Rare Morphological Variant and a Diagnostic Challenge.

Introduction: Giant cell tumor (GCT) with chondroid differentiation is an uncommon entity. We report a case report of a 44-year-old female presenting with GCT of the left distal femur showing chondroid differentiation. Case Report: A 44-year-old female presented to us with complaints of pain and swelling in the left distal femur for 4 months. On further evaluation, X-ray revealed an osteolytic lesion in the distal femur with ill-defined margins and a narrow zone of transition. Magnetic resonance imaging showed a well-defined lesion in epi-metaphyseal lesion measuring 64 × 52 × 38 mm on the left side of distal femur, associated with a large soft tissue mass. Biopsy was done which revealed several fragments of cartilage with only a few fragments showing oval-to-spindle cell proliferation with areas of stromal hyalinization with some tumor fragments showing no unequivocal substantial number of osteoclast-like giant cells. Immunohistochemistry examination showed diffuse and strong nuclear positivity for H3.3G34W. Considering the diagnosis of GCT, the patient underwent curettage and cementing and plating. Final histopathology was also consistent with GCT, showing chondroid differentiation. The patient is 18-month post-surgery and is doing fine without any recurrence. Conclusion: This report details the radiological and pathological findings of the GCT of distal femur exhibiting cartilage matrix. It behaves as a GCT without cartilage matrix and should be treated accordingly. Furthermore, immunohistochemical staining with antibodies against the mutant-specific H3.3 protein or identification of H3.3G34A guides us for confirmation of diagnosis.