Malignant Transformation in a Chondroblastoma-Does it Exist?

Chondroblastoma is currently described as a benign bone tumor, histopathologically characterized by its classical features including chondroblasts, pink cartilage, and a variable number of osteoclast-like giant cells with foci of dystrophic calcification. Although recurrent and metastasizing chondroblastomas are reported, a malignant chondroblastoma is exceedingly rare and somewhat a contentious entity. A 35-year-old male presented with a lump in his ankle of 15 years' duration. Imaging disclosed a lytic destructive lesion involving the lower ends of the tibia and fibula with a soft tissue component, indicative of atypical/"worrisome" features. Microscopic examination of the biopsy revealed distinct foci of chondroblastoma, transitioning to areas of high-grade sarcoma, including pleomorphic cells, increased mitoses, and prominent stromal hyalinization. Immunohistochemically, the entire tumor was positive for H3K36M, while DOG1 highlighted the areas of chondroblastoma. SATB2 highlighted the areas of high-grade sarcoma, sparing the areas of chondroblastoma. Additionally, the areas of a high-grade sarcoma showed multifocal desmin immunostaining. A diagnosis of a malignant transformation in a chondroblastoma was offered. The patient defaulted to further treatment and unfortunately died 8 months, post-diagnosis. The conceptual evolution of a malignant chondroblastoma with H3K36M immunostaining in the few reported tumors is described herewith.