Chondroblastoma is a rare benign cartilaginous tumor mostly confined to the epiphyses and apophyses. Cases outside the epiphyseal region are exceedingly rare. Extramedullary chondroblastomas are exceptional; to our knowledge, only two cases qualified as \"periosteal chondroblastoma\" have been described in the literature. We report two cases of metaphyseal periosteal chondroblastoma both located on the inferior surface of the femoral neck. Both cases were paucicellular with an unusual dense sclerotic reaction. The diagnosis of chondroblastoma was supported by the expression of histone 3.3, K36M mutant in tumor cells.

Periosteal chondrosarcoma is an extremely rare low-grade malignant cartilaginous tumor arising from the external bone surface. Diagnosis of periosteal chondrosarcomas may be challenging, since this condition closely resembles periosteal chondromas. It has been reported that positron emission tomography (PET) is useful in distinguishing benign from malignant cartilaginous tumors using a maximum standardized uptake value (SUVmax) cut-off of 2.0 or 2.3. This report presents the case of a 40-year-old female with an 18-month history of a tender mass in the left distal femur. Radiological findings demonstrated periosteal buttressing. Magnetic resonance imaging (MRI) revealed a chondrogenic tumor of 3 cm in size developing from the external bone surface. It was difficult to differentiate periosteal chondrosarcoma from periosteal chondroma on the basis of size and the radiological and MRI findings. PET/computed tomography (CT) revealed abnormal linear uptake with an SUVmax of 2.7, indicating a malignant tumor. A diagnosis of periosteal chondrosarcoma was made, and wide resection was performed. Tumor histology was consistent with grade II chondrosarcoma. PET/CT is thus useful in differentiating periosteal chondrosarcoma from periosteal chondroma.