A case of subperiosteal clavicular chondromyxoid fibroma in a 52-year-old man: emphasis on diagnostic utility of GRM1 immunohistochemistry.

Chondromyxoid fibroma (CMF) is a benign cartilaginous neoplasm that accounts for less than 1% of all primary bone tumors and typically arises in the metaphysis of long bones in young adults. CMF occurring in the clavicle is uncommon, and preoperative diagnosis can be challenging. We report a case of CMF arising in the subperiosteal region of the clavicular diaphysis in a 52-year-old man. Radiologically, a well-defined, subperiosteal lesion with expansile remodeling but without periosteal reaction was identified, showing marked hyperintensity on T2-weighted magnetic resonance imaging and intense contrast enhancement. The lesion gradually increased in size over a 5-year period, prompting surgical curettage. Histologically, the tumor showed heterogeneous cellularity, with stellate to spindle-shaped cells in a chondromyxoid matrix and densely distributed mononuclear cells with occasional osteoclastic giant cells, arranged in a vaguely lobulated pattern. Immunohistochemical positivity for GRM1 confirmed the diagnosis of CMF. Postoperative follow-up revealed no recurrence at 6 months. Only 14 cases of clavicular CMF have been reported in the literature, to the best of our knowledge. This case is unique because of its subperiosteal diaphyseal location and prolonged radiological observation prior to surgery. Integration of radiological findings with molecular and immunohistochemical evidence, such as GRM1 overexpression, is essential for establishing a definitive diagnosis and avoiding overtreatment in rare tumors with atypical presentations. The present case highlights the diagnostic utility of GRM1 immunohistochemistry as an ancillary tool for confirming CMF, particularly when histological and radiological features are inconclusive because of an unusual anatomical location or limited tissue sampling.