Chondromyxoid fibroma with limited or no chondromyxoid stroma: A clinicopathological study of 3 cases.

Chondromyxoid fibroma (CMF) is a rare benign bone tumor characterized by lobulated growth, chondromyxoid stroma, and GRM1 rearrangement. Here, we describe 3 cases of CMF with limited or no chondromyxoid stroma. The patients (2 males and 1 female) were 14-18 years old and had lesions in the tibial diaphysis, rib, and pubic bone. The tumors measured 24-60 mm, and were central in 2 cases and juxtacortical in 1 case. Treatment included curettage and resection, and the tumors did not recur for 2 to 8 years after surgery. Histologically, all 3 tumors comprised sheets of uniform, non-atypical, ovoid or short spindle cells interspersed with numerous osteoclast-like giant cells. Delicate pericellular eosinophilic matrix was focally deposited in all cases, which showed chicken-wire calcification in 2 cases. Osteoid deposition in a conglomerate pattern was observed in 2 cases. Aneurysmal bone cystic changes were present in all cases. Chondromyxoid stroma was present only in ~ 10% and < 5% of the tumor volume in 2 cases, whereas it was absent in the remaining case. All 3 cases showed diffuse immunohistochemical expression of GRM1, and GRM1 rearrangement was confirmed with fluorescence in situ hybridization. CMF with limited or no chondromyxoid stroma is a rare variant that histologically overlaps with other giant cell-rich tumors. Recognition of this pattern is essential for appropriate clinical management, and the demonstration of GRM1 rearrangement and overexpression provides strong support for this challenging diagnosis.