Cytopathology of Chondromyxoid Fibroma: Report of 2 Cases with Immunocytochemical Expression of GRM1

Introduction: Chondromyxoid fibroma (CMF) is a rare, benign bone tumor that occurs predominantly in the second and third decades of life, more frequently in males. Overexpression of GRM1 as a consequence of tumor-specific gene rearrangement of GRM1 has recently been reported as a useful immunohistochemical marker for histopathological diagnosis of CMF. However, the usefulness of GRM1 staining of cytology specimens has not yet been evaluated. In this report, the cytological findings and GRM1 immunocytochemistry of two cases of CMF are described. Case Presentations: Case 1 was a 15-year-old girl with a rib tumor. Imaging findings suggested a benign neurogenic tumor such as schwannoma. The tumor had increased in size over a 2-year period and was resected. Case 2 was a 14-year-old boy with a metatarsal tumor involving his left first toe. Imaging findings were suspicious of a benign neoplastic lesion. Biopsy findings suggested a benign tumor, and the patient underwent tumor resection. Cytologically, in both cases the tumor cells were predominantly spindle-shaped or stellate, with a myxoid to chondromyxoid background matrix and multinucleated giant cells, and these matrices were metachromatic with Giemsa staining. Cellular atypia was more accentuated in case 2 than in case 1. Immunocytochemical staining for GRM1 was positive in both cases. Conclusion: Due to the overlap in cytological findings, it is often difficult to differentiate CMF from chondroblastoma and chondrosarcoma grade 2. Immunocytochemical staining for GRM1 may support the diagnosis of CMF, and the reuse of Papanicolaou-stained specimens is applicable. The present cases further demonstrated the difficulty of differentiating CMF from other mimicking tumors such as chondroblastoma and chondrosarcoma grade 2. In such instances, immunocytochemistry for GRM1 is applicable to the diagnostic process, the value of which is strengthened by reusing Papanicolaou-stained specimens.