PATH-40. A GIANT CELL LESION RESEMBLING REPARATIVE GIANT CELL GRANULOMA WITH FN1-TEK FUSION, IN THE SPECTRUM OF CALCIFIED CHONDROID MESENCHYMAL NEOPLASM?

Abstract A newly described entity called “calcified chondroid mesenchymal neoplasms” includes chondroid neoplasms with variable amounts of calcification and FN1 fusions [Liu et al, Mod Pathol 2021]. We report a case of a left temporal bone giant cell granuloma-like lesion that also shows an FN1-TEK fusion. Close monitoring with MRI images after surgery has showed no recurrence. Case report: a 33-year-old male developed left jaw pain and he was diagnosed with a temporomandibular joint disorder. He felt left ear fullness and 2 years later experienced severe pain and swelling in the left temporal region. Imaging showed an expansile osseous lesion with multi-cystic components involving the left sphenoid bone as well as all portions of the temporal bone including the mastoid, middle ear cavity, and glenoid fossa. Pathology showed a hemorrhagic giant cell lesion that was negative for H3G34W mutation and USP6 rearrangement, suggesting a reparative giant cell granuloma. A subsequent NGS with the Tempus 648 gene panel did not identify somatic or germline mutations, but an additional Tempus RNA fusion test found a FN1-TEK fusion suggesting the possibility of neoplasia. Close monitoring with brain MRI over past 9 months has shown no signs of recurrence. DISCUSSION A recent small series describes a novel entity called calcified chondroid mesenchymal neoplasm that has FN-1 fusions including a FN1-TEK fusion in one case. The case in that series with FIN1-TEK fusion also showed prominent giant cells. It is unclear whether giant cell granuloma-like lesions with FN1-TEK fusion belong in the calcified chondroid mesenchymal neoplasm spectrum or are a separate non-neoplasm entity that shares the same alteration. More studies are warranted.