Uterine Mesenchymal Neoplasm With BRD8::PHF1 Fusion: Low-grade Endometrial Stromal Sarcoma or Uterine Ossifying Fibromyxoid Tumor?

We report an unusual uterine polypoid mesenchymal tumor in a 52-year-old resembling the soft tissue neoplasm ossifying fibromyxoid tumor (OFMT). The neoplasm was morphologically low-grade with hypocellular areas containing bland spindle cells in a fibromyxoid stroma, cellular areas resembling typical low-grade endometrial stromal sarcoma (LGESS), and abundant mature bone. The cellular areas were ER and CD10 positive and cyclin D1 negative, and the hypocellular areas were ER and CD10 negative, with approximately 50% of the nuclei being cyclin D1 positive. The tumor harbored a BRD8::PHF1 fusion. This fusion has been reported rarely in uterine mesenchymal neoplasms, which have been designated as LGESS or high-grade endometrial stromal sarcoma. In reporting this case, we review previously reported uterine mesenchymal neoplasms with a BRD8::PHF1 fusion. Since OFMT commonly contains PHF1 fusions, we discuss the most appropriate terminology for the neoplasm we report and suggest that it is best classified as an LGESS with OFMT-like morphology.