Ossifying low grade endometrial stromal sarcoma with PHF1-BRD8 fusion

Low-grade endometrial stromal sarcoma and ossifying fibromyxoid tumors are two types of mesenchymal tumors that share no similarities in terms of site, sex, and morphological characteristics. They are rare, low grade tumors of uncertain lineage, with no definite immunological markers. Interestingly, a common PHF1 gene- related rearrangement was observed in these two tumors. Here, we report a case of endometrial stromal sarcoma with distinct ossification. Microscopically, the tumor is composed of bland-looking ovoid cells with low cellularity in the fibromyxoid stroma. Foci of metaplastic bone formation were noted. Using a combination of FISH, transcriptome sequencing, and molecular techniques, we identified a new PHF1-BRD8 fusion transcript, which was previously described, but in its reciprocal fusion form. This case expands the current understanding of low-grade endometrial stromal sarcoma and emphasizes the importance of molecular characterization of unique fusion, which may be related to its distinct morphological features and the possibly chemosensitive target.