Uterine Myxoid Inflammatory Myofibroblastic Sarcoma (MIMS) Harboring Pathogenic KRAS Mutations and Expressing ER, PR, and CD10.

Myxoid inflammatory myofibroblastic sarcoma (MIMS) is a recently described aggressive sarcoma with deceptively bland spindled cells with a myofibroblastic phenotype and myxoid stroma. These tumors are negative for ALK gene rearrangements, but have been shown to harbor gene fusions involving PDGFRA/B , JAK1 , and PML and/or mutations involving KRAS . Only one uterine case has been previously reported. Here we report the second case of uterine MIMS. This tumor arose in a 40-yr-old woman patient and showed a conspicuously whorled architecture with myxoid stroma and so-called organoid aggregates. By immunohistochemistry, the tumor was positive for CD10, estrogen receptor (ER), and progesterone receptor (PR) with focal smooth muscle actin expression and no staining for ALK, desmin, h-caldesmon, HMB-45, or cathepsin-K. Next-generation sequencing analysis revealed the presence of 2 pathogenic mutations in KRAS , as well as pathogenic mutations in RAD51B and LATS1 . No gene fusions in PDGFRA/B , JAK1 , PML , ALK, ROS1, PLAG1 , or any other gene were identified by whole transcriptomic RNA sequencing. Given its deceptively bland appearance and its propensity, at least in the uterus, to express both CD10 and ER with only focal SMA expression, MIMS can be mistaken for an endometrial stromal tumor, an ALK and ROS1 -negative uterine inflammatory myofibroblastic tumor, or other cytologically bland fusion-driven uterine mesenchymal neoplasms.