Myxoid inflammatory myofibroblastic sarcoma with multiple liver metastases showing both PML::JAK1 fusion and KRAS mutation.

Recently, Papke et al. reported two cases of myxoid inflammatory myofibroblastic sarcoma with PML::JAK1 fusion. Smith et al. identified a high-grade PML::JAK1 fusion sarcoma. Here, we present the fourth case of a PML::JAK1 fusion sarcoma, the first to harbor a concomitant KRAS hotspot mutation (KRAS Q61H). Morphological manifestations include prominent myxoid stroma, lobulated but infiltrative growth patterns, and branching blood vessels. In contrast to Papke et al., we observed distinct non-myxoid stromal fascicular herringbone solid architecture and perivascular spaces, thereby expanding the morphological spectrum of PML::JAK1 fusion. The tumor was approximately 80% myxoid and 20% solid in composition. Importantly, a needle biopsy confirmed liver metastases of myxoid inflammatory myofibroblastic sarcoma in this case, suggesting its bland morphology but aggressive clinical behavior, which is a major potential diagnostic pitfall for this tumor. In addition, the presence of both myxoid lobular and herringbone regions in the present case suggests that the PML::JAK1 fusion sarcomas reported by Papke et al. and Smith et al. might be on a biological spectrum.