STRN::NTRK3-fused neoplasm with "monster cells" in the pelvis of a young adult female: expanding the clinicopathologic spectrum of NTRK-rearranged neoplasms.

Gene fusions involving neurotrophic tyrosine receptor kinase (NTRK) genes represent uncommon genetic alterations that may occur across a wide spectrum of tumor types. NTRK-rearranged sarcomas are most frequently identified in pediatric mesenchymal neoplasms, although they have also been described, less commonly, in adult mesenchymal tumors. We present the case of a 28-year-old woman with a large pelvic mass arising in relation to the psoas muscle. It was histologically composed of a dual cell population consisting of small hyperchromatic irregular cells intermixed with markedly pleomorphic "monster cells". Immunohistochemistry showed diffuse p16, Factor XIIIa, and CD163 positivity, while S100 protein, CD34, CD68 and other lineage-specific markers were negative. Pan-TRK immunostaining was negative. Targeted next-generation sequencing (NGS) was performed due to the non-specific findings of the tumor in a young adult patient, and a STRN::NTRK3 gene fusion involving exon 3 and exon 14, respectively, was identified, establishing this tumor as part of the emerging group of NTRK-rearranged neoplasms. Entrectinib was administered, and an excellent response was observed. This case emphasizes the relevance of performing NGS in cases with unusual clinicopathological findings to identify potential treatment options. Importantly, the morphology deviated from the classic fibrosarcoma-like appearance described in most adult NTRK3-fused neoplasms and broadens the morphological spectrum in which these neoplasms should be considered.