Novel NCOA2/3-rearranged low-grade fibroblastic spindle cell tumors: A report of five cases.

Spindle cell mesenchymal neoplasms are a diverse and often challenging diagnostic group. While morphological impression is sufficient for some diagnoses, increasingly immunohistochemical and even molecular data is required to render an accurate diagnosis, which can lead to the characterization of new entities. We describe five cases of novel mesenchymal neoplasms with rearrangements in the NCOA2 and NCOA3 genes partnered with either CTCF or CRTC1. Three tumors occurred in the head and neck (palate, auditory canal), while the other two were in visceral organs (lung, urinary bladder). All cases occurred in adults (range 33-86) with a median age of 42 and fairly even sex distribution = (male-to-female = 3:2). Morphologically, they had similar features consisting of monotonous, bland spindle to ovoid cells with fascicular and reticular arrangements in a myxohyaline to collagenous stroma. However, immunophenotypically they had essentially a null phenotype, with only two tumors staining partially for CD34 and smooth muscle actin. Targeted RNA sequencing detected in-frame CTCF::NCOA2 (one case), CRTC1::NCOA2 (two cases), and CTCF::NCOA3 (two cases) fusions. Treatment was surgical resection in all cases. Local recurrence and/or distant metastases were not observed in any case (median follow-up, 7.5 months; range, 2-19 months). Given their morphologic, immunohistochemical, and molecular similarities, we believe that these cases may represent an emerging family of low-grade NCOA2/3-rearranged fibroblastic spindle cell neoplasms.