Plexiform fibromyxoma is a very rare mesenchymal tumor of the stomach, found almost exclusively in the antrum/pylorus region. The most common presenting symptoms are anemia, hematemesis, nausea and unintentional weight loss, without sex or age predilection. We describe here two cases of plexiform fibromyxoma, involving a 16-year-old female and a 34-year-old male. Both patients underwent complete resection (R0) by distal gastrectomy and retrocolic gastrojejunostomy (according to Billroth 2); for both, the postoperative course was uneventful. Histology showed multiple intramural and subserosal nodules with characteristic plexiform growth, featuring bland spindle cells situated in an abundant myxoid stroma with low mitotic activity. Immunohistochemistry showed α-smooth muscle actin-positive spindle cells, focal positivity for CD10, and negative staining for KIT, DOG1, CD34, S100, β-catenin, STAT-6 and anaplastic lymphoma kinase. One of the cases showed focal positivity for h-caldesmon and desmin. Upon follow-up, no sign of disease was found. In the differential diagnosis of plexiform fibromyxoma, it is important to exclude the more common gastrointestinal stromal tumors as they have greater potential for aggressive behavior. Other lesions, like neuronal and vascular tumors, inflammatory fibroid polyps, abdominal desmoid-type fibromatosis, solitary fibrous tumors and smooth muscle tumors, must also be excluded.
Bone and soft-tissue tumors are in general rare. Diagnosing these tumors is challenging based on the significant number of different tumor entities, the rareness of these tumors, and the considerable morphological heterogeneity which can be found within a single tumor entity. Considering that more than half of the described soft-tissue tumors and approximately 25% of the bone tumors harbor recurrent genetic alterations, the use of auxiliary molecular examinations should be strongly considered. Molecular analyses are important to confirm the diagnosis, to guide treatment, to provide information about prognosis, and to allow patient recruitment for basket trials based on the molecular signature of a tumor. In addition, novel molecular alterations detected by next-generation sequencing (NGS) obtain further insights into the pathogenesis of these rare tumors and allow a more detailed genetic classification. Based on our single-center results of NGS using the Ion AmpliSeq Cancer Hotspot Panel v2 and the Ion AmpliSeq Comprehensive Cancer Panel (Thermo Fisher Scientific) for mutational analyses as well as the Archer FusionPlex Sarcoma Kit (ArcherDX, Inc) to detect gene fusions in 26 genes since early 2016, we have experienced NGS as a very sensitive method to detect genetic alterations. In our experience, the use of the Archer FusionPlex Sarcoma Kit is superior to fluorescent in situ hybridization as an auxiliary tool in the routine workup of soft-tissue and bone tumors.
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