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Introduction. Myofibroblastomas of the breast are rare mesenchymal lesions thought to be derived from a CD34-positive mammary stromal cell that shows a great capacity for multipotency. Cases presentation. We performed a retrospective analysis of 4 cases with confirmed mammary myofibroblastomas. Hematoxylin and eosin slides and immunohistochemical slides were reviewed. For next-generation sequencing studies we used a TruSight Tumor 15 panel which ran in a MiSeq sequencer and was analyzed with illumina VariantStudio 3.0 and IGV (Integrative Genomic Variation). The case series consisted of 4 patients (3 men, 1 woman). The anatomic location distribution was equal, with a 1:1 ratio between the left and right breast and a gross median size of the lesion of 34 mm. Gross features revealed a well circumscribed lesion with a vaguely lobular cut surface and the microscopical features were that of a classic-type MFB composed of benign, spindle-shaped cells interrupted by thick collagen fibers that show extensive hyalinization. CD34 and desmin immunohistochemical markers were diffusely positive in tumor cells, thus confirming the diagnosis. Next generation sequencing used a TruSight Panel 15 and revealed mutations in TP53, p.P72R gene in two cases and in ERBB2 p.1655V, KIT c.2484C>T, GUSB p.V270M genes in one case. No pathogenic mutations were identified. Conclusions. The diagnosis of these tumors is challenging based on the rareness of these lesions. Molecular analyses are important to confirm the diagnosis based on morphology and immunohistochemistry. Given the fact that these tumors are rare and benign, there is little data about the molecular signature of this tumor.

Mammary myofibroblastoma is a rare soft-tissue tumor. Extramammary myofibroblastomas are particularly rare. We here report the case of a 78- year-old man presenting with pelvic pain relieved by defecation or urination. Rectal examination showed a mass in front of the anterior rectal wall. The magnetic resonance imaging (MRI) showed a wellcircumscribed and heterogeneous mass measuring 10 x 6 x 8cm located behind the bladder which was pushed forward in front of the rectosigmoid. Immunohistochemical analyses showed diffuse coexpression on CD34 cells and desmin, Rb expression on most cells, oestrogen receptor expression, intense and diffuse P16 expression and a ki67 proliferation index of 25%. The patient had no recurrence 8 months after radiotherapy followed by surgery. Breast myofibroblastoma is a rare and benign tumor. Recurrence is hardly observed after local treatment. This study highlights the supporting role of radiotherapy in the efficacy of surgery. [ABSTRACT FROM AUTHOR]