Myofibroblastoma of the breast showing rare palisaded morphology and uncommon desmin‐ and CD34‐negative immunophenotype: A case report

Myofibroblastoma is a rare benign mesenchymal tumor typically arising in the breast. We report a diagnostically challenging case of myofibroblastoma of the breast showing a rare palisaded morphology and an uncommon desmin‐ and CD34‐negative immunophenotype. A 73‐year‐old man underwent an excision for an 8 mm‐sized breast mass. Histology revealed that the tumor was composed of fascicles of bland spindle cells showing prominent nuclear palisading and Verocay‐like bodies. First, schwannoma, malignant peripheral nerve sheath tumor, and synovial sarcoma were suspected given the palisaded morphology. However, none of them was confirmed by immunohistochemical or molecular analyses. Next, a palisaded variant of myofibroblastoma was suspected by the morphology and coexpression of estrogen, progesterone and androgen receptors, BCL2 and CD10 in immunohistochemistry. However, the key diagnostic markers, desmin and CD34, were both negative. Finally, the diagnosis of myofibroblastoma was confirmed by detecting RB1 loss in immunohistochemistry and monoallelic 13q14 deletion (RB1 and FOXO1 loss) by fluorescence in situ hybridization assay. For the correct diagnosis of myofibroblastoma, it is important for pathologists to recognize the wide morphological spectrum, including a palisaded morphology, and the immunophenotypical variations, including desmin‐ and CD34‐negative immunophenotypes, and to employ a comprehensive diagnostic analysis through combined histological, immunohistochemical and molecular evaluations.