H-caldesmon expression in myofibroblastoma of the breast: evidence supporting the distinction from leiomyoma

AIMS: The ultrastructural detection of leiomyomatous rather than myofibroblastic features in some cases of myofibroblastoma of the breast led some electron microscopically orientated pathologists to doubt the commonly accepted myofibroblastic nature of such a tumour, so the alternative terms 'myogenic stromal tumour' or 'variant of leiomyoma' have been proposed. The aim of this study was to analyse the immunohistochemical expression of h-caldesmon, a reliable marker in distinguishing smooth muscle versus myofibroblastic cellular differentiation, in a large series of myofibroblastomas of the breast to clarify whether these tumours are basically leiomyomatous. Moreover, cases from primary myofibroblastic lesions of the breast, such as fibromatosis and inflammatory myofibroblastic tumour, were analysed to assess whether h-caldesmon expression parallels that observed in their soft tissue counterparts. METHODS AND RESULTS: Paraffin-embedded sections from 12 cases of myofibroblastoma, seven cases of fibromatosis, and one case of inflammatory myofibroblastic tumour were evaluated immunocytochemically for the expression of h-caldesmon. As expected, all myofibroblastic lesions failed to express h-caldesmon. Conversely, focal staining, ranging from 2% to 10% of neoplastic cells, was detected in myofibroblastomas, even though it was restricted to 50% of analysed cases. CONCLUSIONS: Our results, indicating that smooth muscle differentiation occurs in a minority of the myofibroblastoma cells exclusively in half of the analysed cases, support the separation of myofibroblastoma from leiomyoma. The detection of smooth muscle cells in breast myofibroblastoma is easily explained if we postulate its histogenesis from the CD34+ fibroblasts of mammary stroma capable of multidirectional mesenchymal differentiation, including smooth muscle. We recommend retention of the term myofibroblastoma for all the desmin-positive and/or alpha-smooth muscle actin-positive spindle cell tumours of the breast consistent with the previously well-established morphological criteria for such neoplasms, unless one is dealing with a typical leiomyoma easily recognizable at light microscopy