Breast tumour. Light micrograph (LM) of a section of a breast myofibroblastoma. Myofibroblastoma is a rare benign mesenchymal tumour of mammary stroma composed of fibroblasts and myofibroblasts. The tumour usually occurs in the breast parenchyma of both females and males.Most cases of myofibroblastoma occur most often in women and men aged 40??7 years affecting older men and postmenopausal women. Magnification: x80 when printed 10 centimetres wide.