Myofibroblastoma of the breast is an uncommon but well defined benign stromal tumour. This report describes a case in which the predominant histological component was mature adipose tissue and two further cases with a major adipocytic component. Although small foci of adipose tissue are a recognised feature of this tumour, the dominance of the histological pattern by fat has not been described previously, and the recognition of this variant is important to allow confident diagnosis and avoid confusion with other primary adipocytic or stromal lesions, especially in the setting of potential needle core biopsy of such a lesion.
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We wish to present a myofibroblastoma in which the histological features were dominated by mature adipocytes to such a degree that the diagnosis of myofibroblastoma could only be made after careful assessment and consideration of the diagnosis of an adipocytic lesion, and two further cases with a major adipocytic component, which dominated the histological appearance in some areas.
Case 1. A 60 year old woman with a family history of breast carcinoma presented with a 1.5 cm firm, non-tender, mobile mass in the right axillary tail. Mammography demonstrated a relatively well circumscribed density in the right axillary region. Aspiration cytology was unsatisfactory and therefore the mass was excised.
Case 2. A 51 year old woman presented with a small hard lump in the superomedial part of the right breast. An ultrasound scan of the breast showed a 5 mm diameter hypoechoic focus lying in the subcutaneous tissue medial to the breast tissue. Aspiration cytology was unsatisfactory and the mass was excised.
Case 3. A 68 year old woman underwent a mastectomy for invasive ductal carcinoma. The distant random breast tissue examined included an incidental asymptomatic 3 mm well circumscribed nodule.
The cut surface of the specimens from cases 1 and 2 revealed well defined and circumscribed, firm, grey/white, solid nodules, 20 mm and 12 mm in diameter, respectively. The third case was discovered incidentally on microscopic examination and consisted of a well defined 3 mm nodule. Histologically, the lesions were well delineated from the surrounding breast tissue and composed of irregular nodules and strands of collagenous stroma with intervening fatty stroma (fig 1). The collagenous areas enclosed bland spindle cells with scanty eosinophilic cytoplasm and regular spindle shaped nuclei showing no pleomorphism or mitotic activity (fig 2). Occasional cellular foci with a more epithelioid pattern were present in case 1. No epithelial component was seen in cases 1 or 2 and an occasional small ductule was sited peripherally in case 3. The relative proportion of adipocytic to non-adipocytic elements was approximately 33% in case 2 and approximately 75% in cases 1 and 3. In these last two cases, the relative proportions of fat and cellular stroma varied throughout the lesion, with both cases including areas where the predominant component was fatty.
Immunohistochemistry was performed on cases 1 and 2: a large proportion of the spindle cells in both cases expressed vimentin and desmin (fig 2), with similar expression of α smooth muscle actin in case 1 but only focal expression in case 2. There was no evidence of cytokeratin or S100 protein expression. There was weak focal expression of CD34 in case 1. There was insufficient tissue in case 3 to attempt immunohistochemistry.
The histological and immunohistochemical features of the spindle cell component of these lesions are those described in myofibroblastoma of the breast.1 Although generally believed to be true myofibroblastic tumours, it has been suggested that these tumours are mesenchymal tumours capable of diverse lines of differentiation, or are modified myogenic stromal tumours.2, 3 Several authors have described small lobules of fat scattered throughout these lesions, but only one case had an abundance of fat cells.1, 3 Magro et al have also described a variant of myofibroblastoma that shows atypical spindle cells associated with islands of mature adipocytes.4 The major difference between the cases included in this report compared with those described previously is the unusually large adipocytic component. The pronounced fatty component in our cases is potentially misleading and led to serious consideration of the differential diagnosis of spindle cell lipoma. This is also a circumscribed and painless, firm nodule, found more frequently in men, in the regions of the posterior neck and shoulder.5 The spindle cells in a spindle cell lipoma are closely associated with a mucoid matrix6 and, although immunostaining of the spindle cell in this lesion varies, some are positive for S100 protein.6
A needle core biopsy was not attempted in case 1 or 2 because at the time these biopsies were not available routinely in this centre. However, it is now likely that such lesions would undergo core biopsy on presentation, which would provide specimens showing an apparent infiltrative spindle cell process within fatty breast stroma; this could lead to a false diagnosis of fibromatosis or even sarcoma because myofibroblastomas can show mitotic activity and some degree of cellular pleomorphism. Therefore, it is particularly important in this setting to appreciate the wider range of appearances of this benign lesion than hitherto recognised to avoid potential misdiagnosis and inappropriate treatment.
Predominant fatty variant of myeloblastoma of breast. Baxendine-Jones J, Theaker JM, Baldwin LJ. J Clin Pathol 2001;54:568�9.
The correct author listing should have been Baxendine-Jones J, Baldwin LJ, Bateman AC, Theaker JM.