Fibromatosis-like metaplastic carcinoma: a case report and review of the literature

Abstract

Background: We report an unusual case of low-grade fibromatosis-like metaplastic carcinoma (LG-FLMC) of the breast. This exceedingly rare epithelial breast malignancy has been reported only 68 times in the past 20 years, and is classified as a subtype of metaplastic breast carcinoma (MBC). It is a locally aggressive tumor with a low potential for lymph node and distant metastases, but with a tendency to recur after excision. Here we describe a less common presentation of LG-FLMC, provide its molecular characterization, discuss the major differential diagnosis and bring a short review of the literature.

Case presentation: A 65-year-old woman presented with a self-palpated breast lump that had discordant radio-pathological features. While imaging results were compatible with an infiltrative malignancy, on core needle biopsy (CNB) a sharply delineated lesion composed by a bland-looking population of spindle cells was observed; excision was recommended for final diagnosis. Histology of the resection specimen showed small areas of epithelial differentiation and foci of peripheral invasion. Immunohistochemical analysis revealed a co-immunoreactivity for epithelial and myoepithelial markers in the spindle cell component. Mutation analysis with a capture-based next generation sequencing method revealed pathogenic mutations in GNAS, TERT-promotor and PIK3R1 genes. A diagnosis of LG-FLMC was rendered.

Conclusion: This case highlights the importance of a broad differential diagnosis, exhaustive sampling and the use of a broad immunohistochemical panel whenever dealing with a low-grade spindle cell lesion in the breast, and provides further insights into the molecular background of LG-FLMC.

Keywords: Breast; Low-grade fibromatosis-like metaplastic carcinoma; Metaplastic breast carcinoma; Spindle cell lesion.

Fig. 1

Routine histology with hematoxylin and eosin (H&E) stains. a The breast lesion has a variable cellularity. Large areas consist of dense collagenous stroma with a low cellularity (upper right of the picture). These areas are admixed with more cellular areas that have a myxoid appearance (lower left of the picture). (H&E ×50). b A highly cellular, myxoid area of the lesion. There is clustering of fusiform to discrete epithelioid cells with a round, slightly irregular nucleus that contains a small nucleolus. Occasional mitotic figures are seen (arrow). (H&E × 400). c Within the breast lesion, small and normal-looking ducts are entrapped by the proliferative spindle cells. The latter show no obvious atypia or mitotic activity at high magnification. At the periphery, a limited infiltration of bland-looking spindle cells into the surrounding fat tissue is seen. A scattered infiltrate of lymphocytes is also seen. (H&E × 100). d A view of the assessable border of the breast lesion at low magnification. A nodular and sharply delineated margin in relation to the surrounding fat tissue can be seen. This is not typical for FLMC, where a more infiltrative growth pattern is expected at the border. (H&E × 25). e In the center of the lesion, a distinctive area with a relatively high number of bland-looking ducts surrounded by a striking sclerotic stroma is seen. (H&E × 200)

Fig. 2

Immunohistochemistry. Staining patterns for αSMA (a), p63 (b), CK AE1/AE3 (c) and β-catenin (d). (IHC × 400). There is a co-expression in both the spindle cells and the more epithelioid cells for p63 and CK AE1/AE3