Table 2.

Most common bland-appearing spindle cell lesions of the breast and their key features

Lesion	Key features	Other features
Fibromatosis-like metaplastic breast carcinoma (MBC)	Infiltrative, focal mild atypia and focal epithelioid differentiation	- Low-grade spindle cell proliferation with variable cellularity - Cells with pale eosinophilic cytoplasm and slender nuclei with tapered edges and finely distributed chromatin admixed with plump fusiform and polygonal tumour cells that have more rounded nuclei and are arranged in”epithelioid “ clumps mainly seen centrally in the tumour - Infiltrative with entrapped normal breast structures - Regressive changes: collagenisation, scattered inflammatory cell infiltrate comprised of lymphocytes and plasma cells with occasional lymphoid follicles at the edges of the tumour. May be associated with papillary lesion or radial scar - IHC expression of CKs and/or p63 (and p40). CK + cells are seen in almost all cases and usually appear as cords or sheets of polygonal cells, rarely as isolated positive cells. SMA is often positive particularly in CK negative cells. Typically negative for CD34, hormone receptors and HER2
Scar	Fat necrosis, haemosiderin-laden macrophages	- History of trauma/procedure and presence of biopsy site-associated changes such as hemosiderin deposition, fat necrosis, foamy macrophages, and foreign body giant cells - Distinction of post-operative scar from residual fibromatosis may be extremely difficult but a fascicular growth pattern and entrapment of breast parenchyma are not usually seen in a scar - Reactive spindle cell nodule is likely to represent an exuberant reparative process (i.e., young scar) but may reach a large size and be associated with breast fibro-sclerotic lesions - IHC: shows positive SMA expression but negative immunoreactivity of β-catenin, desmin and CD34
Fibromatosis	Infiltrative, long fascicles, nuclear spacing	- Locally infiltrative, non-metastatic, lesion frequently arises from deep fascia - Usually solitary non-tender ill-defined mass - May be spiculated on mammography mimicking carcinoma. US and MRI are more sensitive for its detection - Long fascicles; variable collagen deposition and cellularity, diffusely infiltrative with entrapped fat at periphery. The lesion nuclei are characteristically spaced - Lymphocytes are often seen at the periphery - IHC: nuclear staining of β-catenin and cytoplasmic staining of SMA. CD34, CKs, p63, desmin and S100 are negative
Myofibroblastoma	No atypia, devoid of breast parenchyma, ropy collagen	- Benign solitary slowly growing often as a circumscribed tumour - Variable morphology but typically fascicular growth of spindle cells with bands of collagen fibres, amianthoid fibres, variable adipocytic component, may be cellular. No regularly spaced nuclei. Devoid of breast glandular elements - IHC: positive for CD34, desmin, SMA, ER, PR, CD99, BCl2 and CD10
Nodular fasciitis	Rapidly growth lesion with short history, devoid of breast parenchyma, extravasation of RBCs	- Rapidly growing, may be tender or painful, self-limiting mass-forming composed of clonal cellular proliferation - Well-circumscribed; tissue culture-like fibroblasts with plump vesicular nuclei; myxoid stroma; extravasated RBCs. Mitoses may be frequent but no abnormal forms - IHC: positivity for actin

CK cytokeratin, MBC metaplastic breast carcinoma. Most of the lesions, apart from nodular fasciitis, show rare or no mitoses. Atypia and necrosis are not features of this category of BSCLs. All entities, apart from MBC and some cases of IMT, typically lack cytokeratin immunoreactivity