Spindle cell lesions of the breast: a diagnostic approach

Abstract

Spindle cell lesions of the breast comprise a heterogeneous group of lesions, ranging from reactive and benign processes to aggressive malignant tumours. Despite their rarity, they attract the attention of breast pathologists due to their overlapping morphological features and diagnostic challenges, particularly on core needle biopsy (CNB) specimens. Pathologists should recognise the wide range of differential diagnoses and be familiar with the diverse morphological appearances of these lesions to make an accurate diagnosis and to suggest proper management of the patients. Clinical history, immunohistochemistry, and molecular assays are helpful in making a correct diagnosis in morphologically challenging cases. In this review, we present our approach for the diagnosis of breast spindle cell lesions, highlighting the main features of each entity and the potential pitfalls, particularly on CNB. Breast spindle cell lesions are generally classified into two main categories: bland-appearing and malignant-appearing lesions. Each category includes a distinct list of differential diagnoses and a panel of immunohistochemical markers. In bland-appearing lesions, it is important to distinguish fibromatosis-like spindle cell metaplastic breast carcinoma from other benign entities and to distinguish fibromatosis from scar tissue. The malignant-appearing category includes spindle cell metaplastic carcinoma, stroma rich malignant phyllodes tumour, other primary and metastatic malignant spindle cell tumours of the breast, including angiosarcoma and melanoma, and benign mimics such as florid granulation tissue and nodular fasciitis.

Keywords: Approach; Breast; Diagnosis; Immunohistochemistry; Spindle cell lesions.

Fig. 1

Low-grade fibromatosis-like metaplastic breast carcinoma on H&E (a, b). Cytokeratin (34betaE12) immunohistochemistry highlights the malignant epithelial cells (c, d)

Fig. 2

Fibromatosis featuring an infiltrative margin (a) with bland appearing spindle cells showing spacing of nuclei (b). Beta-catenin staining shows nuclear positivity (c, d)

Fig. 3

Nodular fasciitis showing a tissue culture like pattern with loose stroma (a). Extravasation of red blood cells and mitotic figures may also be seen (b)

Fig. 4

Tissue reaction to trauma resulting from previous surgery with fat necrosis and a florid histiocytic and myofibroblastic reaction (a). Florid reactive changes and mitotic figures may be seen (b) but inflammatory cells, hemosiderin deposition and occasional multinucleated giant cells are usually also present

Fig. 5

Myofibroblastoma: A needle core biopsy (a) with higher power view (b) showing bland looking spindle cells with intervening thick hyalinised collagen bands. Excision specimen of myofibroblastoma showing increased cellularity and ovoid nuclei (c, d). e and f show an example of myofibroblastoma with palisaded nuclei. g shows loss of nuclear expression of Rb gene on immunohistochemistry

Fig. 6

A case of solitary fibrous tumour (SFT) with a staghorn-like vasculature and a featureless pattern in which spindle cells and collagen bundles are randomly dispersed throughout the tumour (a). The cells are ovoid to fusiform and spindle-shaped with indistinct cell borders (b) arranged haphazardly or in short, ill-defined fascicles. Immunohistochemistry shows nuclear staining of STAT6 (c) and cytoplasmic staining of CD99 (d)

Fig. 7

Pseudoangiomatous stromal hyperplasia (PASH) shows complex inter-anastomosing spaces in dense collagenous stroma (a, b). The spindle-shaped myofibroblasts lining the slit-like spaces simulate endothelial cells. Spaces are usually empty but may contain rare red blood cells. Some cases may be cellular with plump spindle cells, which may obscure the pseudoangiomatous structure

Fig. 8

Malignant appearing spindle cell lesion of the breast with no distinguishing features (A) and CK negative. However, further sampling revealed areas with squamous cell carcinoma component (B) confirming the diagnosis of MBC. C shows a case of malignant appearing spindle cell lesion that is negative for CK and CD34. However, further sampling revealed areas with the biphasic growth pattern and characteristic parenchymal component (D) confirming the diagnosis of phyllodes tumour