I would like to talk a little about the lecture I gave yesterday on spindle cell lesions of the breast at the BGICC 2023 meeting in Cairo, Egypt. Spindle cell lesions of the breast are challenging lesions for the pathologist. Any soft tissue spindle cell lesion may occur in the breast; these are obviously less common than epithelial lesions. The problem with soft-tissue lesions in the breast is that the morphology can be very confusing; benign lesions may look malignant and malignant lesions may look benign. 

I spoke about the classification of these lesions and an approach to diagnosis. A modern approach to classification is to regard these lesions as bland-appearing lesions or malignant-appearing lesions. The advantage of this approach to classification on H&E microscopy is that it encourages us to consider a malignant diagnosis when looking at a bland-appearing lesion and a benign diagnosis which may occur in a malignant-appearing lesion. 

I went on then to discuss the differential diagnosis of these two main categories. I like to consider these really in groups of three. So, taking the bland-appearing lesions first of all we can consider bland-appearing lesions as those that have an irregular outline and this differential diagnosis includes primarily fibromatosis-like and metaplastic carcinoma. That is the diagnosis that we do not want to miss in considering these lesions. Other differential diagnoses include fibromatosis, nodular fasciitis, pseudoangiomatous stromal hyperplasia and low-grade angiosarcoma. 

The second category of bland-appearing lesions we can consider is those that have a round configuration; these include myofibroblastoma, leiomyoma, neural tumours and hamartoma. They are the two main categories, then, of bland-appearing lesions and that approach is useful because it tends to immediately give rise to a set of differential diagnoses.

Malignant-appearing lesions, then, broadly speaking, our main differential diagnosis there is metaplastic spindle cell carcinoma; that is our top differential diagnosis. Other differential diagnoses include malignant phyllodes tumour, sarcoma, myoepithelial tumours and then the benign lesions which can look malignant, in particular nodular fasciitis and granulation tissue with scar formation. 

So when we look at these lesions on a needle core biopsy, we try to take account, first of all, all of the information that is available to us. It is very important that we remember to interpret any pathological findings in context. So we take account of the clinical findings, the radiological findings and, in particular, any relevant past medical history that the patient may have. So our first line, then, when we have considered all that information, to examine our H&E section and to look at the spindle cell proliferation itself – is it pure, is it mixed? If it’s a mixed proliferation usually the diagnosis is easier and more apparent. Then we look at the configuration of the lesions, the growth pattern, the presence or absence of necrosis and any particular clues such as pigment position or calcification that may help us to narrow down our differential diagnosis.

Having considered the H&E then, our next line, really, is consideration of immunohistochemistry. There is a large number of immunohistochemical stains at our disposal; the one that we really most commonly use is cytokeratin. It is important when using cytokeratin to use a panel of antibodies as staining in metaplastic carcinoma can be focal or even absent. If we find this on a needle core biopsy we should repeat those stains on excision. Other useful stains are beta catenin for the diagnosis of fibromatosis, but we have to be careful because sometimes we can get non-specific staining in a metaplastic carcinoma. Other useful stains are CD34 and p63. 

In today’s world of pathology, molecular studies are becoming increasingly important in diagnosis and there are specific mutations in a number of conditions, such as fibromatosis, nodular fasciitis and in many different types of sarcoma, that may help us to make a more precise diagnosis.