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It is the pathologist who will confirm the breast cancer diagnosis and identify the characteristics of the tumour. This effectively provides an 'identity card' for the cancer, which is essential when it comes to choosing treatment options
The pathologist begins with a macroscopic examination, i.e. by the naked eye. The test area is covered by indelible ink so that the edges of the sample can be identified on the slides. He measures the size of the tumour and the distance from the excision boundary (the excision margin). The fragment of tissue is then cut into ultrathin slices which are placed on glass slides and coloured using different chemical products. The pathologist analyses these slides under the microscope and makes a diagnosis.
Some tissue is preserved in order to answer any questions arising in the future.
What the pathologist sees
Analysis of the tumour margins
When the entire tumour is removed (lumpectomy), the pathologist analyses the edges (called 'excision margins') to determine if the cancer has reached the edge of the sample area or not.
- If the cancer is too close to the edges, it may be possible that there are remaining tumour cells in the breast, and it may be necessary to intervene again (further surgery).
- If the cancer does not touch the margins, the pathologist measures the distance of healthy tissue surrounding the cancer – this is the safety margin.
In situ or Invasive cancer
The breast is composed of cells that are arranged in lactiferous ducts and in lobules. These two elements are separated from the supporting tissue (connective tissue) by a 'basement membrane'
Breast cancers occur mostly in the lactiferous (milk) ducts and lobules.
- If the basement membrane is not crossed by the tumour cells, the cancer is known as 'in situ' and is called a carcinoma in situ (CIS). If the cells are found in the ducts, this is a ductal carcinoma in situ (DCIS), while if the cancer originated in the lobules, it is a lobular carcinoma in situ (LCIS). Lobular carcinoma in situ is not considered to be a cancer, but seen as lobules that could become cancerous.
- The cancer is said to be invasive or even infiltrating if the tumour cells have crossed the basement membrane. Around 80% of Invasive cancers are ductal cancers, and 15% are lobular cancers. The remaining 5% are rare forms of breast cancer.
In the case of Invasive cancer, the pathologist also examines the tissues located around the tumour to look for possible malignant cells in the fat, blood vessels and lymphatic channels.
Analysis of tumour cells
The pathologist determines if the cancer is differentiated or undifferentiated, depending on how much the tumour cells resemble normal breast cells. The more a cancer cell resembles a normal cell, the more it is said to be differentiated (it possesses characteristics similar to those of a normal breast cell) and the less aggressive it is.
Based on this criterion and other observations, the pathologist determines the grade of cancer (known as 'Elston Ellis histoprognostic grading').
- Grade I : the tumour is not very aggressive.
- Grade II : the tumour is of intermediate grade (between I and III).
- Grade III : the tumour is aggressive.
Determination of biomarkers
The pathologist also looks for the presence of biomarkers on the surface of the cancer cells. These are the biological characteristics of the cell, which, when identified, helps to determine the most appropriate treatment for the patient.
- Determination of hormone receptors. If the cancer cell has hormone receptors for oestrogen and/or progesterone, the tumour is known as hormone-sensitive. An appropriate treatment (hormone therapy) will therefore be proposed.
- Determination of HER2 status. This protein is involved in cell proliferation. In the case of HER2 overexpression (an excess of this protein in cancer cells), a specific post-operative treatment is administered: trastuzumab, whose trade name is Herceptin R. We talk about 'targeted therapy' because this medicine acts selectively on the cancer cells carrying the receptor.
- Measurement index for protein Ki67. This protein is a marker for cell proliferation. As for other biomarkers, studying it enables us to characterise the cell and can help to select the best therapeutic strategy.
The final pathology report allows the pathologist to summarise what he has seen and make a diagnosis about :
- the size of the tumour ;
- the safety margin (has the surgeon removed enough of the tissue surrounding the cancerous area?) ;
- the type of tumour (ductal or lobular) ;
- the presence or absence of invasion; non-invasive cancer (in situ) or invasive;
- the grade of the cancer, or histoprognostic grade ;
- le grade du cancer, ou grade histo-pronostique ;
- the presence or absence of hormone receptors, with the percentage of cells which exhibit hormone receptors ;
- the HER2 status (overexpression or not) ;
- the Ki67 index to specify the rate of cancer cell division.
The findings of the histological examination make up the cancer's identity. They are fundamental when choosing treatment.