Breast Cancer Pathology and Histology

By Frances Mather, MD, dept of Pathology, Herlev University Hospital and Anne-Vibeke Lænkholm, MD, dept of Pathology, Zealand University Hospital

Introduction

In this chapter you will learn how breast carcinoma is diagnosed and histologic graded by the pathologist using the microscope. You will be introduced to the normal breast tissue, ductal carcinoma in situ, ductal- and lobular carcinoma and microscopy of axillary sentinel lymph nodes.

The normal breast

Breast tissue is composed of epithelial, stromal and adipose tissue components.
The epithelium component consists of ducts and lobules. Large ducts branch into successive smaller ducts, eventually forming the terminal duct lobular unit.
The epithelium consists of a double-layered epithelium; inner luminal cuboidal-columnar epithelium and an outer myoepithelium. This two-layered epithelium is surrounded by a basement membrane, separating it from the surrounding stroma.
In invasive carcinoma it is the outer myoepithelial cell layer and the basement membrane which are breached by tumor cells, and thus no longer intact.

Fig 1: Normal breast ducts and lobules surrounded by stroma and a little fatty tissue.

Macroscopic handling of breast tissue specimens

Lumpectomy

Lumpectomy specimens should preferably be assessed whilst they are fresh and unfixed (not in formaline) in order to preserve certain essential features such as mitotic count and expression of molecular biomarkers.
The specimen is weighed, measured, orientated and then inked with different colored markers according to specified markings, before it is sliced into parallel slices.
The tumor diameter and the distance from the tumor to the resection margins are measured, and the information is communicated to the breast surgeon perioperatively. Inking with different markers is essential in order to maintain an overview of the tumor’s relation to all margins, both macroscopically and microscopically.

Fig 3: Lumpectomy slices containing a 21 mm tumor with sufficient distance to the inked resection margins (radical resection).

Mastectomy

Mastectomy specimens should also preferably be assessed whilst they are fresh and unfixed. The specimen is weighed, measured, orientated according to stated markings and the deep fascial plane is assessed and inked. The specimen is then sliced into parallel slices and the tumor diameter and the distance from the tumor to the resection margins are measured.

Breast Cancer

The most common types of breast cancer are invasive carcinoma of no special type (also called ductal carcinoma), comprising about 75% of cases of breast cancer and lobular carcinoma (comprising about 5-15%). There are several other less common types of breast cancer, including mucinous and cribriform carcinoma.

Invasive ductal carcinoma

Macroscopically these tumors often are relatively well defined, firm or irregular and stellate (star formed) in outline.
Arises from the terminal duct lobular unit
Tumor morphology can vary. The tumor cells are often arranged in islands/groups of varying size, trabeculae or cords, surrounded by a desmoplastic stroma. The appearance of tumor cells can also vary, from small regular nuclei to large pleomorphic nuclei with several nucleoli.
The vast majority of tumor cells display positive expression for E-Cadherin (Epithelial calcium-dependent cell adhesion protein), a cellular adhesion molecule
Tumors can be well differentiated where many tubular structures can be seen, or poorly differentiated with no tubular structures.

Fig 4: Ductal carcinoma grade 1 with a moderate degree of tubular formation, few mitoses and moderate nuclear pleomorphism.

Fig 5: Ductal carcinoma grade 3 with a poor degree of tubular formation, numerous mitoses and pronounced nuclear pleomorphism.

Invasive lobular carcinoma

Macroscopically these tumors are often more diffuse, irregular and poorly circumscribed.
Tumor morphology can also vary. In the classic pattern the tumor cells often lack cohesion, are small, regular, with little pleomorphism.
They are arranged in infiltrating linear cords with single file configuration and individually, and can surround normal ducts.
The majority display loss of E-Cadherin expression.

Other types of breast cancer

Cribriform carcinoma – tumor cells exhibit a ’cribriform’ appearance, where they are arranged in groups with arches of tumor cells creating cribriform spaces. The tumor cells are often small and regular, displaying only mild pleomorphism. The prognosis is generally better for this type of breast cancer.

Mucinous carcinoma

Mucinous carcinoma – groups of tumor cells are seen in lakes of extracellular mucin. Macroscopically the tumors have a different appearance – they’re softer, well defined and gelatin-like. The tumor cells are often small and regular, displaying only mild pleomorphism. The prognosis is favourable.

Resection margins

When performing a lumpectomy procedure, to obtain a radical tumor resection, there must be a margin of minimum 5 mm macroscopically, and microscopically there cannot be tumor ’on the ink’ (ie. directly on the margin).

In cases of non-radical resection further resection must be performed.

Fig 9: Mixed ductal-lobular carcinoma with tumor cells on the green inked resection margin (Non-radical excision!).

Histologic Grading

Tumor grade conveys significant prognostic information.
Invasive ductal and lobular carcinomas are graded according to three criteria:
1) Degree of tubular formation,
2) Mitotic count
3) Nuclear pleomorphism.
Each criterion is assigned a score from 1-3, and these values are added together to determine the grade.
Grade 1 corresponds to well differentiated carcinoma and conveys a better prognosis, while grade 3 corresponds to poorly differentiated carcinoma and conveys a worse prognosis.

Fig 10: Ductal carcinoma grade 1 with a moderate degree of tubular formation, few mitoses and moderate nuclear pleomorphism.

Fig 11: Ductal carcinoma grade 3 with a poor degree of tubular formation, numerous mitoses and pronounced nuclear pleomorphism.

Tumor receptor status

Estrogen receptor (ER) and HER2 status are molecular biomarkers which are routinely assessed on immunohistochemical analysis.

ER is a strong predictive factor in breast cancer in terms of response to anti-estrogen hormonal treatment.
The percentage of tumor cell nuclei which are ER positive is assessed and ER positivity is defined as ≥1%.
The majority of breast cancers (approximately 80%) are ER positive.

Fig 12: Ductal carcinoma where tumor cells display 100% estrogen receptor positivity

Studies have shown that progesterone receptor status in breast cancer is NOT a predictive factor, which is why this biomarker is omitted from routine analysis.

HER2 = Human Epidermal growth factor Receptor 2. HER2 gene codes for a growth factor receptor on the surface of normal breast epithelial cells.
HER2 over-expression occurs in approximately 10-15% of breast carcinomas
It is a strong predictive and prognostic factor. Patients with carcinomas with HER2 over expression are candidates for anti-HER2 therapy.
HER2 status is assessed by either immunohistochemistry or ‘In Situ Hybridisation’

Fig 13: Ductal carcinoma with HER2 receptor over-expression.

Ki67 is a marker of cell proliferation, which has prognostic and predictive significance in breast cancer. The higher Ki67 expression the more cells are proliferating.
There are not universally accepted cut off values regarding low/middle/high proliferation rates for Ki67 index.

Fig 14: Ki67 index assessed as 90% in tumor cell hot spots

Sentinel node

The Sentinel node is identified using radiotracer and patent blue dye coloer injected around the tumor (or beneath the areola).
Frozen section evaluation are performed intraoperatively with the pathologist and breast surgeon in close cooperation.
If macrometastases are found, axillary lymph node dissection is subsequently performed (all lymph nodes are removed).
A macrometastasis is where the largest metastatic area in the lymph node measures >2 mm.

Fig 15: A lymph node with macrometastasis and perinodal growth, where the tumor cells infiltrate the surrounding nodal tissue.

Micrometastasis is where the largest tumor area measures >0.2 mm and/or ≤2 mm or contains >200 cells.
Isolated tumor cells are defined as isolated/small groups of tumor cells which collectively measure ≤0.2mm or contain ≤200 cells. These cells are hard to see, especially on frozen sections and may only be detected with immunohistochemical stains eg. CK7/19.

Fig 16: Isolated CK7/19 positive tumor cells in a lymph node

Ductal carcinoma in situ

Neoplastic proliferation of epithelial cells, confined to the ductal-lobular system and does not breach the basement membrane and invade the surrounding stroma.
Can present as a lump, but is often asymptomatic and discovered mammographically.
Incidence has increased with the introduction of screening programmes accounting for approximately 15% of the screening detected malignant lesions.
Different architectural types eg. solid, comedo, micropapillary.
One classification system is the Van Nuys classification system, which incorporates nuclear pleomorphism and the presence/absence of intraluminal necrosis.
The basement membrane is intact with a preserved myoepithelial layer, which excludes an invasive carcinoma.
Microscopically to obtain a radical resection, there must be minimum 2 mm to the nearest resection margin.

Fig 18: DCIS with SMMS-1 staining, demonstrating the intact myoepithelial layer.

Paget’s disease of the nipple

Presence of malignant epithelial cells (Paget cells) in the epidermis of the nipple.
Most commonly associated with underlying invasive carcinoma or ductal carcinoma in situ, can rarely occur without an underlying neoplastic cause.
Paget cells are large with lots of pale cytoplasm, large nuclei and pronounced nucleoli.
May be arranged in the epidermis individually, in clusters or rarely in glandular formation.
Differential diagnosis includes melanoma and Bowen disease (immunohistochemical stains can help here).