Understanding Your Breast Cancer Pathology Report

A Guide for Breast Cancer Patients — Y-ME National Breast Cancer Organization

What Is a Pathology Report?

A pathology report is a medical document prepared by a pathologist — a physician who specializes in examining tissue and cells under a microscope — after analyzing a sample of tissue taken from your breast during a biopsy or surgery.

The report describes the biological characteristics of the tissue sample in precise medical language. If the tissue is cancerous, these characteristics tell your health care team a great deal about how your cancer behaves, how far it has progressed, and which treatments are most likely to be effective.

Being diagnosed with breast cancer is a life-changing event, and the pathology report can feel overwhelming — written in dense medical terminology that seems designed only for specialists. This guide is here to help you make sense of it.

One important note: most pathology reports contain a mix of findings — some that look favorable, some that may seem concerning. This is typical. It is the overall picture that matters, not any single element read in isolation. Your oncologist is trained to interpret your complete report in the context of your full clinical picture.

Sections of the Pathology Report

Pathology reports are not written in a single standard format — the layout can vary between institutions and laboratories. However, most reports contain some version of the following sections:

Demographics

The opening section contains identifying information: your name, date of birth, medical record number, the name of the physician who ordered the biopsy or surgery, and the date the specimen was received and analyzed. Always verify that this information is yours — errors, while rare, do occur.

Gross Description

This section describes the tissue sample as it appeared to the naked eye before microscopic examination — its size, weight, shape, color, and texture. It also documents how the sample was handled and prepared in the laboratory for microscopic analysis. This section is primarily technical and is most relevant to the pathologist and your surgeon.

Microscopic Description

Here the pathologist describes the features of the tissue as seen under a microscope. This is where the specific characteristics of the cancer cells — how they look, how they are arranged, how they compare to normal breast tissue — are documented. These observations lead directly to the diagnosis and to many of the findings described in the sections below.

Prognostic Report

This section contains the results of specific laboratory tests performed on the tissue sample, including hormone receptor testing, HER2 testing, Ki-67, and others. These test results have direct implications for your treatment options and prognosis.

Summary and Diagnosis

This is the most important section for patients. It brings together all of the data from the preceding sections into a consolidated diagnostic summary — a list of the key characteristics and findings specific to your tumor. This is the section your oncologist and surgeon will refer to most frequently when discussing your treatment plan.

Information Found in the Report — Term by Term

Anatomic Site

This tells you exactly which part of the breast was biopsied or removed, and the type of procedure performed. Common examples include “right breast, upper outer quadrant, core needle biopsy” or “left breast, mastectomy.” This confirms that the correct specimen is being analyzed.

Tumor Size

Tumor size is reported in centimeters (cm) or millimeters (mm). One inch equals approximately 2.54 centimeters. Size is one of the primary factors used in staging your cancer and influences surgical decisions.

In general, smaller tumors are associated with better outcomes — but size is only one factor among many. A small, high-grade, hormone receptor-negative tumor may require more aggressive treatment than a larger, low-grade, hormone receptor-positive one. Context is everything.

Invasive vs. Non-Invasive (In Situ)

One of the most critical distinctions in your report is whether the cancer is invasive or non-invasive (in situ).

• Non-invasive (in situ) — cancer cells are confined within the milk ducts or lobules and have not broken through into surrounding breast tissue. The most common non-invasive type is ductal carcinoma in situ (DCIS), which is classified as Stage 0. DCIS is highly treatable and, when managed appropriately, carries an excellent prognosis.
• Invasive (infiltrating) — cancer cells have broken through the wall (basement membrane) of the duct or lobule and entered the surrounding breast tissue, with the potential to spread to lymph nodes and beyond. The most common invasive type is invasive ductal carcinoma (IDC), followed by invasive lobular carcinoma (ILC).

The distinction between invasive and non-invasive has major implications for staging, treatment intensity, and prognosis.

Histologic (Tumor) Type

This tells you where in the breast the cancer originated and how the cells appear under the microscope. The most common types include:

• Invasive ductal carcinoma (IDC) — originates in the milk ducts; accounts for approximately 70–80% of invasive breast cancers
• Invasive lobular carcinoma (ILC) — originates in the milk-producing lobules; accounts for approximately 10–15% of invasive cancers
• Ductal carcinoma in situ (DCIS) — non-invasive, confined to the ducts
• Lobular carcinoma in situ (LCIS) — not technically a cancer but a marker of increased future risk
• Less common types — including inflammatory breast cancer, mucinous carcinoma, tubular carcinoma, medullary carcinoma, and metaplastic carcinoma

Histopathologic Grade

Grade describes how abnormal the cancer cells appear under the microscope and how quickly they are likely to grow and divide. It is one of the most important indicators of tumor aggressiveness.

The most widely used grading system is the Nottingham Grading System (also called the Scarff-Bloom-Richardson or Elston-Ellis system). It evaluates three microscopic features, each scored from 1 to 3:

Nuclear Grade

This assesses the appearance of the nucleus — the control center of the cell — including its size, shape, and how much it varies from cell to cell.

• Grade 1 (low) — nuclei look relatively normal; small, uniform, with little variation
• Grade 2 (intermediate) — nuclei show moderate abnormality
• Grade 3 (high) — nuclei are large, irregular, and markedly abnormal — the higher the nuclear grade, the more aggressive the tumor

Mitotic Rate

This counts the number of cancer cells that are actively dividing (undergoing mitosis) in a given field under the microscope. A higher mitotic rate means cells are dividing more rapidly — generally associated with faster tumor growth.

• Score 1 — few cells dividing (low)
• Score 2 — moderate number dividing
• Score 3 — many cells dividing (high) — more aggressive behavior

Tubule Formation (Cellular Differentiation)

This measures how closely the cancer cells resemble normal breast duct cells, which naturally form tube-like structures called tubules.

• Grade 1 — most cells form tubules; tissue is well-differentiated and resembles normal breast tissue
• Grade 2 — moderate tubule formation; moderately differentiated
• Grade 3 — few or no tubules; cells are poorly differentiated and look very different from normal tissue

Overall Nottingham Grade

The three scores are added together for a total between 3 and 9:

• Grade 1 (score 3–5) — well differentiated; slow-growing; generally more favorable prognosis
• Grade 2 (score 6–7) — moderately differentiated; intermediate behavior
• Grade 3 (score 8–9) — poorly differentiated; faster-growing; may require more aggressive treatment

Important note: Grade describes the tumor’s biology — how the cells look and behave. It is separate from stage, which describes how far the cancer has spread. A Grade 3 tumor caught at Stage I can have a very good prognosis; a Grade 1 tumor at Stage III presents different challenges. Both pieces of information are needed together.

Surgical Margins

When a tumor is removed surgically — whether by lumpectomy or mastectomy — the pathologist examines the edges of the removed tissue to determine whether cancer cells are present at or near the boundaries. These edges are called surgical margins, and the laboratory inks them before sectioning so they can be precisely evaluated under the microscope.

• Clear (negative) margins — no cancer cells are found at the inked edges of the removed tissue. This indicates that the surgery likely achieved complete local removal of the tumor. Current guidelines define a clear margin as no ink on tumor for invasive breast cancer.
• Close margins — cancer cells are present near but not at the inked edge. Whether further surgery is recommended depends on how close the cells are and other clinical factors.
• Positive margins — cancer cells are found at the inked edge of the specimen, suggesting that some cancer may remain in the breast. Re-excision surgery is typically recommended to achieve clear margins.

Achieving clear margins on the first surgery is one of the most important goals of breast cancer surgery. Preoperative MRI, particularly for invasive lobular carcinoma, can help improve margin outcomes by better defining tumor extent before the operation.

Lymph Node Status

The lymphatic system is a network of vessels and nodes throughout the body that carries fluid, filters waste, and houses white blood cells that fight infection. In breast cancer, cancer cells can travel from the tumor through nearby lymph vessels to the axillary lymph nodes (the nodes in the armpit) and from there to other parts of the body.

Lymph node status is one of the most important prognostic factors in breast cancer. The pathology report will document how many nodes were removed and how many — if any — contain cancer cells.

• Node-negative (0/N) — no cancer cells found in any of the lymph nodes examined; generally associated with better prognosis
• Node-positive (e.g., 3/15) — cancer cells found in the specified number of nodes out of the total removed. In general, fewer positive nodes is better than more, and the extent of nodal involvement influences both staging and systemic treatment recommendations.

Sentinel Lymph Node Biopsy (SLNB)

Rather than removing all axillary lymph nodes — a procedure called axillary lymph node dissection (ALND) — most patients with early-stage invasive breast cancer now undergo sentinel lymph node biopsy (SLNB) first. This minimally invasive technique identifies the first lymph node(s) that drain from the tumor site (the sentinel nodes) by injecting a blue dye, a radioactive tracer, or both near the tumor.

If the sentinel node(s) are negative, it is highly unlikely that cancer has spread further into the axilla, and full ALND can be avoided — significantly reducing the risk of lymphedema (arm swelling). Current guidelines from the American Society of Breast Surgeons and NCCN (2024) support avoiding ALND in many node-positive patients with early-stage disease as well, based on updated clinical trial data including the ACOSOG Z0011 and AMAROS trials.

If cancer is found in a sentinel node, your surgeon will discuss whether additional nodes need to be removed based on your specific clinical situation.

Hormone Receptor Status (ER and PR)

Breast cancer cells are tested for the presence of receptors that bind to the female hormones estrogen and progesterone. If these receptors are present, the hormones can bind to them and stimulate the cancer cells to grow.

• Estrogen Receptor-Positive (ER+) — cancer cells have estrogen receptors and use estrogen as a growth signal. Approximately 75–80% of breast cancers are ER+. These cancers are likely to respond to hormone-blocking (endocrine) therapy.
• Progesterone Receptor-Positive (PR+) — cancer cells have progesterone receptors. PR status is usually evaluated alongside ER status. Being PR+ in addition to ER+ generally indicates a higher degree of hormone sensitivity.
• ER-negative / PR-negative — cancer cells do not have these receptors and will not respond to hormone therapy. Treatment relies on chemotherapy, HER2-targeted therapy, or immunotherapy depending on other tumor characteristics.

Hormone receptor status is reported as a percentage (the proportion of positive cells) and an intensity score (how strongly the receptors are expressed), combined into an Allred score or similar scale. A result is considered positive when at least 1% of cells stain positive, per current ASCO/CAP guidelines (2020 update).

Endocrine therapy options for ER+ breast cancer include:

• Tamoxifen — a selective estrogen receptor modulator (SERM); used in premenopausal and postmenopausal women; blocks estrogen from binding to receptors
• Aromatase inhibitors (AIs) — anastrozole, letrozole, exemestane; used primarily in postmenopausal women; reduce estrogen production throughout the body
• Ovarian suppression — in premenopausal women at higher risk, combining ovarian suppression with an AI or tamoxifen may provide additional benefit
• CDK4/6 inhibitors — abemaciclib, ribociclib, palbociclib; used in combination with endocrine therapy in metastatic disease and, in the case of abemaciclib, in high-risk early-stage disease

HER2 Status

The HER2 gene (Human Epidermal Growth Factor Receptor 2) produces a protein that sits on the surface of cells and acts as a receptor for growth signals. In some breast cancers, the HER2 gene is amplified — meaning too many copies of the gene are present — causing the cell surface to be overloaded with HER2 receptors and the cancer to grow faster.

How HER2 is tested

Current ASCO/CAP HER2 testing guidelines (2018, updated 2023) recommend two primary testing methods:

• Immunohistochemistry (IHC) — measures the amount of HER2 protein on the cell surface, scored 0 to 3+:
  • 0 — HER2-negative
  • 1+ — HER2-negative (low expression)
  • 2+ — equivocal; requires reflex ISH testing to confirm
  • 3+ — HER2-positive (strong overexpression)
• In Situ Hybridization (ISH) — including FISH (fluorescent) and CISH (chromogenic); measures the number of HER2 gene copies in the cell; result reported as positive or negative based on the ratio of HER2 gene copies to chromosome 17

HER2-positive

Approximately 15–20% of breast cancers are HER2-positive. While historically more aggressive, HER2+ breast cancer is now one of the most treatable subtypes thanks to highly effective targeted therapies including:

• Trastuzumab (Herceptin) — the original HER2-targeted antibody
• Pertuzumab (Perjeta) — used with trastuzumab in early-stage and metastatic disease
• Trastuzumab emtansine (T-DM1/Kadcyla) — antibody-drug conjugate for residual disease after neoadjuvant therapy
• Trastuzumab deruxtecan (T-DXd/Enhertu) — a newer antibody-drug conjugate with remarkable efficacy in HER2+ and HER2-low metastatic disease
• Tucatinib (Tukysa) and neratinib (Nerlynx) — oral HER2-targeted tyrosine kinase inhibitors

HER2-low — an important new classification

Recent research has established a clinically meaningful category called HER2-low, defined as IHC 1+ or IHC 2+/ISH-negative. HER2-low tumors were previously classified simply as HER2-negative, but the antibody-drug conjugate trastuzumab deruxtecan (T-DXd) has demonstrated significant efficacy in this population. As of 2022–2024, HER2-low is now an important designation in metastatic breast cancer treatment planning. Ask your oncologist whether your HER2 result falls into the HER2-low category.

Lymphovascular Invasion (LVI)

Lymphovascular invasion refers to the presence of cancer cells inside the walls of blood vessels or lymphatic channels within or around the tumor — visible under the microscope as small clusters of cancer cells within vessel spaces.

LVI is an indicator that cancer cells have begun the process of entering the body’s transport systems, which could potentially carry them to other sites. Its presence is associated with:

• Higher likelihood of lymph node involvement
• Increased risk of distant recurrence
• May influence decisions about the extent of surgery, radiation field, and systemic therapy

The absence of LVI is generally a favorable finding. The presence of LVI does not automatically mean cancer has spread, but it is a factor your oncologist will weigh alongside other findings when recommending treatment.

Ki-67 Proliferation Index

Ki-67 is a protein present in cells that are actively dividing. The Ki-67 index measures what percentage of cancer cells are in an active state of division at the time of the biopsy — it is essentially a measure of how fast the tumor is growing.

Results are reported as a percentage:

• Low Ki-67 (below 15–20%) — slower-growing tumor; often associated with better prognosis in hormone receptor-positive cancers
• Intermediate Ki-67 (15–30%) — intermediate proliferation rate
• High Ki-67 (above 30%) — faster-growing tumor; may be associated with greater chemotherapy sensitivity but also higher recurrence risk

Ki-67 correlates strongly with tumor grade — Grade 3 tumors almost invariably have high Ki-67. ER-negative and triple-negative cancers also tend to have elevated Ki-67. The index is one factor considered when making decisions about the addition of chemotherapy, particularly in the context of genomic testing results.

Note: Ki-67 measurement has known variability between laboratories, and its cutoff thresholds are not universally standardized. It should be interpreted as one part of the overall picture, not as a standalone decision-making tool.

Genomic Testing — Beyond Standard Pathology

For patients with early-stage, hormone receptor-positive, HER2-negative invasive breast cancer, standard pathology findings alone may not fully predict whether chemotherapy will provide meaningful additional benefit beyond hormone therapy. Genomic tests analyze the activity of specific genes within the tumor tissue to provide additional prognostic and predictive information.

Oncotype DX Recurrence Score

Analyzes 21 genes and produces a Recurrence Score (RS) from 0 to 100:

• Low score (0–17) — low risk of recurrence; chemotherapy unlikely to provide benefit
• Intermediate score (18–30) — intermediate risk; decision individualized based on age, menopausal status, and clinical risk
• High score (31+) — higher risk; chemotherapy likely to provide meaningful benefit

Validated by the landmark TAILORx and RxPONDER clinical trials. Recommended by NCCN, ASCO, and ESMO for eligible patients.

MammaPrint

Analyzes 70 genes and classifies tumors as genomically low-risk or high-risk for distant recurrence. Validated by the MINDACT trial. Patients classified as clinical high-risk but genomic low-risk may safely forgo chemotherapy. FDA-cleared and recognized by NCCN guidelines.

Your oncologist will advise you whether genomic testing is appropriate for your specific diagnosis based on your pathology results, stage, and nodal status.

Staging — Putting It All Together

Staging combines the findings from your pathology report with clinical information from imaging studies to produce an overall assessment of how far your cancer has advanced. Staging drives treatment planning and provides a framework for discussing prognosis.

The standard system is the AJCC TNM system, where:

• T = Tumor size and local extent
• N = Lymph Node involvement
• M = Metastasis to distant organs

These factors combine to assign a stage. The 8th edition of the AJCC staging system (2018) also incorporates biological factors — tumor grade and biomarker status (ER, PR, HER2) — into the pathological stage, reflecting the importance of tumor biology alongside anatomy.

This table is a simplified overview. Modern staging is more granular — substages (IA, IB, IIA, IIB, IIIA, IIIB, IIIC) exist and incorporate additional biological factors. Your oncologist will discuss your specific stage with you.

In general, a lower stage correlates with better prognosis — but this is not absolute. Biological factors such as receptor status, grade, and genomic score can significantly modify the outlook within any given stage. A Stage II, Grade 1, ER+/PR+, HER2-negative cancer with a low Oncotype DX score has a very different outlook than a Stage II, Grade 3, triple-negative cancer.

A Sample Checklist — What to Find in Your Report

Use this checklist to ensure you have all the key information from your pathology report. If any of these items are missing, ask your oncologist whether they are applicable to your case and, if so, whether testing should be requested:

• ☐ Tumor type (histologic type — ductal, lobular, other)
• ☐ Invasive or non-invasive (in situ)
• ☐ Tumor size (in centimeters)
• ☐ Nottingham grade (1, 2, or 3) including nuclear grade, mitotic rate, and tubule formation
• ☐ Surgical margins status (clear, close, or positive)
• ☐ Lymph node status (number removed, number positive)
• ☐ Lymphovascular invasion (present or absent)
• ☐ Estrogen receptor (ER) status — percentage and intensity
• ☐ Progesterone receptor (PR) status — percentage and intensity
• ☐ HER2 status — IHC score and/or ISH result; HER2-low classification if applicable
• ☐ Ki-67 proliferation index (percentage)
• ☐ Genomic testing result (Oncotype DX Recurrence Score or MammaPrint) — if applicable
• ☐ Overall stage (TNM)