Medical Oncologist
Mutation & Targeted Treatment
HER2-Low Breast Cancer: — A New Category That Changes Treatment
HER2-low breast cancer is a category that did not exist in oncology a decade ago. It now opens access to a class of targeted treatment — antibody-drug conjugates — that was previously unavailable to most breast cancer patients.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed August 2026
- A newly defined category — HER2-low means your tumour shows a small amount of the HER2 protein — enough to matter for treatment, but less than HER2-positive.
- Previously called HER2-negative — Until recently, HER2-low tumours were grouped with HER2-negative and received no HER2-targeted therapy at all.
- A specific drug is now approved — Trastuzumab deruxtecan (T-DXd) is approved by the FDA and listed in NCCN and ASCO guidance for HER2-low metastatic breast cancer.
- Testing decides eligibility — Your pathology report — specifically the IHC score and ISH result — is the document that determines whether you qualify.
17+
Cancer Specialists
on Panel
on Panel
96.9%
Breast Cancer
Survival Rate*
Survival Rate*
15,000+
Patients
Treated
Treated
4.8★
Google Rating
(800+ reviews)
(800+ reviews)
HER2-low breast cancer means your tumour scores IHC 1+ or IHC 2+/ISH-negative — previously classified as HER2-negative. Trastuzumab deruxtecan (T-DXd), an antibody-drug conjugate, is now approved by the FDA and listed in NCCN and ASCO guidance specifically for HER2-low metastatic breast cancer.
What is HER2-low, and how is it different from HER2-positive?
HER2 is a protein on the surface of some cancer cells. It is measured on a scale: 0 (none detected), 1+, 2+, and 3+. HER2-positive means 3+, or 2+ confirmed by a second test called ISH.
HER2-low falls in the middle: IHC 1+, or IHC 2+ where the ISH test did not confirm amplification. These tumours were previously grouped with HER2-negative and were not eligible for any HER2-targeted drug.
That changed when trastuzumab deruxtecan (T-DXd) was developed. It is designed to reach tumours with even small amounts of HER2 on their surface. NCCN and ASCO guidelines added it to their recommendations for HER2-low metastatic breast cancer following data from the DESTINY-Breast04 trial.
What to check if your report says HER2-low
- Find the IHC score on your pathology report.It should say 0, 1+, 2+, or 3+. HER2-low is 1+, or 2+ with a negative ISH result alongside it.
- Ask whether ISH was done if your score is 2+.The ISH test is needed to confirm HER2-low versus HER2-positive. Without it, eligibility cannot be determined from a 2+ score alone.
- Check whether your cancer is metastatic or locally advanced.Current approvals for T-DXd are for metastatic or advanced HER2-low breast cancer. Early-stage HER2-low is being studied but is not yet an approved indication.
- Tell your oncologist your hormone receptor status.Whether your cancer is hormone receptor positive or negative affects which drugs are given alongside T-DXd and in what order.
- Ask if older biopsy results are still valid.HER2-low status can differ between the primary tumour and a metastatic site. Your team may recommend re-testing a more recent biopsy.
- Do not assume HER2-low and HER2-positive receive the same drugs.T-DXd is the approved agent for HER2-low. Classic HER2-positive therapies such as trastuzumab and pertuzumab are not indicated for HER2-low tumours.
Meet the Specialists
17+ senior cancer specialists. One panel for your case.
Trained at AIIMS, Tata Memorial, and leading international centres. Combined 150+ years of experience. Every complex case is reviewed by 3+ of them — together.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
Surgical Oncologist
Surgical Oncologist
Dr. Vinay Mamidala
MBBS, MS(General Surgery), M.Ch(Surgical Oncology), FMAS, FARIS(Ongoing)
Surgical Oncologist
Radiation Oncologist
Radiation Oncologist
Radiation Oncologist
Hematologist
Interventional Radiologist
Dr. Mohammed Imran
Surgical Oncologist
Dr. Vajja Sandeep Kumar
MBBS, MS (General Surgery), DrNB (Surgical Oncology), FALS Oncology
Surgical Oncologist
Want a specific doctor for your case? Mention them when booking.
Book Free ConsultationBook an appointment with our specialist
Share your name and number — we'll call you back within 30 minutes to schedule your consultation.
You do not have to work this out alone
A 45-minute consultation with a specialist who treats this every week.
What treatment is used for HER2-low metastatic breast cancer?
Trastuzumab deruxtecan — T-DXd — is an antibody-drug conjugate. The antibody portion seeks out cancer cells that carry HER2 on their surface and delivers a chemotherapy payload directly inside them. This is different from standard chemotherapy, which travels freely through the bloodstream.
T-DXd is given by intravenous infusion, usually as day-care treatment. NCCN guidance recommends it for patients with HER2-low metastatic breast cancer who have received prior chemotherapy.
One side effect to be aware of is interstitial lung disease (ILD), a form of lung inflammation. It is manageable when caught early. Report any new breathlessness or cough to your team the same day — not at your next scheduled appointment.
At CION, antibody-drug conjugates including T-DXd are administered as day care across our centres. Response-assessment scans are coordinated with partner imaging centres.
What do the medical terms on your pathology report and treatment plan mean?
- HER2-low
- A tumour that scores IHC 1+, or IHC 2+ with a negative ISH result. Eligible for trastuzumab deruxtecan (T-DXd). Not the same as HER2-positive, and treated differently.
- IHC (immunohistochemistry)
- A laboratory stain applied to tumour tissue that measures how much HER2 protein is present on the cell surface. Reported as 0, 1+, 2+, or 3+.
- ISH (in situ hybridisation)
- A second test that checks whether the HER2 gene itself is amplified. Done when IHC is 2+. A negative ISH result confirms HER2-low status.
- ADC (antibody-drug conjugate)
- A drug class that links a targeted antibody to a chemotherapy payload. The antibody finds cancer cells carrying the target protein and delivers the payload inside them. T-DXd is an ADC targeting HER2.
- Trastuzumab deruxtecan (T-DXd)
- The ADC approved by the FDA for HER2-low metastatic breast cancer. Listed in NCCN and ASCO guidance. Given by intravenous infusion as day-care treatment. Brand name: Enhertu.
- ILD (interstitial lung disease)
- Lung inflammation that can occur with T-DXd. Manageable when reported early. Report any new breathlessness, cough, or fever to your oncology team the same day.
Explore 119 more Targeted Therapy Basics, Testing & Your Cancer Type topics
Mutation & Target-Specific Pages
- ALK-Positive Lung Cancer: Drugs, Response and What Comes Next
- Antiangiogenic Drugs (VEGF Inhibitors): How Starving a Tumour Works
- BCR-ABL and the Philadelphia Chromosome in CML
- BRAF V600E Mutation: Targeted Therapy Across Multiple Cancers
- BRCA vs HRD Testing: Which One Determines PARP Inhibitor Eligibility?
- BRCA1 and BRCA2 Mutations in Cancer Treatment: PARP Inhibitors
- BTK Inhibitors in CLL and Lymphoma
- CD20, CD38 and Other Antibody Targets in Blood Cancers
- CDK4/6 Inhibitors in Hormone-Positive Breast Cancer
- Co-Mutations: When You Have Two Mutations at Once
- EGFR Exon 20 Insertion: Why It's Different and Harder to Treat
- EGFR Mutation in Lung Cancer: Complete Patient Guide
- ESR1 Mutation: Why Your Hormone Therapy Stopped Working
- FGFR Alterations in Bladder and Bile Duct Cancer
- FLT3 Mutation in AML: What It Means for Your Prognosis
- HER2 Mutation in Lung and Gastric Cancer
- HER2-Low Breast Cancer: A New Category That Changes Treatment
- HER2-Positive Breast Cancer: The Full Targeted Therapy Roadmap
- IDH1 and IDH2 Mutations in Glioma, AML and Bile Duct Cancer
- KIT and PDGFRA Mutations in GIST
- KRAS G12C Mutation: The Target That Was Undruggable for 40 Years
- KRAS Mutation but Not G12C: What Are My Options?
- MET Exon 14 Skipping and MET Amplification Explained
- MSI-High and Mismatch Repair Deficiency: Testing and Treatment
- NRG1, MET, RET and Other Rare Fusions: Where to Get Tested
- NTRK Fusion: One Drug for Many Different Cancers
- Osimertinib: What to Expect, Side Effects and How Long It Works
- PALB2, ATM and Other BRCA-Like Genes: Do They Get the Same Drugs?
- PIK3CA Mutation in Breast Cancer: Alpelisib and What to Expect
- PSMA and AR-Targeted Therapy in Prostate Cancer
- RET Fusion Cancer: Lung, Thyroid and Beyond
- ROS1-Positive Cancer: A Rare Mutation with Excellent Options
- TP53 Mutation: Why There's Still No Targeted Drug for It
- Understanding Variant Allele Frequency (VAF) in Your Report
- What If My Mutation Has No Approved Drug Yet?
- mTOR Inhibitors in Cancer: Everolimus and Temsirolimus
Before You Start - Preparation & Baseline
- 25 Questions to Ask Your Oncologist Before Starting Targeted Therapy
- Before You Start Targeted Therapy: The Complete Preparation Checklist
- Dental Check-Up Before Cancer Treatment: Why It Matters
- Financial Planning Before Starting Long-Term Targeted Therapy
- How Long After Surgery Can You Start Targeted Therapy?
- How to Store Your Targeted Therapy Tablets Correctly
- Setting Up a Dosing Routine You Won't Forget
- Should Targeted Therapy Be Taken With Food or on an Empty Stomach?
- Vaccinations Before and During Cancer Treatment
- What to Expect in Your First Month on Targeted Therapy
- Which Baseline Tests Are Done Before Starting a TKI?
- Which of Your Existing Medicines Must Stop Before Targeted Therapy
- Why You Need an ECHO and ECG Before Certain Cancer Drugs
Biomarker & Molecular Testing
- Can Mutation Testing Be Done on an Old Biopsy Block or Slides?
- Cost of NGS and Mutation Testing in India: 2026 Price Guide
- Do I Need Genetic Testing Before Starting Targeted Therapy?
- Germline vs Somatic Testing: The Difference Nobody Explains Properly
- Getting a Second Opinion on Your Genomic Report
- How Long Do Mutation Test Results Take in India? Realistic Timelines
- How to Read Your Molecular Pathology Report, Line by Line
- IHC, FISH, PCR or NGS: Which Mutation Test Is Right for Your Cancer?
- Is Mutation Testing Worth It If I Can't Afford the Drug?
- Liquid Biopsy vs Tissue Biopsy: Which Test Do You Need?
- My Report Says a Mutation Is 'Actionable': What Does That Mean?
- No Mutation Found in My Report: What Are My Options Now?
- Should I Start Chemotherapy While Waiting for Mutation Results?
- Should You Repeat Molecular Testing After Your Cancer Progresses?
- Single-Gene Test or Full Panel? How to Choose Without Wasting Money
- What Are PD-L1, TMB and MSI, and Do They Affect Targeted Therapy?
- What Does 'Variant of Uncertain Significance' (VUS) Mean?
- What Happens If There Isn't Enough Tissue for Mutation Testing?
- What Is NGS (Next-Generation Sequencing) Testing in Cancer?
- What Is a Molecular Tumour Board and Why Should You Want One?
- Which Cancers Should Always Be Tested for Mutations? A Checklist
- ctDNA and MRD Testing: Tracking Cancer Through a Blood Test
Cancer-Type Specific Targeted Therapy
- CML Treatment with TKIs: Living a Normal Life Span
- Cancer of Unknown Primary: Can Molecular Testing Help?
- GIST Treatment: Why This Cancer Changed Targeted Therapy Forever
- Targeted Therapy for Bile Duct Cancer (Cholangiocarcinoma)
- Targeted Therapy for Bladder and Urothelial Cancer
- Targeted Therapy for Brain Tumours and Glioma
- Targeted Therapy for Breast Cancer: HER2, HR+ and Triple Negative
- Targeted Therapy for CLL and Lymphoma
- Targeted Therapy for Cervical and Endometrial Cancer
- Targeted Therapy for Childhood Cancers
- Targeted Therapy for Colorectal Cancer: RAS, BRAF and HER2
- Targeted Therapy for Head and Neck Cancer
- Targeted Therapy for Kidney Cancer (RCC)
- Targeted Therapy for Liver Cancer (HCC)
- Targeted Therapy for Lung Cancer: Every Mutation and Drug Explained
- Targeted Therapy for Melanoma: BRAF and MEK Inhibitors
- Targeted Therapy for Multiple Myeloma
- Targeted Therapy for Neuroendocrine Tumours
- Targeted Therapy for Ovarian Cancer: PARP Inhibitors and Beyond
- Targeted Therapy for Pancreatic Cancer: Limited but Real Options
- Targeted Therapy for Prostate Cancer
- Targeted Therapy for Sarcoma
- Targeted Therapy for Stomach and Gastroesophageal Cancer
- Targeted Therapy for Thyroid Cancer
- Targeted Therapy in AML: Beyond Standard Chemotherapy
- Tumour-Agnostic Drugs: When the Mutation Matters More Than the Organ
Foundations & How Targeted Therapy Works
- Adjuvant and Neoadjuvant Targeted Therapy: Treatment Before or After Surgery
- Antibody-Drug Conjugates (ADCs): The Smart Bombs of Cancer Treatment
- Biosimilars and Generic Targeted Therapy Drugs: What They Are
- Can Targeted Therapy Be Combined with Chemotherapy or Radiation?
- Can Targeted Therapy Cure Cancer? An Honest Answer
- First-Line, Second-Line and Beyond: How Treatment Lines Work
- How Doctors Decide Between Chemo, Immunotherapy and Targeted Therapy
- How Does Targeted Therapy Actually Work? Explained Without Jargon
- How Long Does Targeted Therapy Take to Start Working?
- Is Targeted Therapy Painful? What the Treatment Actually Feels Like
- Is Targeted Therapy a Type of Chemotherapy? Clearing Up the Confusion
- Oral Tablets vs IV Infusion Targeted Therapy: What's the Difference?
- Precision Oncology vs Personalised Medicine: Are They the Same Thing?
- Targeted Therapy Success Rates: What the Numbers Really Mean
- Targeted Therapy for Stage 4 Cancer: What You Can Realistically Expect
- Targeted Therapy vs Chemotherapy: 12 Differences That Actually Matter
- Targeted Therapy vs Immunotherapy: Which One Is Right for You?
- The 8 Main Types of Targeted Therapy Drugs (With Examples)
- What Do Cancer Drug Endings Like -nib, -mab and -tinib Mean?
- What Is Targeted Therapy for Cancer? A Complete Patient Guide
- Which Cancers Can Be Treated with Targeted Therapy? Full List
- Why Didn't My Doctor Recommend Targeted Therapy for Me?
Next step
Still not sure what applies to you?
Send your reports across and a senior medical oncologist will go through what they mean, what is known, and what the options actually are.
Common questions
Frequently asked questions
Is HER2-low the same as HER2-positive?
No — they are separate categories with different treatments. HER2-positive means IHC 3+, or IHC 2+ with a positive ISH test. Those patients receive drugs such as trastuzumab and pertuzumab. HER2-low is IHC 1+, or IHC 2+ with a negative ISH, and the relevant drug is trastuzumab deruxtecan (T-DXd). The two groups do not share the same treatment pathway, so confirming exactly which category you are in is the first step.
What does IHC 1+ or 2+ mean on my pathology report?
IHC measures how much HER2 protein is present on the surface of your cancer cells. A score of 1+ means a small amount is detectable. A score of 2+ is borderline — a second test called ISH is then done to check whether the HER2 gene is amplified. If ISH comes back negative, the combined result is HER2-low. If ISH is positive, the result is HER2-positive. Ask your oncologist to walk you through both numbers together, because one without the other does not give the full picture.
My cancer is hormone receptor positive — does HER2-low still apply to me?
Yes. HER2-low status is a separate question from hormone receptor (ER/PR) status, and both can be positive at the same time. NCCN and ASCO guidance covers HER2-low patients regardless of hormone receptor status. In practice, your hormone receptor result affects the sequence of treatments — which drugs come before or alongside T-DXd — and your oncologist will plan that sequence based on both results together.
What are the side effects of trastuzumab deruxtecan?
Common side effects include nausea, fatigue, hair thinning, and low blood counts. The most important side effect to know about is interstitial lung disease (ILD) — lung inflammation that can become serious if not caught early. Your team will monitor for it throughout treatment. Report any new or worsening breathlessness, cough, or fever the same day you notice it, rather than waiting for your next appointment. Your oncologist will explain the full side-effect profile before treatment starts.
Does HER2-low mean my cancer is more aggressive?
No. HER2-low is a classification that describes how much HER2 protein is on the tumour surface — it tells your oncologist which treatment may help, not how the cancer will behave. Aggressiveness depends on other factors such as tumour grade, hormone receptor status, and Ki-67. The significance of a HER2-low result is that it opens access to T-DXd, which was not available to these patients before. It is not a marker of a more dangerous cancer.
Should HER2-low testing be done at diagnosis, or only when cancer spreads?
HER2 testing including the IHC score is part of standard pathology at diagnosis. The HER2-low category becomes clinically important when breast cancer is metastatic, because that is where T-DXd is currently approved. If your original test was done some years ago, ask whether the 1+ or 2+ distinction was recorded — some older reports noted only 'negative' without preserving the exact score. Re-testing on a more recent biopsy may also be worth discussing, because HER2 expression can differ between the original tumour and a metastatic site.