Breast cancer is the most common invasive cancer in women and the second leading cause of cancer death in women after lung cancer. In the United States, breast cancer accounts for 30% of all new cancer diagnoses in women. An estimated 281,550 new cases of invasive breast cancer are expected to be diagnosed in women in the US in 2022. Understanding which breast is most often affected by cancer can help women be aware of changes and get screened appropriately.
Quick Facts
- Breast cancer can occur in either breast, but is slightly more common in the left breast.
- Some studies have found a 3-6% higher incidence of breast cancer in the left breast compared to the right.
- Reasons for the higher left breast cancer rates are not fully understood, but may relate to breast size, handedness, or lymphatic drainage patterns.
- All women should be aware of changes in either breast and get regular mammograms as recommended by their doctor.
Left Breast Cancer More Common
Several large studies show that breast cancer occurs slightly more frequently in the left breast compared to the right. A 2014 analysis of over 1.5 million breast cancer cases found that 53% occurred in the left breast, while 47% were in the right. A similar study in China examined over 200,000 cases of breast cancer and found 52% were left-sided and 48% right-sided.
The higher incidence of cancer in the left breast has been a consistent finding, but the exact reasons for this imbalance are still uncertain. Some key theories include:
- On average, the left breast is slightly larger, which means more cells at risk for mutation.
- Most people are right-handed, so the left breast receives less mechanical protection from the right hand and arm.
- The main lymphatic drainage from the left breast is into the thoracic duct, while the right breast drains to the right lymphatic system. Different drainage patterns could influence metastasis.
However, more research is still needed to determine if any of these factors definitively contribute to the left-sided predominance of breast cancer.
Small But Significant Difference
Although breast cancer is consistently more common on the left side, the difference in rates between the two breasts is relatively small. Most studies have found only a 3-6% higher incidence of cancer in the left vs right breast.
This means that while statistically significant, the predominance of left breast cancer is not overwhelmingly lopsided. Breast cancer remains a substantial risk in both breasts. All women should continue standard breast cancer screening and be vigilant about changes in either breast.
Percentage Difference in Left vs Right Breast Cancer
| Study | Left Breast Cancer | Right Breast Cancer | Percentage Difference |
|---|---|---|---|
| SEER Database | 53% | 47% | 6% higher in left |
| Chinese Database | 52% | 48% | 4% higher in left |
| MD Anderson Cancer Center | 51.6% | 48.4% | 3.2% higher in left |
As seen in major studies, the left breast cancer rate exceeds the right, but only by a few percentage points in most cases. The higher left breast cancer risk is considered statistically significant due to large sample sizes, but the difference remains small.
Awareness for Both Breasts Essential
Although there is a slightly elevated risk of developing cancer in the left breast, the chance of right breast cancer remains substantial. All women should maintain awareness about potential changes in either breast:
- Conduct regular self-exams on both breasts and note any lumps, skin changes, nipple discharge, or pain
- Get mammograms as recommended by your physician, usually starting yearly at age 40
- Make sure your clinician examines both breasts thoroughly during annual well-woman visits
- Learn your own breast tissue and report any concerning changes promptly
Being vigilant about screening and symptoms is crucial for early detection. Breast cancers detected at an early stage when they are small and localized have a 99% five-year relative survival rate. Staying on top of breast health is key, whether you are at average or increased risk.
Risk Factors Affect Both Breasts
Known risk factors for breast cancer apply equally to both the left and right breasts. These include:
- Increasing age
- Reproductive history – Early period, late menopause, never pregnant
- Family history of breast cancer
- Breast density on mammogram
- Inherited genetic mutations – BRCA1, BRCA2
- Previous chest radiation treatment
Having one or more risk factors does not mean breast cancer is inevitable, but it does warrant extra vigilance. Speak to your doctor about tailoring your screening approach if your risk is increased.
When to Seek Evaluation
Being familiar with your breasts is key to noticing any abnormalities and seeking prompt evaluation. You should contact your physician if you notice any of the following in either breast:
- A new lump or mass
- Thickening or swelling
- Changes to the skin – Redness, warmth, dimpling, puckering
- Nipple changes – Turning inward, scaliness, rash
- Nipple discharge (particularly bloody discharge)
- Constant pain or tenderness
While most breast changes are non-cancerous like cysts or fibroadenomas, it is essential to see your doctor for evaluation of any persistent findings. Catching cancer early when it is small and local provides the best prognosis.
Screening Recommendations
Guidelines for breast cancer screening aim to detect cancer in its earliest stages before symptoms develop. Recommendations from major health organizations include:
- Mammograms every 1-2 years starting at age 40-45. Yearly mammograms after age 55.
- Clinical breast exams by a doctor every 1-3 years starting at age 25-40.
- Breast self-exams for awareness, starting in your 20s.
- Consider MRI screening for women at high risk.
These approaches facilitate early detection for women of average risk. But screening needs can vary for women at increased risk – make sure to discuss a tailored plan with your doctor.
Breast Cancer Screening Guidelines By Age
| Age Group | Recommended Screening |
|---|---|
| Ages 20-39 | Clinical breast exam every 1-3 years Breast self-exam awareness |
| Ages 40-44 | Mammogram every 1-2 years Clinical breast exam every year |
| Ages 45-54 | Mammogram every year Clinical breast exam every year |
| Age 55+ | Mammogram every year Clinical breast exam every year |
Following screening guidelines facilitates finding breast cancers when they are small and most treatable. Discuss your personal screening plan with a clinician.
Treatment Targets Both Breasts
Breast cancer treatment depends on the specific type and stage of cancer, but often includes:
- Surgery – Lumpectomy or mastectomy
- Radiation therapy
- Chemotherapy
- Hormone therapy
- Targeted therapy
Even when cancer is limited to one breast, treatment often impacts both sides. For example, radiation is often given to the affected breast and adjacent lymph node areas.
Additionally, preventive mastectomy of the unaffected breast may be recommended for some women at very high genetic risk. Discuss all treatment options thoroughly with your multi-disciplinary medical team.
Prognosis Depends on Specific Diagnosis
Breast cancer prognosis varies substantially based on the specific details of your cancer diagnosis, including:
- Cancer type – Such as ductal, lobular, triple-negative
- Stage – Early vs late stage
- Tumor grade – Well vs poorly differentiated
- Lymph node involvement – Yes or no
- Hormone receptor status – ER/PR positive vs negative
In general, being diagnosed at an early stage with a lower grade, hormone receptor positive cancer carries the most favorable prognosis. Prognosis is not specifically affected by whether the cancer is in the left or right breast.
5-Year Relative Breast Cancer Survival Rates By Stage
| Stage | 5-Year Survival Rate |
|---|---|
| Stage 0 | 100% |
| Stage 1 | 100% |
| Stage 2 | 93% |
| Stage 3 | 72% |
| Stage 4 | 27% |
Being diagnosed at an early stage has a significant impact on outlook. This underscores the importance of early detection through regular screening.
Prevention Involves Both Breasts
All women can take steps to lower their breast cancer risk through lifestyle approaches like:
- Maintaining a healthy weight
- Engaging in regular physical activity
- Minimizing alcohol intake
- Breastfeeding if possible
Preventive medications like tamoxifen or raloxifene may be options for some women at increased risk. Preventive mastectomy may also be considered for those at very high genetic risk.
Risk-reducing approaches affect both breasts equally. Discuss your personal risk profile with a doctor to develop an appropriate prevention plan.
Conclusion
In summary, breast cancer occurs slightly more frequently in the left breast compared to the right. However, the difference is relatively small at just a few percentage points in most research. All women should remain vigilant about cancer risk in both breasts.
Being aware of changes, getting regular age-appropriate screening, and knowing your risk factors are crucial for early detection when breast cancer is most treatable. If you notice any persistent breast changes, schedule an evaluation promptly, whether it is your left or right side.