What is the role of adjuvant radiotherapy (RT) in a patient with T1 breast cancer who has undergone a partial mastectomy with no lymph node involvement?

Adjuvant Radiotherapy for T1 Node-Negative Breast Cancer After Partial Mastectomy

For most patients with T1 node-negative breast cancer after partial mastectomy (breast-conserving surgery), adjuvant whole breast radiotherapy is standard of care and should be administered, as it significantly reduces local recurrence rates and improves breast cancer-specific survival. 1

Standard Recommendation

• Radiotherapy after breast-conserving surgery is systematically required for infiltrating carcinoma, regardless of tumor characteristics, because it decreases local recurrence rates and thereby reduces breast cancer-specific mortality. 2
• Guideline-adherent adjuvant RT is associated with significantly improved recurrence-free survival (p<0.001) and overall survival (p<0.001) compared to patients who do not receive RT. 3
• The reduction in local recurrence translates to improved survival outcomes, with RT reducing the 10-year incidence of locoregional recurrence by approximately 8% in patients receiving tamoxifen. 1

Exception: Older Women with Favorable Biology

RT may be omitted in a highly selected subset of older women meeting ALL of the following criteria: 1

• Age ≥70 years 1
• ER-positive tumor 1
• Clinically node-negative (T1) 1
• Will receive adjuvant endocrine therapy (tamoxifen or aromatase inhibitor) 1
• This is a Category 1 recommendation from NCCN 1

Evidence Supporting Omission in Select Elderly Patients

• In the CALGB 9343 trial of women aged ≥70 years with clinical stage I, node-negative, ER-positive breast cancer receiving tamoxifen, RT reduced 10-year locoregional recurrence from 10% to 2% (HR 0.18, p<0.001), but showed no differences in overall survival, disease-free survival, or need for mastectomy. 1
• The PRIME II study demonstrated similar findings in women aged ≥65 years with node-negative breast cancers <3 cm: ipsilateral breast tumor recurrence was lower with RT (1.3% vs 4.1%), but no differences in overall survival, regional recurrence, or distant metastases were observed. 1

Critical Caveats

Do NOT omit RT if any of the following apply, even in elderly patients: 1

• Age <70 years - younger patients derive greater absolute benefit from RT 1
• ER-negative tumors - these patients were excluded from trials showing safety of RT omission 1
• Patient declines or is unable to take endocrine therapy - RT omission is only safe when combined with hormonal treatment 1
• High-risk features present: close margins, lymphovascular invasion, high grade, or triple-negative biology 1

Radiation Technique When Indicated

• Whole breast radiotherapy (WBRT) is the standard approach with typical doses of 45-50 Gy in 25-28 fractions or hypofractionated regimens of 40-42.5 Gy in 15-16 fractions. 1, 2
• Hypofractionated schedules (2.5-2.67 Gy per fraction over 15-16 fractions) show similar effectiveness and comparable side effects to conventional fractionation (Category 1 evidence). 1
• Boost to the tumor bed may be considered for younger patients or those with higher-risk features, though data are limited for T1 tumors. 1
• Accelerated partial breast irradiation (APBI) may be considered in select patients meeting strict criteria (age ≥50 years, unifocal, node-negative, non-lobular, ≤3 cm, negative margins), but long-term data are still maturing. 1

Algorithm for Decision-Making

Step 1: Confirm pathology shows T1 (≤2 cm), node-negative, invasive breast cancer after partial mastectomy with negative margins. 1

Step 2: Assess patient age:

• If age <70 years → Recommend adjuvant RT (standard of care) 1
• If age ≥70 years → Proceed to Step 3 1

Step 3: For patients ≥70 years, verify ALL favorable criteria:

• ER-positive tumor 1
• Clinically node-negative 1
• Patient will comply with endocrine therapy 1
• No high-risk features (close margins, LVSI, high grade) 1

Step 4: If ALL criteria in Step 3 are met:

• Discuss RT omission as an option with patient, explaining 2-3% absolute increase in local recurrence risk over 10 years but no impact on survival 1
• If ANY criterion is not met → Recommend adjuvant RT 1

Common Pitfalls to Avoid

• Do not extrapolate the elderly omission data to younger patients - the trials specifically enrolled women ≥65-70 years, and younger patients have longer life expectancy with greater cumulative recurrence risk. 1
• Do not omit RT based solely on small tumor size - even T1a-b tumors benefit from RT in standard-risk patients under age 70. 2, 3
• Ensure adequate axillary staging was performed - clinical node-negative status must be confirmed pathologically with sentinel lymph node biopsy. 4
• Verify ER status is truly positive - ER-negative tumors were excluded from omission trials and should receive RT regardless of age. 1