What is the recommended radiation therapy regimen for a middle-aged woman with T1/T2 N0 (tumor size/stage, lymph node status) Estrogen Receptor (ER)/Progesterone Receptor (PR)-positive, Human Epidermal growth factor Receptor 2 (HER2)-positive breast cancer, based on the Radcomp trial?

Radiation Therapy for T1/T2 N0 ER+/PR+/HER2+ Breast Cancer: RADCOMP Trial Context

Critical Note on the RADCOMP Trial

The RADCOMP trial is not referenced in the provided evidence base, and no specific radiation regimen from this trial can be recommended based on the available guidelines and research.

Standard Radiation Therapy Recommendations

After Breast-Conserving Surgery (Lumpectomy)

Whole breast irradiation is the standard of care following lumpectomy for T1/T2 N0 disease, with a dose of 50-50.4 Gy in 1.8-2.0 Gy fractions delivered over 5-6 weeks. 1

• Standard fractionation: 50 Gy in 25 fractions over 35 days is the established regimen 1
• Hypofractionated alternative: 42.5 Gy in 16 fractions over 22 days has been prospectively validated and provides comparable disease-free and overall survival in node-negative early-stage breast cancer 1
• Tumor bed boost: An additional 10-16 Gy boost to the tumor bed should be strongly considered, particularly in younger patients or those with high-grade disease, lymphovascular invasion, or close margins 1

Technical Delivery Requirements

• CT-based treatment planning is mandatory to minimize cardiac and pulmonary exposure 1
• Tissue wedging, forward planning with segments, or intensity-modulated radiation therapy (IMRT) are recommended techniques 1
• The radiation field should include most of the breast tissue 1

After Mastectomy

For T1/T2 N0 disease, post-mastectomy radiation therapy is considered optional and should be individualized based on additional risk factors. 1

• Chest wall irradiation: 50-50.4 Gy in 1.8-2.0 Gy fractions if indicated 1
• Regional nodal irradiation is not routinely recommended for N0 disease 1

Special Considerations for HER2-Positive Disease

Concurrent Therapy

• Endocrine therapy and trastuzumab can be administered concurrently with radiation therapy 1
• This allows for uninterrupted systemic therapy during the radiation treatment period 1

Radiation Field Decisions

• For patients receiving neoadjuvant chemotherapy, radiation therapy decisions must be based on pre-chemotherapy tumor characteristics (clinical stage), not post-treatment pathology 2, 3
• This principle applies even if pathologic complete response is achieved 2, 3

Accelerated Partial Breast Irradiation (APBI)

APBI is NOT recommended as standard therapy outside of clinical trials for this patient population. 1

• APBI may be considered only for highly selected patients: age ≥60 years, unifocal T1N0 ER-positive disease with negative margins 1
• Recent studies document inferior cosmetic outcomes compared to whole breast radiation 1
• Your patient's HER2-positive status and middle age make her a suboptimal candidate for APBI 1

Key Clinical Pitfalls to Avoid

• Never omit radiation after lumpectomy in HER2-positive disease: Even with excellent systemic therapy response, local control requires radiation 4
• Do not delay radiation excessively: Complete radiation within 6-8 weeks of surgery or completion of chemotherapy 1
• Monitor cardiac function: HER2-targeted therapy combined with radiation requires vigilant cardiac monitoring 2
• Ensure adequate margins: If margins are positive after lumpectomy, re-excision is mandatory before radiation; mastectomy is required if margins remain positive 1

Treatment Sequencing

1. Complete neoadjuvant chemotherapy with pertuzumab + trastuzumab + taxane if indicated 2
2. Perform surgery (lumpectomy with sentinel node biopsy or mastectomy) 2
3. Deliver radiation therapy to breast/chest wall as outlined above 1
4. Continue trastuzumab-based therapy to complete 1 year total 1, 2
5. Initiate aromatase inhibitor therapy (can be concurrent with radiation and trastuzumab) 1, 2