What are the recommended dose constraints for the left anterior descending (LAD) coronary artery in a patient with left‑sided breast cancer receiving adjuvant radiotherapy?

LAD Artery Dose Constraints for Left-Sided Breast Cancer Radiotherapy

Keep the LAD mean dose ≤10 Gy and maximum dose as low as reasonably achievable, ideally <20 Gy, using modern cardiac-sparing techniques during adjuvant radiotherapy for left-sided breast cancer.

Rationale for LAD Dose Minimization

The LAD is the most exposed coronary artery during left breast irradiation and represents a critical target for dose reduction 1. Higher radiation doses to the LAD are directly associated with increased risk of major coronary events, with this risk beginning within the first 5 years after treatment and continuing for decades 1. The ESC position paper identifies radiation-induced coronary artery disease as causing a 2-7 fold increased relative risk of myocardial infarction, with ostial lesions being particularly frequent and potentially life-threatening 1.

Specific Dose Constraints

Primary Constraints

• LAD mean dose: Aim for ≤2-3 Gy when possible, with an upper limit of 10 Gy 2, 3, 4
• LAD maximum dose: Target <10 Gy, with acceptable range up to 20 Gy depending on clinical scenario 3, 5, 4
• LAD V20 (volume receiving ≥20 Gy): Minimize to <50% of LAD volume 5

Supporting Cardiac Constraints

• Mean heart dose: Keep ≤1 Gy using modern techniques, with upper acceptable limit of 3-4 Gy 1, 2, 6
• Heart V25: Minimize volume receiving ≥25 Gy 3

Techniques to Achieve LAD Dose Reduction

Mandatory Planning Approaches

• Prospective LAD contouring: Always contour the LAD before treatment planning, as this reduces mean LAD dose by approximately 1 Gy and maximum dose by 3-5 Gy compared to retrospective contouring 4
• Deep inspiration breath-hold (DIBH): Reduces LAD mean dose from ~18 Gy to ~6 Gy by increasing distance between chest wall and heart 6
• CT-based simulation: Required to visualize cardiac structures and minimize cardiac irradiation 1, 7

Field Design Specifications

• Tangential field placement: Position medial field edges at least 2.5 mm from the contoured LAD 2
• Minimize lung in field: Limit to 3.0-3.5 cm of lung projected on beam radiograph to reduce cardiac exposure 1, 7, 8
• High-energy photons: Use ≥10 MV photons for large-breasted patients to improve dose homogeneity and reduce cardiac dose 7, 8, 9

Position Considerations

• Prone positioning: May achieve lower mean heart doses (0.69 Gy) but can paradoxically increase LAD exposure (mean 33.5 Gy prone vs 25.6 Gy supine for left-sided treatment) 2, 5
• Supine with DIBH: Generally preferred for left-sided breast cancer as it reduces both heart and LAD doses while maintaining target coverage 6

Clinical Context and Dosimetric Data

Recent dosimetric studies demonstrate achievable LAD doses with modern techniques 2, 3:

• Prone hypofractionated WBRT: LAD mean 2.20 Gy, LAD max 4.44 Gy 2
• Supine conventional fractionation: LAD mean 18.1 Gy without DIBH, reduced to 6.4 Gy with DIBH 6
• Standard tangential fields: LAD mean 24 Gy (range 8-34 Gy) without cardiac-sparing techniques 3

Critical Pitfalls to Avoid

Common Errors

• Failing to contour LAD prospectively: This single omission can increase LAD dose by 30-50% 4
• Using prone position without LAD evaluation: Prone positioning may worsen LAD exposure despite improving lung sparing 5
• Accepting high LAD doses for internal mammary coverage: When treating internal mammary nodes, LAD max doses can reach 20 Gy; strict optimization is required to maintain LAD constraints while achieving nodal coverage 4

Technical Considerations

• No bolus use: Bolus should not be applied during whole-breast irradiation as it increases superficial dose without benefit 7, 8
• Avoid cardiac overlap: For left-sided lesions, minimize heart volume in tangential fields through careful field design 1, 7

Long-Term Surveillance Implications

Given that radiation-induced CAD typically manifests 5-10 years after treatment 1, achieving these dose constraints is critical for reducing long-term cardiac mortality. The ESC recommends screening for cardiac disease starting 10-15 years after chest irradiation and continuing lifelong 1. Patients with cardiovascular risk factors (hypertension, diabetes, dyslipidemia, obesity) have significantly magnified risk of radiation-induced cardiac complications, making strict LAD dose constraints even more critical in these populations 1.