What are the dose volume constraints for organs at risk (OARs), including the heart, lungs, and spinal cord, in a patient undergoing breast cancer radiation therapy?

Dose Volume Constraints for Organs at Risk in Breast Cancer Radiotherapy

For breast cancer radiotherapy, the most critical OAR constraints are: lung exposure limited to 3-3.5 cm maximum on radiograph with V20 <40%, heart dose minimized with mean dose <2.5 Gy for left-sided lesions, and spinal cord maximum dose <50 Gy for conventional fractionation. 1

Lung Dose Constraints

The lung constraints are well-established across multiple NCCN guidelines and should be strictly adhered to:

• Maximum lung tissue in treatment field: 3-3.5 cm as projected on radiograph at isocenter 2, 1
• Minimum lung tissue required: 1-1.5 cm 2, 1
• V20 (volume receiving ≥20 Gy): <40% for both lungs combined 1
• Mean lung dose: ≤20 Gy 1
• Preferred mean lung dose with advanced techniques: <8.5 Gy 1

These constraints are designed to minimize the risk of radiation pneumonitis, which can be a serious complication. 2 When using regional nodal irradiation, the ipsilateral lung V20 should be kept below 35% when possible. 3

Heart Dose Constraints

For left-sided breast lesions, cardiac sparing is paramount given the long-term survival of most breast cancer patients:

• Minimize heart volume in tangential fields to the greatest extent possible 2, 1
• Volume-based constraints: 60 Gy to <1/3 of heart volume, 45 Gy to <2/3 of heart volume, 40 Gy to <100% of heart volume 1
• Mean heart dose: <2.5 Gy (strongly preferred for left-sided lesions) 4
• For younger long-term survivors: total heart dose limited to 30 Gy 1
• When using adaptive treatment planning algorithms: mean heart dose ≤500 cGy (5 Gy) 3

The emphasis on cardiac constraints reflects the recognition that even modest cardiac doses can increase late cardiac mortality in long-term survivors. 4 Modern techniques including IMRT, deep inspiration breath-hold (DIBH), and prone positioning can significantly reduce cardiac exposure compared to conventional techniques. 2

Spinal Cord Dose Constraints

Spinal cord tolerance is critical when treating regional nodes:

• Maximum spinal cord dose: 50 Gy for once-daily conventional fractionation (including scatter) 1
• Maximum spinal cord dose: 41 Gy for twice-daily accelerated hyperfractionation 1

These constraints apply primarily when treating supraclavicular and axillary nodal regions. 2

CT-Based Treatment Planning Requirements

CT-based treatment planning is strongly encouraged (essentially mandatory in modern practice) to accurately identify and minimize exposure to OARs. 2, 1, 4

This allows for:

• Precise delineation of lung and heart volumes 2, 4
• Evaluation of dose distribution to critical structures 2
• Optimization of treatment techniques to meet dose constraints 2

Treatment Delivery Specifications

Standard delivery parameters that impact OAR exposure:

• Bolus should NOT be used for standard whole breast treatment 1, 4
• High-energy photons (≥10 MV) indicated for large-breasted women to improve dose homogeneity 1
• Weekly imaging for setup verification 4
• Daily imaging NOT routinely recommended 4

Common Pitfalls and Caveats

Critical considerations to avoid OAR complications:

• Avoid field overlap between adjacent treatment areas to prevent hot spots 4
• For left-sided lesions, cardiac irradiation techniques must be minimized due to known increases in late cardiac mortality 4
• When treating internal mammary nodes, CT planning is mandatory to evaluate heart and lung dose 2
• Respiratory control techniques (DIBH, prone positioning) should be considered to further reduce OAR doses, particularly for cardiac structures 2

When 3D conformal techniques cannot meet OAR constraints, transition to IMRT is clinically indicated without compromising locoregional control. 3 This adaptive approach maintains the therapeutic ratio by preserving cancer control outcomes while meeting critical OAR constraints. 3