For radiotherapy planning, which intercostal levels should be contoured for the internal mammary node (IMN) chain when nodes are positive versus negative, and what margins should be added to the clinical target volume (CTV) to generate the planning target volume (PTV)?

Internal Mammary Node Contouring Guidelines for Breast Cancer Radiotherapy

For positive internal mammary nodes, contour the first 3 intercostal spaces as a minimum, with consideration for extending cranially to the confluence of the internal mammary vein with the brachiocephalic vein and caudally to the 4th-5th intercostal spaces; for negative nodes requiring prophylactic treatment, contour the first 3 intercostal spaces using a 7mm expansion around the internal mammary vessels, with standard PTV margins of 5mm. 1, 2

Intercostal Levels to Contour

When Internal Mammary Nodes Are Positive (Clinically or Pathologically)

• Contour the first 3 intercostal spaces as the mandatory minimum target volume, as 78% of internal mammary node metastases occur within these levels 2
• Consider cranial extension to the confluence of the internal mammary vein with the brachiocephalic vein when known involvement exists, as 8% of metastases occur cranial to the first intercostal space 2
• Consider caudal extension to the 4th or 5th intercostal spaces in patients with known involvement, as 14% of metastases are located below the third intercostal space, provided normal tissue constraints are met 2
• The distribution of positive nodes follows this pattern: 40.4% in the first intercostal space, 30.2% in the second, 24.3% in the third, 4.4% in the fourth, and 0.7% in the fifth 3

When Internal Mammary Nodes Are Negative (Prophylactic Treatment)

• Contour the first 3 intercostal spaces for elective nodal irradiation, as this captures the vast majority of potential microscopic disease 1, 2
• This approach is supported by the MA.20 trial, which demonstrated improved disease-free survival (HR 0.68, p=0.003) with regional nodal irradiation including internal mammary nodes in patients with 1-3 positive axillary nodes 1
• Strong consideration for prophylactic treatment should be given to patients with positive axillary lymph nodes, especially with medial or central tumor location 1

Clinical Target Volume (CTV) Margins

For Positive Nodes

• Generate the CTV by expanding the internal mammary vessels by 7mm in all directions 1
• This 7mm expansion is insufficient for comprehensive coverage; for 90% coverage of known metastases, an additional expansion is needed 3, 2
• For optimal coverage of gross disease, use a 4mm medial and lateral expansion from the internal mammary vessels, which encompasses 90% of lymph nodes within the first 3 intercostal spaces 2
• The mean distance of metastases from vessels is 2.2mm medially (SD 2.9mm) and 3.6mm laterally (SD 2.5mm) 2

For Negative Nodes (Prophylactic)

• Use a 7mm expansion around the internal mammary vessels to generate the CTV 1
• This standard expansion is based on consensus guidelines but may require adjustment based on individual anatomy 1
• The DBCG (Danish Breast Cancer Cooperative Group) guideline provides better coverage than RTOG guidelines, with 60.3% versus 18.4% of sentinel node central points included 3
• To achieve 90% coverage of potential nodal sites, the DBCG-based CTV requires a 5mm expansion, while RTOG-based CTV requires an 8mm expansion 3

Planning Target Volume (PTV) Margins

• Add a standard 5mm margin from CTV to PTV to account for setup uncertainty and organ motion 1
• CT-based treatment planning is mandatory to minimize cardiac and pulmonary toxicity 1, 4
• Modern planning techniques should prioritize heart dose reduction through careful field design and dose constraints 1

Dose Prescription

• Deliver 50-50.4 Gy in 1.8-2.0 Gy fractions to the internal mammary node CTV, matching the dose to other regional nodal basins 4, 1
• An alternative hypofractionated regimen of 42.5 Gy in 2.55 Gy fractions is acceptable 4, 1
• All dose schedules are given 5 days per week 4

Critical Technical Considerations

Target Delineation Technique

• Use the internal mammary artery and vein location as a surrogate for nodal locations, which are usually not visible on imaging 4
• CT-based treatment planning is the gold standard for target delineation 4, 1
• The location of internal mammary nodes is variable from patient to patient, making individualized imaging-based planning essential 5

Common Pitfalls to Avoid

• Do not rely solely on RTOG consensus guidelines without expansion, as only 18.4% of sentinel node central points are included in the standard RTOG CTV 3
• Avoid using tangential fields with arbitrary medial borders, as 17% of patients have at least one internal mammary node not included within tangential fields positioned 3cm across midline 5
• Do not omit the first intercostal space, as it contains 40.4% of internal mammary sentinel nodes 3
• Ensure adequate coverage while respecting cardiac constraints, as the benefit of treatment must be balanced against potential cardiac toxicity 4

Integration with Other Regional Nodal Treatment

• When internal mammary nodes are treated, include the infraclavicular and supraclavicular regions if concurrent axillary disease is present 1
• Regional nodal irradiation should include both the internal mammary nodes and supraclavicular-axillary apical nodes in addition to the chest wall when postmastectomy radiotherapy is used for patients with positive axillary lymph nodes 4
• The minimum mandatory target volumes for postmastectomy radiotherapy are the chest wall and supraclavicular-axillary apical nodes, with controversy remaining over when internal mammary nodes should be deliberately included 4

Evidence Quality and Controversies

• There is substantial disagreement among experts regarding routine internal mammary node irradiation, representing one of the most contentious areas in breast cancer radiotherapy 1
• The EORTC and Danish studies showed 1-5% reductions in relapse rates and improvements in overall survival with regional nodal irradiation including internal mammary nodes 4
• Arguments supporting treatment cite the MA.20 trial's disease-free survival improvement and inclusion of internal mammary nodes in trials demonstrating survival benefit 1