What parameters are important for evaluating an adjuvant radiotherapy plan for breast cancer?

Evaluation Parameters for Adjuvant Radiotherapy Plans in Breast Cancer

When evaluating an adjuvant radiotherapy plan for breast cancer, prioritize assessment of target volume coverage, dose homogeneity, and organs-at-risk (OAR) constraints, with specific attention to cardiac and pulmonary dose limits, particularly for left-sided tumors.

Target Volume Coverage Parameters

Primary Target Metrics

• V95 (volume receiving 95% of prescribed dose): Should achieve ≥93% coverage of the planning target volume (PTV), with mean values around 93% (±4%) being acceptable 1
• V90 (volume receiving 90% of prescribed dose): Target mean coverage of 95% (±3.3%) of the PTV 1
• Minimum dose to 95% of breast PTV: Should be at least 45-46 Gy when prescribing standard doses, with tangential beamlet techniques achieving approximately 45.9 Gy 2

Dose Homogeneity

• Maximum dose: Should not exceed 112% of prescribed dose (typically 47.7 Gy for 42.5 Gy prescription, range 46.2-48.5 Gy) 1
• Dose uniformity: IMRT and volumetric arc therapy (VMAT) techniques provide superior dose homogeneity compared to 3D-CRT 3

Regional Nodal Target Volumes

Supraclavicular and Infraclavicular Nodes

• Include only the most caudal lymph nodes surrounding the subclavicular arch and base of jugular vein, following ESTRO guidelines 4
• Supraclavicular irradiation is indicated for patients with positive axillary nodes 4, 5

Axillary Coverage

• After axillary lymph node dissection: The resected part of the axilla should NOT be irradiated except in cases of residual disease 4
• Comprehensive nodal RT is recommended for node-positive patients 4

Internal Mammary Nodes

• Inclusion should be discussed case-by-case based on benefit/risk ratio, particularly considering cardiac toxicity 5
• Coverage is substantially better with beamlet IMRT techniques than segmental blocked techniques 2

Organs-at-Risk Dose Constraints

Cardiac Dose Limits (Critical for Left-Sided Tumors)

• Mean V25 to heart: Should be ≤6.6% (±4.8%) for left-sided tumors vs 0% for right-sided 1
• Heart V30: Target ≤10% with IMRT, compared to 13-16% with other techniques 3
• Mean heart dose: Tangential beamlet techniques achieve significantly lower doses than 9-field or segmental techniques (mean paired difference of 15.1 Gy, p<0.001) 2
• Left anterior descending coronary artery: Minimize dose exposure, particularly with tangential beamlet and segmental blocked techniques 2

Pulmonary Dose Constraints

• Mean lung dose: Target ≤10.2 Gy (±3.5 Gy) 1
• Lung V20: Should be ≤20.9% (±6%), with RapidArc achieving approximately 17% 1, 3
• Lung V2: Avoid substantial contralateral lung exposure; 9-field techniques can result in unacceptable 75.3% right lung V2 2

Contralateral Tissue Exposure

• Contralateral breast V2: Should be minimized; 9-field techniques result in unacceptable 58.9% exposure 2
• Tangential beamlet and segmental blocked techniques substantially reduce contralateral tissue exposure 2

Fractionation Schedule Verification

Standard Moderate Hypofractionation

• Recommended regimen: 15-16 fractions of 2.5-3 Gy per fraction (e.g., 42.5 Gy in 16 fractions) 4, 6, 1
• This carries Level I, Grade A evidence for equivalent oncologic outcomes 6

Conventional Fractionation (When Applicable)

• 45-50 Gy in 25-28 fractions of 1.8-2.0 Gy 4
• Boost dose: 10-16 Gy in 2 Gy fractions 4

Treatment Technique Assessment

Preferred Techniques

• 3D-conformal radiotherapy: Recommended standard technique 5
• IMRT: Should be proposed only in specific clinical situations requiring improved dose distribution 5, 3
• Tangential beamlet IMRT: Optimal for comprehensive locoregional irradiation, reducing normal tissue exposure while maintaining target coverage 2
• VMAT/RapidArc: Offers improved target coverage and dose homogeneity with efficient treatment delivery 3

Technique-Specific Considerations

• Respiratory gating: Useful for decreasing cardiac dose, particularly in left-sided tumors 5
• Monitor units: 3D-CRT requires approximately 527 MU, followed by VMAT, then IMRT 3
• Treatment time: Similar for 3D-CRT and VMAT, but doubles with IMRT 3

Clinical Indication Verification

Post-Breast Conserving Surgery

• RT is mandatory regardless of disease characteristics 5
• Boost to tumor bed required if patient <50 years old 4

Post-Mastectomy Radiotherapy (PMRT)

• Always indicated: T3-T4 tumors, ≥4 positive nodes, positive deep margins 4, 5
• Consider for: 1-3 positive nodes with additional risk factors (young age, vascular invasion, grade 3) 4

Common Pitfalls to Avoid

• Do not irradiate resected axilla after ALND unless residual disease present 4
• Avoid 9-field IMRT techniques for comprehensive breast/nodal irradiation due to excessive contralateral tissue exposure 2
• Do not use partial breast irradiation in young patients (<50 years) as it has not been sufficiently studied 4
• Ensure cardiac constraints are met for left-sided tumors; segmental techniques without blocking can result in 15 Gy higher mean heart dose 2
• Verify internal mammary coverage if indicated, as segmental blocked techniques provide substantially inferior coverage compared to beamlet techniques 2