Surface-Guided Radiotherapy (SGRT) in Breast Cancer

What is SGRT?

Why SGRT Matters in Breast Cancer

• The breast and chest wall lie in close proximity to the heart, left anterior descending artery (LAD), and lungs
• Respiratory motion significantly displaces target volumes and organs at risk (OARs)
• Conventional tattoo/laser-based setup has positional isocenter variation up to 4.4 mm in DIBH
• Cardiac dose exposure, especially in left-sided breast cancer, drives long-term radiation-induced cardiotoxicity

Clinical Applications

1. Deep Inspiration Breath Hold (DIBH)

• Patient takes a deep breath to a pre-defined breath-hold level (BHL)
• SGRT cameras detect the chest surface displacement in real time
• A gating window (typically ±2–3 mm) is set; the linear accelerator beam fires only when the patient is within tolerance ("Beam ON") and holds automatically if they move outside ("Beam Hold")
• Visual biofeedback (goggle displays or ceiling-mounted monitors) guides patients to maintain the target BHL

• Mean heart dose significantly reduced vs free breathing
• LAD max dose reduced (e.g., 14.6 ± 12.0 Gy → 7.7 ± 7.1 Gy in one RCT)
• Left ventricle V5Gy reduced (2.4% → 0.8%)
• Left lung V20Gy and V30Gy both reduced
• Risk of late cardiac toxicity with SGRT: +1% vs +3.6–20.5% with daily kV-CBCT (due to imaging dose)

2. Patient Setup and Positioning

• SGRT setup is superior to conventional tattoo/laser-based setup in isocenter reproducibility
• In a large comparative study (n=141 patients, 1692 image pairs), AlignRT® achieved the smallest positional variance, followed by Catalyst™, both superior to laser-tattoo setup (Laaksomaa et al., 2022, PMID 36523797)
• Systematic and random errors below 2 mm in all orthogonal directions with SGRT vs CBCT
• Mean setup error between SGRT and CBCT: <0.08 cm (DIBH) and <0.30 cm (free breathing)
• SGRT reduces total treatment time by approximately 20% due to less frequent CBCT requirement

3. Tattoo-Free Radiotherapy

• SGRT setup was equivalent or faster than tattoo-based setup
• Free-breathing setups: SGRT was 11 seconds faster per fraction
• DIBH setups: equivalent time; SGRT 23 seconds faster for free-breathing
• No significant difference in positional shifts on pre-treatment imaging
• Fiscally feasible and non-inferior to permanent tattoos

SGRT Systems in Clinical Use

SGRT in Locoregional Breast Radiotherapy

• Arm position significantly affects lymph node dose; the arms are outside the field of view of most SGRT systems, creating a limitation
• Locoregional lymph node positions cannot always be accurately predicted from surface anatomy alone
• SGRT shows 6D monitoring of chest position, posture, and movement throughout treatment
• For right-sided breast + RNI, SGRT-DIBH reduces liver mean dose dramatically (from 916.9 cGy to 281.2 cGy), as well as heart and right coronary artery dose (Lai et al., 2023, PMID 37087557)

Key Technical Considerations and Challenges

Region of Interest (ROI) Selection

• Chest-only ROI is standard but may miss abdominally-driven DIBH patients
• ~15% of left-sided breast cancer patients are "abdominal DIBH" (A-DIBH) with minimal vertical chest elevation (≤5 mm) — these patients require an abdominal ROI or surrogate to ensure adequate heart sparing; conventional chest-only SGRT will incorrectly trigger beam-on while the heart remains in an elevated position (Zeng et al., 2023, PMID 38047221)

Systematic Errors

• Reference surface capture timing matters: tissue swelling after CT simulation can cause systematic errors if surface was captured at simulation
• Acquiring a new reference surface after the first IGRT-corrected fraction reduces systematic posture errors
• Setup images (CBCT/kV) are still recommended, particularly in the first 2 weeks; a combined protocol of daily SGRT + CBCT 3×/week (mandatory in weeks 1–2) optimizes accuracy and efficiency

Breath-Hold Level (BHL) Management

• If the BHL is inconsistently achieved, SGRT detects this and withholds beam
• Clinicians can raise/lower BHL windows or re-acquire DIBH surfaces mid-course

Skin Dressings

• Prophylactic skin dressings (for radiation dermatitis) can disrupt SGRT surface tracking; studies confirm this effect must be accounted for in workflow design (PMID 38525921)

Comparison with Mechanically-Assisted DIBH

• In a 2023 RCT (Vander Veken et al., PMID 36898583), mechanically-assisted non-invasive ventilation DIBH (MANIV-DIBH) achieved equivalent positional breast accuracy to voluntary SGRT-guided DIBH (sDIBH), but provided better OAR sparing (LAD near-max dose 7.7 vs 14.6 Gy; left ventricle V5Gy 0.8% vs 2.4%). This suggests SGRT-guided voluntary DIBH, while excellent, may be surpassed by mechanically-assisted techniques for high-risk cardiac anatomy.

SGRT vs CBCT: Imaging Dose Comparison

Current Guidelines and Standards

• SGRT for initial patient positioning (all sites)
• SGRT as the standard for breast DIBH treatment guidance
• Commissioning and ongoing QA requirements for all video-based SGRT systems
• Workflow integration with IGRT (CBCT/kV imaging), not replacement of it

Clinical Summary

• AAPM TG-302: Al-Hallaq et al., Med Phys 2022 [PMID: 35179229]
• Rudat et al., Sci Rep 2024 (DIBH + SGRT OAR impact) [PMID: 39537683]
• Laaksomaa et al., Rep Pract Oncol Radiother 2022 (AlignRT vs Catalyst vs laser) [PMID: 36523797]
• Vander Veken et al., Radiother Oncol 2023 RCT (voluntary vs mechanical DIBH) [PMID: 36898583]
• Zeng et al., Adv Radiat Oncol 2023 (abdominal DIBH pitfall) [PMID: 38047221]
• Lai et al., J Appl Clin Med Phys 2023 (right-sided breast + RNI) [PMID: 37087557]
• Malhotra et al., Tech Innov Patient Support Radiat Oncol 2025 (tattoo-free) [PMID: 41211452]
• Bellala et al., J Cancer Res Ther 2023 (clinical review, 533 patients) [PMID: 37787279]