Beam's-eye-view imaging during non-coplanar lung SBRT.

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-Guided Therapy

Background:

• Beam's-eye-view (BEV) imaging using electronic portal imaging devices (EPIDs) enables real-time tumor monitoring during lung stereotactic body radiation therapy (SBRT).
• Tumor localization accuracy is influenced by patient anatomy and beam angles (gantry and couch).

Purpose of the Study:

• To investigate the angular dependence of automatic tumor localization during non-coplanar lung SBRT.
• To identify optimal gantry and couch angles for accurate BEV lung tumor localization.

Main Methods:

• Amorphous silicon EPID images acquired at 12 Hz.
• Markerless lung tumor localization algorithm validated against manual localization.
• Analysis of 12,987 image frames from 123 sequences across 48 gantry-couch rotations in 10 SBRT patients.

Main Results:

• Best tracking performance (δ = 1.40 mm) observed with gantry angles near 250°, irrespective of couch angle.
• Gantry angles of 150°, 210°, and 350° also showed good tracking (δ = 1.77-2.00 mm).
• Specific combinations like (θ=30°, ϕ=330°) yielded high accuracy (δ=1.19 mm), while others (θ=90°, 230°, ϕ=10°) showed significant errors (δ > 3.5 mm).
• Patient-specific variability in tracking performance was noted for 35% of gantry-couch combinations.

Conclusions:

• Non-coplanar beams at specific angles (e.g., θ=30°, ϕ=330°) offer the highest accuracy for BEV lung tumor localization.
• Gantry angles around 150°, 210°, 250°, and 350° generally provide good tracking performance.
• Certain beam geometries are associated with substantial localization errors, and performance is patient-dependent.