Evidence-Based Planning Target Volume Margin Reduction for Modern Lung Stereotactic Ablative Radiation Therapy Using...

Area of Science:

• Radiation Oncology
• Medical Physics
• Thoracic Oncology

Background:

• Standard planning target volume (PTV) margins for lung stereotactic ablative radiation therapy (SABR) are typically 5 mm.
• Advancements in high-dose-rate volumetric modulated arc therapy (VMAT) with flattening filter-free (FFF) beams and modern immobilization systems may permit PTV margin reduction.

Purpose of the Study:

• To evaluate the feasibility of reducing PTV margins from 5 mm to 3 mm in lung SABR.
• To assess the impact of margin reduction on internal gross tumor volume (IGTV) coverage and organ-at-risk (OAR) doses.

Main Methods:

• Analysis of intrafractional motion from cone beam computed tomography (CBCT) scans in 33 patients treated with 5-mm PTV margins.
• Deformable image registration to assess volume changes and intrafractional movement in 36 patients.
• Re-planning with 3-mm margins and recalculation to evaluate d-IGTV coverage and OAR doses.

Main Results:

• Reducing PTV margins to 3 mm maintained high d-IGTV coverage (≥95% V100% and D99.9% in 99.3% and 98.6% of fractions, respectively).
• Significant reductions in lung dose (V50%, V80%, mean lung dose) and chest wall dose were observed with 3-mm margins (P < .001).
• Theoretical models showed a decrease in radiation pneumonitis probability and maintained a 2-year local control probability of >95%.

Conclusions:

• A 3-mm PTV margin is acceptable for lung SABR using modern treatment and immobilization techniques.
• Margin reduction modestly decreases toxicity to organs at risk.
• A 3-mm PTV margin maintains a high calculated 2-year local control rate.