Patient Positional Uncertainty and Margin Reduction in Lung Stereotactic Ablative Radiation Therapy Using Pneumatic...

Area of Science:

• Radiation Oncology
• Medical Physics

Background:

• Motion management is crucial in thoracic stereotactic ablative radiation therapy (SABR).
• Current standard planning target volume (PTV) margins of 5.0 mm may be excessive with advanced techniques.
• Reassessing PTV margins is warranted due to advancements in tumor localization and motion management.

Purpose of the Study:

• To analyze intrafraction repositioning data for lung SABR under forced shallow breathing.
• To determine appropriate planning target volume (PTV) margin requirements for modern lung SABR delivery techniques.

Main Methods:

• Retrospective analysis of 607 lung SABR fractions from 124 patients treated with pneumatic abdominal compression and 4-dimensional (4D) cone beam computed tomography (4D CBCT)-guided intrafraction repositioning.
• Calculation of margin requirements using the 2-standard deviation (SD) method and an analytical model accounting for systematic and random errors.

Main Results:

• 85.7% of fractions had 3D repositioning shifts under 5.0 mm.
• Margins calculated by the 2-SD method were 3.62 mm (right-left), 4.34 mm (inferior-superior), and 3.50 mm (posterior-anterior).
• Analytical modeling suggested margins of 4.01 mm, 4.37 mm, and 3.95 mm; intrafraction repositioning reduced margin needs by 0.73 mm.

Conclusions:

• The standard 5.0-mm PTV margin is likely conservative for workflows utilizing advanced techniques like 4D CT and 4D CBCT-guided intrafraction repositioning.
• These modern techniques can significantly reduce required margins, thereby decreasing radiation dose to healthy tissues.
• Caution is advised when reducing margins; a formal risk assessment is recommended before adopting new PTV margin standards.