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

Katie Jasper; Baochang Liu; Robert Olson; Quinn Matthews

doi:10.1016/j.adro.2021.100750

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

Adv Radiat Oncol. 2021 Jul 3;6(6):100750. doi: 10.1016/j.adro.2021.100750. eCollection 2021 Nov-Dec.

Authors

Katie Jasper^{1

2}, Baochang Liu^{3

4}, Robert Olson^{2

3}, Quinn Matthews³

Affiliations

¹ BC Cancer-Vancouver, Vancouver, British Columbia, Canada.
² Division of Radiation Oncology, Department of Surgery, University of British Columbia, Vancouver, British Columbia, Canada.
³ BC Cancer-Prince George, Prince George, British Columbia, Canada.
⁴ Radiation Medicine Program, Walker Family Cancer Centre, St. Catharines, Ontario, Canada.

Abstract

Purpose: Standard planning target volume (PTV) margins for lung stereotactic ablative radiation therapy (SABR) are 5 mm. High-dose-rate volumetric modulated arc therapy delivered using flattening filter-free (FFF) beams with modern immobilization systems may allow for PTV margin reduction. This study assesses whether PTV margins can be reduced from 5 to 3 mm.

Methods: Target intrafractional motions derived from pretreatment and posttreatment cone beam computed tomography (CBCT) scans for 33 patients receiving lung SABR treated with 10XFFF energy and 5-mm PTV margins from 2016 to 2019 were used to calculate the required PTV margin. Deformable registration of the planning CT scan and internal gross tumor volume (IGTV) contour to posttreatment CBCT scans for 36 consecutive patients with 4 fraction schedules was completed to capture volume changes and intrafractional movement. Plans were replanned with 3-mm margins and recalculated on each deformed CT scan to assess deformed IGTV (d-IGTV) coverage and organ-at-risk doses.

Results: Margin analysis showed PTV margins may be reduced to 3 mm. The mean d-IGTV coverage (percentage of the d-IGTV receiving ≥100% of the prescription dose [V100%] and the minimum dose covering 99.9% of the d-IGTV volume [D99.9%]) over 4 fractions for each patient was >95% with both margins. With 5-mm PTV margins, all 144 fractions had a d-IGTV V100% of >95% and a D99.9% >95%. With 3-mm PTV margins, the d-IGTV V100% was >95% in 99.3% of fractions (143 of 144) and the D99.9% was >95% in 98.6% of fractions (142 of 144). With 3-mm PTV margins, significant reductions in body V50%, body V80%, the volume of the lung receiving ≥20 Gy, and the mean lung dose and chest wall dose to 0.035 cm³ and 30 cm³ were observed (all P < .001). Using theoretical models, the normal tissue complication probability for radiation pneumonitis decreased by a mean of 0.8% (range, 0.1%-2.7%), and the mean 2-year tumor control probability was 96.1% and 95.2% with 5-mm and 3-mm PTV margins, respectively.

Conclusion: With modern treatment and immobilization techniques in lung SABR, 3-mm PTV margins maintain acceptable IGTV coverage, modestly reduce toxicity to organs at risk, and maintain a calculated 2-year local control rate of >95%.