Internal target volume reproducibility and dose coverage for lung stereotactic body radiotherapy using spine-based positioning and tracking

Free-breathing spine-based positioning/tracking in lung stereotactic body radiotherapy is required when direct tumor positioning is not possible. Respiratory baseline, motion and target position reproducibility interact and remain uncertainty sources. This results in conflicting margins but also clinical control. The internal target volume geometric reproducibility, spine distance dependency and four-dimensional per-fraction gross tumor volume (GTV) dose was analyzed in 340 fractions. While the target would leave a projected isotropic 5 mm planned target volume in 11% of fractions, ΔD 98% (4D-GTV) > 10% occurred in 2.6% of fractions. Risk mitigation is suggested through additional off-line 4DCT, surface guidance, fractionation and increased inferior-superior and posterior-anterior margins.