Characteristics and dosimetric impact of intrafraction motion during peripheral lung cancer stereotactic radiotherapy: is a second midpoint cone beam computed tomography of added value?

BACKGROUND: In our department, during lung stereotactic body radiation therapy (SBRT), all patients receive an intra-fractional midpoint cone beam computed tomography (CBCT). This study aimed to quantify the benefit of adding a second midpoint CBCT over a course of peripheral lung SBRT. MATERIALS AND METHODS: Six-hundred-sixty-four CBCTs from 166 patients were retrospectively analyzed. Treatments were based on the internal target volume (ITV) approach. An isotropic 0.5 cm margin was used to create the planning target volume (PTV) around the ITV. The prescribed dose was 48 Gy in 4 fractions to the PTV. Patients were divided into two groups: patients for whom the 3D-intra-fractional-variation (IFV) was < 0.5 cm (105 patients, low risk group) and patients with at least one 3D-IFV ≥ 0.5 cm (61 patients, high-risk group). Plans simulating the dosimetric impact of the IFV were created as follows: the original 2 arcs (ARC ) were copied into a new plan consisting of 4 times ARC 1 and 4 times ARC 2. The delivery of ARC 1 was always assumed to have occurred with the isocenter initially coordinated, whereas the positions of ARC 2 were modified for each arc by the measured the 3D-IFV. RESULTS: For the PTV, we obtained: D99% (Gy) = 45.2 vs. 48.2 Gy (p < 0.0001); Dmean = 53 vs. 54 Gy (p < .0001) for the reconstructed vs. planned dose values, respectively. For the ITV, the changes are less pronounced: D99% (Gy) = 52.2 vs. 53.6 Gy (p = 0.0007); Dmean = 56 vs. 56.8 Gy (p = 0.0144). The V48 Gy(%)-ITV coverage did not statistically change between the delivered vs. planned dose (p = 0.1803). Regarding the organs at risk for both groups, dose-volume-histograms were near-identical. CONCLUSION: We demonstrated that a single CBCT is sufficient and reliable to manage the IFV during peripheral lung SBRT.