Intrafractional Target Position Accuracy for Lung Stereotactic Body Radiotherapy (SBRT) using Cone-beam CT (CBCT)

Abstract

To assess intrafractional geometric accuracy of lung SBRT treated under image-guidance, and impact of immobilization method and performance status (PS). Setup accuracy using CBCT was analyzed in 108 SBRT lung patients treated on an REB-approved protocol. Tumor matching was performed on the internal target volume (ITV) contour. Couch position was adjusted when positional discrepancies exceeded ± 3 mm in any direction, and verified with a second CBCT. Repeat imaging was performed at the mid-treatment and end-treatment of each SBRT fraction. Patient positioning data, matched with time stamps from all acquired CBCTs, were recorded and analyzed to identify systematic (Σ) and random (σ) errors, and total treatment duration. Data were stratified to assess the impact of immobilization method (evacuated cushion [EC]; n = 67), evacuated cushion + abdominal compression (COMP; n = 27), chestboard (CB; n = 14), and PS (ECOG 0; n = 26), 1 (n = 61), 2 (n = 21). Five hundred localization, 399 verification, 401 mid-treatment, and 360 post-treatment CBCTs were analyzed. The mean time from localization to end fraction CBCT was 31:50 ± 7:21 minutes, with no significant difference between immobilization or PS. After initial setup, CBCT assessment of ITV coverage showed 15% of all fractions were within ± 3 mm, and 39% within ± 5 mm. After correction of discrepancies and acquisition of a verification CBCT, 88% were within ± 3 mm, and 99% within ± 5 mm. At mid- CBCTs, 68% of all fractions were within ± 3 mm, and 94% within ± 5 mm. Similarly, at the end-CBCTs, 65% were maintained within ± 3 mm, and 93% within ± 5 mm. At the end of treatment, Σ ranged from 1.1 to 1.9 mm across all immobilization subgroups. The largest σ (2.2 mm) was observed in the superior/inferior (S/I) direction of the COMP group. In the left/right (L/R) and anterior/posterior (A/P) direction, σ was highest in the CB group (1.9 compared to 1.4 and 1.2 mm for EC, 1.4 and 1.3 mm for COMP, respectively). In PS 0, Σ was stable throughout treatment, with a noted increase in the A/P direction from verification (0.8 mm) to mid- and end-CBCTs (1.3 mm). An increase in Σ and σ from verification to mid- and end-CBCTs was observed in both PS 1 and 2. The largest Σ increase was observed in PS 2, where values increased from 1.0, 1.4, 1.0 mm (L/R, S/I, A/P) at verification to 1.2, 1.8, 1.4 mm at mid-treatment, and 1.7, 2.0, 1.7 mm at end-treatment. Image-guidance is essential for lung SBRT. Once target match is performed, the intrafraction positional changes through a course of lung SBRT are small, regardless of immobilization used. As the overall geometric accuracy for PS 1 and 2 deteriorates at a faster rate then PS 0, increased frequency of image-guidance throughout treatment may be more pertinent for these cohorts to ensure accurate target localization.