Room: Exhibit Hall
Purpose: Investigate the effectiveness and suitability of two 2-step alignment protocols used in thorax/abdomen SBRT to align tumors after the initial tattoo-based setup: â€œSTâ€?-surface, then tumor (AlignRT, CBCT); â€œBTâ€?-bony, then tumor (2D-kV OBI, CBCT).
Methods: 77 ST and 70 BT alignments were processed (39 patients; 21 female, 18 male; ABC in 21 ST and 19 BT). Motion management strategies: ITV planning or ABC. Execution time was recorded for the first-step alignment. Average shifts in each direction, for each alignment step as well as for the total shift were calculated for the entire population and sub-groups.
Results: First-step alignment: Shifts were similar between protocols when averaged over all data. Tumor alignment (â€œfine-tuningâ€?): Shifts larger by ~2 mm following AlignRT-guided first-step. ABC vs. non-ABC: First-step corrections larger by ~5 mm for ABC group (significant differences in all directions with AlignRT, along LNG with 2D-kV); fine-tuning corrections larger by ~2 mm for ABC group for both protocols. Male vs. Female: Initial and fine-tuning shifts larger for female patients, and larger with AlignRT. First-step largest shifts occurred for ABC males with AlignRT. Fine-tuning shifts were largest in female ABC population after AlignRT. Alignment time: The two alignment protocols are comparable, with significantly larger time required with ABC.
Conclusion: Surface-tumor and bony-tumor alignments perform similarly. Shifts are larger, albeit not significantly, for AlignRT group (that is, tumor location correlates better with bony anatomy). ABC and female sub-groups underperform (larger shifts from tattoos necessary to align the tumor), likely due to uncertainties in ABC breath-hold reproducibility, which is more consequential when body surface is used to guide alignment, especially in female patients. To minimize radiation exposure, surface-tumor protocol is recommended, except when using ABC (particularly for female patients), as the uncertainty in the breath-hold level may also be present during treatment delivery.