Dosimetric Impact of Deviations from Planned Breath-Hold In Surface-Guided Liver SBRT
Abstract
Purpose
Surface-guided radiation therapy (SGRT) uses external surface motion as a surrogate for internal target position. While strict SGRT tolerances (typically ±1mm) are common, they may still be permissive for patients with small surface excursions. In these cases, a patient could drift into respiratory phases outside the expected breathing range predicted by the miniMIP built based on the skin excursion tolerances. This study evaluates internal motion and the dosimetric impact of small deviations beyond the clinically selected miniMIP.
Methods
Ten liver SBRT patients treated with end-expiration breath-hold and <2 mm surface excursion within the clinically selected miniMIP were analyzed. To simulate a scenario where breathing extends slightly past the planned range, the phases included in the miniMIP were expanded to allow for an additional 1mm of surface motion. Internal fiducial displacement was measured between the original and extended ranges. The maximum displacement along the dominant direction was applied as an isocenter shift to the original plan without reoptimization. PTV D95% and GTV D99% were calculated for the original and shifted plans to assess coverage compromise.
Results
Extending the miniMIP by 1mm revealed additional fiducial motion in all patients, with a mean internal amplitude increase of 3.6 ± 1.6mm (maximum: 6.1mm). Worst-case shifts reduced PTV D95% by a mean of 7.3 ± 3.4% (maximum reduction: 33.6%). GTV D99% changed by a mean of 2.0 ± 1.9%. While some cases demonstrated improved GTV coverage, the maximum reduction was 13.9%.
Conclusion
Internal fiducial motion can be disproportionately large relative to surface motion, especially for patients with minimal surface excursion. While PTV-level dosimetry was highly sensitive to worst-case motion, GTV coverage remained relatively robust, suggesting standard planning margins effectively protect the gross tumor volume. However, these findings underscore the importance of patient-specific characterization of surrogate-internal motion relationships to ensure total coverage.