Purpose: External surrogates are commonly used for respiratory gating during pancreas stereotactic body radiation therapy (SBRT). Other factors, such as peristalsis and deformation, also contribute to intrafraction tumor motion and can be accounted for through the use of implanted fiducial markers. Target localization errors- due to disagreement between external surrogates and true tumor motion- can lead to dose delivery errors. In this work, we evaluate correlation between external surrogate motion and implanted marker motion to determine whether an external surrogate alone is adequate to monitor intrafraction motion for pancreas SBRT.
Methods: Data from 19 pancreas SBRT patients, each with 1-4 fiducial markers implanted in or near the tumor was analyzed. 15 patients were treated in expiration breath hold, and 4 patients were treated free-breathing gated at exhale. Respiratory amplitude was monitored using an abdominal block as an external surrogate (RGSC, Varian Medical Systems, Palo Alto). Implanted markers were manually segmented offline in orthogonal kV/MV intrafraction images. Through kV/MV triangulation, each marker’s left-right (LR), superior-inferior (SI) and anterior-posterior (AP) positions were calculated. Correlation between respiratory amplitude and the LR, SI and AP displacements of the markers were calculated using Pearson’s correlation coefficient, R, and the associated statistical significance, p.
Results: Insignificant correlation (R < 0.3) was observed for the LR and AP displacements. SI correlations ranged between R = 0.11, p = 0.488 and R = 0.57, p < 0.001 for EBH patients and R = 0.08, p = 0.721 and R = 0.46, p < 0.001 for FB patients, hence the correlation between respiratory amplitude and SI displacement has significant variation between patients.
Conclusion: Pancreas SBRT treatments using external surrogates alone for intrafraction motion management do not sufficiently model the intrafraction motion of pancreas tumors. Online intrafraction monitoring of tumor position is recommended for the safe delivery of pancreas SBRT.
Funding Support, Disclosures, and Conflict of Interest: Patient data used for this study was collected under an ethically approved trial (ID: NCT03505229).
Target Localization, Image Guidance
TH- External Beam- Photons: Motion management - intrafraction