Room: Exhibit Hall | Forum 6
Purpose: In linac-based respiratory-gated stereotactic body radiotherapy (SBRT) of the pancreas, the treatment is delivered assuming that the relationship between an external breathing signal and the tumor position is consistent. This is known, however, to not always hold true. The purpose with this study is to investigate potential predictors for relationship changes.
Methods: We investigated 42 treatment fractions from 11 pancreatic cancer patients with implanted fiducials. Fluoroscopic images in anterior-posterior (AP) and lateral (LR) directions before and after treatment delivery were acquired simultaneously to the external breathing signal. The fiducial positions were retrospectively found via template-based fiducial tracking in Matlab. For each fraction, we established linear relationships between the LR, AP, and superior-inferior (SI) fiducial positions and the AP breathing signal. The intrafractional relationship change (IRC) was defined as the absolute distance between the before and after treatment linear curves. This was calculated for each direction as well as a total 3D change. The treatment delivery time and the baseline drift (BLD) were investigated as potential predictors of IRC. We calculated the squared Pearson correlation coefficient (RÂ²) and the p-value of the linear relationship between the predictor and the IRC.
Results: The averageÂ±SD (maximum) IRCs were 1.3Â±1.2 (5.3) mm, 0.7Â±0.7 (2.9) mm, and 1.4Â±1.2 (3.6) mm, in the LR, AP, and SI directions, respectively. Treatment delivery time was not a statistically significant predictor. BLD was statistically significant of the AP and SI directions, with RÂ² 0.39 (p<0.001) and 0.11 (p=0.03), respectively. For the 3D IRC, the RÂ² for BLD was 0.32 (p<0.001). For each mm of BLD, there was about 1 mm of IRC.
Conclusion: BLD indicates internal-external relationship changes, and, therefore, observation and measurement during treatment should be performed. Further research needs to be done to find action levels.
Funding Support, Disclosures, and Conflict of Interest: Work funded by Varian Medical Systems.
Not Applicable / None Entered.