Dosimetric and clinical effects of interfraction and intrafraction correlation errors during marker-based real-time tumor tracking for liver SBRT

Correlation model error (CME) between the internal target and the external surrogate, and marker–tumor correlation error (MTCE) between the tumor and the implanted marker occur during marker-based real-time tumor tracking. The effects of these intrafraction and interfraction errors on the dose coverage in the clinical target volume (CTV) and on tumor control probability (TCP) for hepatocellular carcinoma (HCC) were evaluated in this study. Eight HCC patients treated with non-isocentric dose delivery by a robotic radiosurgery system were enrolled. The CMEs were extracted from the treatment log file, and the MTCEs were calculated from the preceding study. The CMEs and MTCEs were randomly added to each beam's robot position, and the changes in the TCP and the 2%, 95% and 99% dose coverage values for the CTV (D2, D95 and D99) were simulated. The data were statistically analyzed as a function of the CTV to planning target volume (PTV) margin, the dose fraction and the marker–tumor distance. Significant differences were observed in the majority of the CTV D2, D95 and D99 values and the TCP values. However, a linear regression revealed that ∆CTV D2, D95 and D99 have a weak correlation with ∆TCP. A dose-difference metric would be unable to detect a critical error for tumor control if the coverage changes for the CTV and ∆TCP were weakly correlated. Because the simulated TCP-based parameter determination was based on the dose simulation, including predicted interfraction and intrafraction errors, we concluded that a 95th percentile TCP-based parameter determination would be a robust strategy for ensuring tumor control while reducing doses to normal structures.