ESTRO 2024 - Abstract Book

S4306

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

Results of residual errors and margins are shown in Table 1. Tracking errors do not yield important systematic errors, while intrafraction random errors are predominant when compared to interfraction random errors. It is shown that margins can increase from 0.5mm to 1mm when fiducials are not placed within the tumor, and that the effect of rotations is predominant in the lateral direction.

Fiducial tracking error

Rotational error

Tumor Tracking error

SI(mm)

LR(mm)

AP(mm)

Yaw(°)

Pitch(°)

Roll(°)

SI(mm)

LR(mm)

AP(mm)

μ

0.12

0.01

0.05

0.18

0.29

0.41

0.06

0.04

0.02

Σ

0.31

0.17

0.19

1.13

1.04

1.09

0.53

0.44

0.35

σ inter

0.33

0.22

0.21

1.07

0.94

1.38

0.54

0.63

0.37

σ intra

1.71

0.75

0.94

1.93

2.02

2.00

1.75

1.17

0.96

σ total

1.74

0.78

0.98

2.22

2.14

2.43

1.83

1.33

1.01

Margin

1.60

0.6

0.8

-

-

-

2.2

1.6

1.2

Table 1. Geometrical errors and margins due to tracking errors

Comparison between margins in the first fraction and the rest of the treatment yielded mean differences (SD) of - 0.1mm (0.4mm), 0.0mm (0.22mm) and 0.0mm (0.24mm) in the SI, LR and AP directions if only fiducials were considered and values of -0.2mm (0.6mm), -0.2 (0.8mm) and 0.2mm (0.5mm) in SI, LR and AP directions when rotations are taken into account. Figure 1 shows differences between margins calculated in the first fraction and during the rest of the treatment for those lesions from which rotational errors are available. It is shown that margins due to dispersion of tracking errors is important in some patients and that they would benefit for adaptive margins.

Conclusion:

Geometrical errors due to respiratory tracking with Cyberknife have a minor impact on treatment margins compared to other uncertainties present during liver SBRT treatment. However, these errors might be important in some of the patients that present a large dispersion of residual errors. In those patients, information gathered during the first fraction could help to adapt margins during the rest of the treatment.

Keywords: Respiratory tracking, Cyberknife, Liver SBRT

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