ESTRO 2024 - Abstract Book

S4074

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

easily expect that variations of the amplitude occur and the corresponding duty cycle may be unstable. This may deny the validity of pre-defined margin setting. In this study, we investigated the impact of internal tumor motion derived from respiratory phase variations.

Material/Methods:

A total of 30 fractions in twenty-five patients with respiratory-gated VMAT for lung and esophageal cancer were evaluated. 4DCT acquisition was performed for simulation CT to assess target respiratory motion and the gated widow was then determined for internal motion. The CTV was expanded to account for the internal motion, and a PTV was created with a 5 mm margin. Subsequently, the treatment plan was generated. For daily respiratory management, the gated window for treatment was adjusted at the first treatment using fluoroscopic images and the surrogate of tumor motion such as the diaphragm. Daily gated treatment was managed using the duty cycle (DC), which is calculated from the amplitude of abdominal wall in Varian RPM system and the gated window. The tolerable variations of DC were determined from a part of PTV margin. If the DC exceeded the tolerable variation, the gated window was re-adjusted after confirmation with fluoroscopic images and the surrogate. The variations in internal target motion were estimated using the relation between tumor motion measured at the 4DCT simulation and respiratory phase changes from the change of gated irradiation timing in consecutive daily treatments. The RMS (root mean square) was used to calculate the variation in respiratory phase for all patients from the standard deviation of respiratory phase changes. The change in internal motion was also evaluated using RMS (inclusion coefficient 2).

Results:

The tumor motion under planned respiratory gating was 4.5 to 10.8 mm in three dimensions. The planned DC was 14 to 71%. The average DC during daily treatments for all patients was 9.4 to 70.9%. The difference between the average DC and the planned DC was -3.8 ± 8.0% (mean ± 1 SD), with a maximum of 12.1% and a minimum of -23.9%. The RMS of DC variation during treatment for all cases was 3.8%. In addition, the average DC during daily treatments was shifted to increase internal target motion compared to the planned DC in 8 of the 25 cases. The RMS of internal motion variation estimated from DC variation was 1.1 mm for all patients (k = 2).

Conclusion:

In daily respiratory-gated VMAT treatment managed using DC, the variations of respiratory motion estimated from DC was less than 2 mm. This means that respiratory motion management was performed in approximately 70% of cases to suppress internal target motion, resulting in safer and more effective treatment. On the other hand, our data should be used to resolve the difficult issue of balancing safety and treatment efficiency.

Keywords: gating VMAT, internal motion, motion management

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