Evaluation of reproducibility of tumor repositioning during multiple breathing cycles for liver stereotactic body radiotherapy treatment

Abstract

Aim

To evaluate the tumor repositioning during gated volumetric modulated arc therapy (VMAT) for liver stereotactic body radiotherapy(SBRT) treatment using implanted fiducial markers and intrafraction kilovoltage (kV) images acquired during dose delivery.

Materials and methods

Since 2012, 47 liver cancer patients with implanted fiducial markers were treated using the gated VMAT technique with a Varian Truebeam STx linear accelerator. The fiducial markers were implanted inside or close to the tumor target before treatment simulation. They were defined at the maximum inhalation and exhalation phases on a 4-dimensionnal computed tomography (4DCT) acquisition. During the treatment, kV images were acquired just before the beam-on at each breathing cycle at maximum exhalation and inhalation phases to verify the fiducial markers positions. For the five first fractions of treatment in the first ten consecutive patients, a total of 2705 intrafraction kV images were retrospectively analyzed to assess the differences between expected and actual positions of the fiducial markers along the cranio-caudal (CC) direction during the exhalation phase.

Results

The mean absolute intrafractional fiducial marker deviation along the CC direction was 1.0[[ce:hsp sp="0.25"/]]mm at the maximum exhalation phase. In 99%, 95% and 90% cases, the fiducial marker deviations were ≤4.5[[ce:hsp sp="0.25"/]]mm, 2.8[[ce:hsp sp="0.25"/]]mm and 2.2[[ce:hsp sp="0.25"/]]mm, respectively.

Conclusion

Intrafraction kV images allowed us to ensure the consistency of tumor repositioning during treatment. In 99% cases, the fiducial marker deviations were ≤4.5[[ce:hsp sp="0.25"/]]mm corresponding to our 5[[ce:hsp sp="0.25"/]]mm treatment margin. This margin seems to be well-adapted to the gated VMAT SBRT treatment in liver disease.