Background: The management of breath-induced tumor motion is a major challenge for lung stereotactic body radiation therapy (SBRT). Three techniques are currently available for these treatments: tracking (T), gating (G) and free-breathing (FB). Aim: To evaluate the dosimetric differences between these three treatment techniques for lung SBRT. Materials and methods: Pretreatment 4DCT data were acquired for 10 patients and sorted into 10 phases of a breathing cycle, such as 0% and 50% phases defined respectively as the inhalation and exhalation maximum. GTV ph , PTV ph (=GTV ph + 3 mm) and the ipsilateral lung were contoured on each phase. For the tracking technique, 9 fixed fields were adjusted to each PTV ph for the 10 phases. The gating technique was studied with 3 exhalation phases (40%, 50% and 60%). For the free-breathing technique, ITV FB was created from a sum of all GTV ph and a 3 mm margin was added to define a PTV FB. Fields were adjusted to PTV FB and dose distributions were calculated on the average intensity projection (AIP) CT. Then, the beam arrangement with the same monitor units was planned on each CT phase. The 3 modalities were evaluated using DVHs of each GTV ph , the homogeneity index and the volume of the ipsilateral lung receiving 20 Gy (V 20Gy). Results: The FB system improved the target coverage by increasing D mean (75.87 (T)-76.08 (G)-77.49 (FB) Gy). Target coverage was slightly more homogeneous, too (HI: 0.17 (T and G)-0.15 (FB)). But the lung was better protected with the tracking system (V 20Gy : 3.82 (T)-4.96 (G)-6.34 (FB) %). Conclusions: Every technique provides plans with a good target coverage and lung protection. While irradiation with free-breathing increases doses to GTV, irradiation with the tracking technique spares better the lung but can dramatically increase the treatment complexity.

a b s t r a c tAim: 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 mm at the maximum exhalation phase. In 99%, 95% and 90% cases, the fiducial marker deviations were ≤4.5 mm, 2.8 mm and 2.2 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 mm corresponding to our 5 mm treatment margin. This margin seems to be well-adapted to the gated VMAT SBRT treatment in liver disease.

We provide a subjective critical analysis of minimal requirements and ideal technique for liver SBRT planning.

a b s t r a c tBackground: Motion management is a major challenge in abdominal SBRT. We present our

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