ESTRO 38 Abstract book

S48 ESTRO 38

the peak regions, traverse air cavities without substantial energy loss and deposit a higher dose distant to the peak in the valley regions. Our results show that the microstructure of the lung cannot be neglected for treatment planning of MRT in lung tissue. Currently, these findings are experimentally validated.

Proffered Papers: RTT 1: Motion management and adaptive strategies

OC-0094 Retrospective evaluation of motion effects in robotic radiosurgery treatments of lung cancer S. Trivellato 1,2 , E. Rondi 1 , S. Vigorito 1 , E. Miglietta 3 , F. Castellini 3 , G. Piperno 3 , A. Ferrari 3 , B.A. Jereczek- Fossa 3,4 , F. Cattani 1 1 IEO- European Institute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy; 2 University of Milan, Department of Physics, Milan, Italy; 3 IEO- European Institute of Oncology IRCCS, Division of Radiation Oncology, Milan, Italy ; 4 University of Milan, Department of Oncology and Hemato-oncology, Milan, Italy Purpose or Objective Stereotactic body radiation therapy (SBRT) has emerged as a non-invasive standard treatment modality for early- stage lung cancer [1-3]. Respiratory management should be individually assessed to ensure SBRT dose conformity [4]. A treatment planning system for a robotic radiosurgery system is now offering the 4D treatment planning and optimization feature to consider organ motion recorded in a 4D computed tomography (4DCT) [5]. The aim of the study is to exploit this 4D module for a retrospective analysis of motion effects on target coverage and organs at risk (OARs) sparing during robotic radiosurgery in lung cancer treatment. Material and Methods Ten consecutive lung cancer treatments, 5 in upper and 5 in lower lobes, were retrospectively selected. The robotic system allows treating lesions that cannot be fully tracked. In this so-called 0-view modality, the internal target volume is defined as the envelope of the lesion volume in the full-exhale and full-inhale CT series. A set- up margin of 5 mm is added to get the planning target volume (PTV). The conventional 3D module optimizes and calculates dose distributions on the reference full-inhale phase. The 4D module uses b-splines deformable image registration to accumulate dose distributions calculated on each static 3D-CT image of the 4DCT dataset. For each patient, 3D optimization was performed and followed by both 3D and 4D calculations (3D opt+calc Plan and 3D opt 4D calc Plan). A complete 4D optimization and calculation (4D opt+calc Plan) were performed when the 3D opt 4D calc Plan resulted clinically suboptimal. All dose distributions were obtained using an 80% isodose prescription, the Ray Tracing algorithm, and 8 respiratory The range of lesion displacement was 0-6 mm and 4-19 mm for upper and lower lobe, respectively. This difference resulted statistically significant at the Wilcoxon Mann Whitney test ( p -value < 0.02). 3D opt+calc Plans showed a median volume covered by the 100% isodose (V 100% ) of 98.2% with a median minimum dose (D min ) of 93.5%. 3D opt 4D calc Plans resulted in a median V 100% of 95.6% and a D min of 87.6%. Variations in V 100% metric were in the range (-14.9%, 0%). Seven out of 10 patients were re-optimized with the 4D Module, all lower lesions and 2 upper lesions. 4D opt+calc Plans showed a median V 100% of 97.3% with a median D min of 93.8%. Among OARs, most relevant variations were registered in the heart maximum dose. Details are reported in Table 1 and 2. phases. Results

Conclusion Due to recorded variations, 70% of plans have been re- optimized. The 4D module is a powerful tool to manage organ motion when lesion tracking is not possible, especially for lesions in lower lobes or close to moving OARs. Further studies will increase the number of patients and a Monte Carlo calculation will permit dose prescription discussion. Results will be validated with film measurements in a 4D anthropomorphic phantom. OC-0095 Intrafraction motion management in VMAT breast radiotherapy with AlignRT system: comparison of ROIs V. Favrel 1 , O. Ruiz Abrard 1 , F. Chabbert 1 , C. Garcia 1 , L. Gonzague Casabianca 1 , H. Mailleux 1 1 Institut Paoli Calmettes, radiotherapy oncology, Marseille, France Purpose or Objective Intra-fraction motion represents a crucial issue in the era of precise radiotherapy in VMAT breast irradiation. Continuous surface imaging offers the capability to monitor patient movements in three-dimensional space without any additional radiation exposure. During the implementation phase of 3D surface imaging system (AlignRT, Vision RT™, London, UK) ) in our institution, we noticed that the video camera-based patient localization