Abstract Book

S1069

ESTRO 37

EP-1965 Predictors of cardiac dose reduction achieved with deep inspiration breath hold C. Grondelli 1 , G. Benecchi 2 , M. Maddalo 2 , S. Nurmohamed 1 , S. Gianni 1 , R. Rossi 1 , M. Manicone 1 , S. Andreani 1 , F. Ghetti 1 , F. Salaroli 1 , E. Calabri 2 , R. Rossi 2 , I. Moschini 3 , C. Dell'Anna 1 , G. Ceccon 1 , M. Bergamini 1 , P. Losardo 1 , C. Ghetti 2 , N. D'Abbiero 1 1 Azienda Ospedaliero Universitaria di Parma, Radiotherapy, Parma, Italy 2 Azienda Ospedaliero Universitaria di Parma, Medical Physics Unit, Parma, Italy 3 Azienda AUSL Piacenza, Radiotherapy, Parma, Italy Purpose or Objective Deep inspiration breath-hold (DIBH) has been routinely applied in left breast radiotherapy to reduce cardiac exposure. The purpose of this study was to compare deep inspiration breath hold and free breathing techniques (FB) in terms of heart and left-lung doses and to determine possible predictors of the dose reduction achieved with DIBH (ΔD BH-FB ). Material and Methods Ten patients with left breast cancer have been treated with postoperative radiotherapy in Parma Hospital. For each patient both the FB and the DIBH planning CT were acquired. 3DCRT treatment plans using tangential field- in-field beams were developed for both FB and DIBH. FB and DIBH dose distributions were compared in terms of D mean , V 5Gy , V 20Gy , V 30Gy of the heart and V 5Gy , V 20Gy of the left-lung. Many geometric/anatomical parameters were measured in the FB condition: heart and left-lung volumes (V heart , V lung-L ), maximum thickness of the same organs inside the tangential fields (d heart , d lung-L ), axial and para-sagittal cardiac contact distances (CCDax, CCDps), maximum tangential distance between beam-entrance and beam-exit (d entrance-exit ), maximum anteroposterior diameter of the chest. Results DIBH plans showed a significant reduction of D mean (- 60.9%), V 5Gy (-82.5%), V 20Gy (-93.6%) and V 30Gy (-95.8%) of the heart (p<0.01, Wilcoxon signed-rank test), while no significant difference in target coverage and left-lung V 5Gy , V 20Gy . The parameters which significantly correlated with ΔD BH-FB were: V heart (0.041≤p≤0.049, 0.63≤R≤0.65), d heart (p<0.001, 0.96≤R≤0.97), CCDps (0.036≤p≤0.045, 0.64≤R≤0.66) and d entrance-exit (0.036≤p≤0.048, 0.64≤R≤0.67). Conclusion This study confirmed how DIBH for left breast cancer treatment can decrease cardiac dose. The benefit of DIBH over FB strictly depends on chest shape, heart size and heart position. However, most of the parameters showed a weak correlation with ΔD BH-FB , so they can’t be used to effectively predict ΔD BH-FB . Only d heart seems to be a good predictor of ΔD BH-FB , but more cases must be included to confirm/reject these findings. EP-1966 Motion management in RT planning: 4D-MRI retrospective automatic sorting based on internal surrogate S. Ken 1 , Z. Celicanin 2 , O. Bieri 2 , P. Cattin 3 , L. Parent 1 1 Institut Claudius Regaud, Department of Engineering and Medical Physics, Toulouse, France 2 University of Basel Hospital, Department of Radiology- Division of Radiological Physics, Basel, Switzerland 3 University of Basel, Medical image analysis center, Basel, Switzerland Purpose or Objective At our institute, MR abdominal motion imaging is managed in clinical routine with acquisitions triggered on exhale phase and in treatment position. MR images are registered with 4D-CT for tumor delineation as better soft tissue contrast is given by MR modality. However in this configuration, only the exhale phase of the

respiratory cycle is registered. Therefore the dynamic behavior and the organ deformation are not captured. This study aims at presenting a novel retrospective gating approach for dynamic MR imaging during free breathing. Material and Methods 4D-MR liver images are acquired on a 1.5T MR scan using an 18-channel body flex coil. 4D-MRI acquisition is performed on healthy volunteers who gave their informed consent. This innovative sequence is based on a modified bSSFP sequence and consists of an interleaved acquisition of 2D image slices and navigators. The navigator is set and acquired at a fixed position in sagittal orientation, while axial image position is changing in order to cover the entire volume of interest. 4D-MR images are acquired over 40 slices with a 2.5mm thickness and a 0.44ms slice- time-resolution. The sequence is repeated 20 times to cover the entire respiratory cycle. Audio-coaching in the MR scan was provided and volunteers were asked to breathe at a constant period of 6 sec. Total acquisition time is less than 12 min. For 4D-MR images reconstruction, the period of the respiratory cycle is divided in 6 bins: 0%, 16%, 33%, 50%, 66% and 83%. This strategy is copied from 4D-CT, in order to prepare consistent registration for accurate lesion contouring in the application cases of liver SBRT. Motion position is given by the mean intensity in a rectangular region of interest (ROI) placed at the level of the diaphragm on the sagittal navigator (Fig. 1).

Results Mean intensity time course within the ROI is able to identify automatically the inspiration position corresponding to the phase 0%. Acquisition time interval between two consecutive inspirations defines each experimental respiratory cycle for the 20 repeated MR acquisitions. 2D MR image slices are automatically binned according to the 6 respiratory phases and stacked in 3D volumes (Fig. 2). As the ROI is defined by the user, 5 ROI voluntarily positioned differently, but still at the level of the diaphragm, were tested. For all ROI, mean respiratory cycles is 5.68 sec [1.76-7.92] and 94.48% of the inspiration positions are found each time for the same navigator slices. The volunteer for whom these results are given recognizes that her respiration was not always synchronized with the audio-coaching.

Conclusion This study confirms that retrospective automatic sorting of 4D-MR images acquired during free breathing is possible by using a ROI placed over the diaphragm on