ESTRO 37 Abstract book

S1070

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

thoracic thermoplastic mask was used with manual abdominal compression. CT images were exported to the TPS Oncentra External Beam v.4.5 (Elekta, Crawley) to contour pulmonary lesions and mediastinal nodes as Gross Tumor Volume (GTV). A preliminary isocenter at the center of the GTV was defined to allow the 4D-CBCT acquisition on Elekta Agility Linear Accelerator. The patients underwent 4D-CBCT with Symmetry software that sorted in 10 breath phases. Subsequently, GTVs were delineated on all phases of 4D-CBCT to define Internal Target Volume (ITV). ITVs were expanded of 5 mm to define PTV. Definitive VMAT treatment planning were created; 4 patients received 50 Gy (10 Gy/fraction) and 2 patients received 35 Gy (7 Gy/fraction). During treatment course a total of 30 4D-CBCT were acquired. Translations were measured in medio-lateral (x), supero- inferior (y) and antero-posterior (z) directions, as well as in rotation around axes. Translational displacements ≥ 2 mm were corrected on-line. Cut-off rotation should be ≤ 3°; if rotation was > 3°, the patients were repositioned. Results ITV was obtained from union of GTVs delineated on all breath phases of 4D-CBCT (Table 1). The mean setup error was 0.18 cm (range 0.18-0.33) in lateral direction, 0.24 cm (range 0.12-0.46) in longitudinal direction, and 0.23 cm (0.08-0.53) in vertical direction. Mean values distribution of translation displacements for each patient were reported in Figure 1.

Conclusion From our preliminary experience, acquisition of 4D-CBCT with Symmetry Elekta X-Ray software, seems to be a useful method for evaluating organ motion in thoracic stereotactic radiotherapy. It is certainly necessary to increase the number of patients in order to confirm these data and in the optics of PTV margin personalization.