ESTRO 36 Abstract Book

S892 ESTRO 36 _______________________________________________________________________________________________

reproduce the measured shift (Figure 1). Water equivalent thickness (WET) has been computed for each voxel of the 3 MidPCTs and revealed that the baseline shift between W1-CT and W2-CT led to a root mean square error (RMSE) of 0.52 mm in the GTV. This WET RMSE was reduced to 0.18 mm between W2-CT and the simulated BS-W1-CT. In addition, a proton therapy plan was optimized on the average W1-CT scan and recomputed on the average W2- CT and BS-W1-CT scans. Figure 2 compares the resulting DVH for all dose distributions. The dose distribution computed on BS-W1-CT reproduces the dose degradation observed on W2-CT. The degradations for D95 are 10.6% and 11.8% for W2-CT and BS-W1-CT, respectively. Similarly, the D90 degradations are 3.2% and 4%. Conclusion Our model was validated by comparing the WET and dosimetric deviations between a simulated scenario and the real data set. As a future work, we will use our model to automatically generate uncertainty scenarios to feed a TPS for robustness evaluation and optimization of proton therapy plans. For instance, the new 4D robust optimizer of the RayStation can easily consider multiple 4D-CT series during the optimization process.

EP-1644 Deep inspiration breath hold respiratory gated 3DRT for left breast cancer:Our clinical experience. M. Pinto Monedero 1 , M. Castanedo Álvarez 1 , J. Martínez Ortega 1 , N. Gómez González 1 1 Hospital Universitario Puerta de Hierro Majadahonda, Medical Physics, Madrid, Spain Purpose or Objective The purpose of this work is to describe Deep Inspiration Breath Hold (DIBH) Respiratory Gated 3D Radiotherapy Treatments (RT) in left breast cancer patients in our clinic and to compare the results to free breathing (FB) treatments. Material and Methods Patients were trained prior to simulation to evaluate suitability for DIBH technique. Varian Real Time Position Management (RPM) System (Varian Medical Systems, USA) was used to monitor the patients’ respiratory motion. They were asked to take a deep breath and hold it repeatedly. A deep breath amplitude was set as a reference level for the treatment sessions. For those patients eligible to this technique, two CT scans were acquired, under FB and DIBH conditions using a Toshiba Aquilion LB CT (Toshiba Medical Systems, Japan). Two treatment plans prescribed to 50 Gy in 25 fractions were computed for each patient: one in FB and one in DIBH conditions with XiO 5.02 treatment planning system (Elekta AB, Sweden). Average dose and V20 for heart and left lung as well as V95 for PTV were evaluated with the physician to decide the treatment technique. Patients were treated in a Varian Clinac-21 EX (Varian Medical Systems, USA) with analogous Varian RPM System. Patient position was verified through AP and lateral planar images in DIBH conditions. The same amplitude which was set as reference level at the simulation must be reached. Results 25 patients underwent this procedure since the technique was introduced at our clinic. Only one patient was found not eligible. 20 of them were finally treated under DIBH conditions, whereas 5 of them were treated in free breathing conditions. Table I summarize average V20 for heart and left lung as well as V95 for PTV. A significant heart dose sparing was achieved in every patient, as V20 was reduced by 77.4%. Moreover, the left lung benefits of a 24% reduction in V20. Table I: Main Dose volume histogram results: average PTV V95, V20 Left Lung, V20 Heart. V95 PTV V20 Left Lung V20 Heart DIBH 90,0% 21,65% 2,98% FB 89,0% 28,47% 13,17%